How a Faulty Capacitor Affects an Air Conditioning System

How a Faulty Capacitor Affects an Air Conditioning System

climate control systems

Air conditioning systems are complex machines that rely on several components to function effectively. Older systems often require HVAC repair cost estimate to restore comfort to your home as soon as possible.. Among these, capacitors play a crucial role in starting the motors and maintaining a steady flow of electricity. When capacitors become faulty, they can severely affect the performance of an air conditioning unit. Understanding the common signs of a faulty capacitor is essential for timely maintenance and repair.


One of the most noticeable signs of a faulty capacitor is when the air conditioning unit fails to start altogether. Capacitors provide the initial jolt of electricity needed to start the compressor and fan motors. Without this jolt, the system may struggle to kick into action or may not start at all. This issue often manifests as a humming noise coming from the unit, indicating that power is reaching it but cannot initiate motor activity due to insufficient electrical charge.


Another indicator of capacitor issues is intermittent or inconsistent cooling performance. If you find that your AC system frequently cycles on and off or doesn't maintain a consistent temperature, a failing capacitor might be at fault. This inconsistency occurs because an unreliable capacitor can't store enough energy to sustain motor operation, leading to frequent interruptions in cooling.




How a Faulty Capacitor Affects an Air Conditioning System - energy efficiency upgrades

  1. residential HVAC systems
  2. radiant heating systems
  3. ventilation services

A further symptom can be observed through physical inspection: bulging or leaking capacitors. Over time, heat and electrical stress can cause capacitors to swell or leak their internal chemical compounds. While examining your unit, if you notice any bulging or leaking components, it's likely time for replacement.


Additionally, increased energy bills without a corresponding increase in usage could signal capacitor problems. A malfunctioning capacitor causes the AC system to work harder than necessary, drawing more power as it struggles to perform its usual tasks efficiently. As such inefficiencies accumulate over time, you'll see them reflected in higher utility costs.


Lastly, you might experience strange noises emanating from your air conditioning system when it's running with a bad capacitor. These sounds could range from clicking noises when attempting to start up to rattling sounds during operation-both indicative of electrical components struggling under strain.


In conclusion, recognizing these common signs of a faulty capacitor-such as startup failures, inconsistent cooling performance, physical damage like bulging or leaks, heightened energy bills, and unusual operational noises-can help diagnose issues early on before they lead to more significant problems within your air conditioning system. Timely intervention not only restores efficient functionality but also protects other components from undue stress caused by an impaired capacitor's inability to perform its critical role efficiently.

Air conditioning systems are a critical component of modern comfort, especially in regions with extreme temperatures. These intricate systems rely on various components working harmoniously to provide the desired indoor climate. Among these components, capacitors play a pivotal role, and their malfunction can significantly degrade system performance.


Capacitors in an air conditioning unit serve as energy storage devices that help start and run the compressor and fan motors. They store electrical energy and release it when needed to maintain the smooth operation of motors. When a capacitor becomes faulty, its ability to perform these functions is compromised, leading to several noticeable impacts on the air conditioning system's performance.


One of the most immediate effects of a faulty capacitor is difficulty in starting the air conditioner. The capacitor provides the initial jolt of electricity required for starting up the compressor and fans. If it fails, these components might struggle to start or may not start at all. This often results in a humming noise as the motor attempts to start without success, commonly referred to as "hard starting." Over time, this hard starting can cause further damage to other parts of the system due to increased stress.


Even if a faulty capacitor allows the air conditioner to start, it can lead to inefficient operation. The compressor may not run smoothly or at full capacity, resulting in inadequate cooling. You might notice that your air conditioning system takes longer than usual to cool down your space or struggles to reach set temperatures during peak heat times. This inefficiency not only affects comfort levels but also leads to higher energy consumption and increased utility bills.


Moreover, a failing capacitor can contribute to uneven cooling within different areas of your home or building. This occurs because when motors do not receive adequate support from capacitors, airflow may become inconsistent, causing some rooms to be cooler than others.


In severe cases, ignoring a faulty capacitor can lead to complete system failure. As other components compensate for impaired functionality over time, they undergo additional wear and tear which could result in costly repairs or even replacement.


Therefore, it's essential for homeowners and maintenance personnel alike to pay close attention to signs that may indicate capacitor issues such as unusual noises during startup or fluctuating performance levels. Routine maintenance checks by professionals should include examining capacitors for any signs of wear or damage before they escalate into more significant problems.


In conclusion, while small in size compared with other parts of an air conditioning unit, capacitors wield great influence over its overall performance capabilities. Addressing any issues promptly ensures that your cooling system operates efficiently without unnecessary strain on both individual components and financial resources associated with elevated energy use or repair costs due largely from neglecting seemingly minor faults like those found within this crucial device known simply enough: The Capacitor!

Long-term Consequences of Ignoring Capacitor Issues

In the realm of modern comfort, air conditioning systems stand as one of the most cherished inventions, providing relief from sweltering heat and creating a pleasant indoor environment. At the heart of these systems lies a component that often goes unnoticed until it fails: the capacitor. This humble part plays a crucial role in ensuring the efficient operation of an air conditioner. However, ignoring issues related to capacitors can lead to long-term consequences that are both costly and inconvenient.


Capacitors in air conditioning units serve as temporary storage devices for electrical energy, releasing it when needed to start the compressor and fan motors. This initial boost is essential for overcoming inertia and getting these components up to speed. When a capacitor becomes faulty, its ability to store and release energy diminishes, leading to significant operational challenges.




How a Faulty Capacitor Affects an Air Conditioning System - energy efficiency upgrades

  1. climate control systems
  2. duct cleaning
  3. energy efficiency upgrades

One of the immediate consequences of a faulty capacitor is reduced efficiency. The compressor might struggle to start or fail to run at optimal levels, causing the system to consume more electricity than necessary. This inefficiency not only increases utility bills but also places unnecessary strain on other components within the system.


Over time, this strain can lead to further mechanical failures. For instance, if the capacitor fails completely and is not replaced promptly, it can cause the compressor motor or fan motor to overheat due to repeated failed attempts at starting. Overheating may lead to permanent damage or even complete failure of these motors, resulting in expensive repairs or replacements.


Moreover, ignoring capacitor issues could shorten the lifespan of your entire air conditioning system. A system that operates under stress due to faulty components will undoubtedly wear out faster compared to one that is well-maintained. In essence, what starts as a minor issue with a small part can snowball into major complications affecting overall system longevity.


From an environmental perspective, inefficient systems contribute more significantly to carbon emissions due to increased energy consumption. By addressing capacitor issues promptly and ensuring regular maintenance checks are conducted by professionals, homeowners can reduce their carbon footprint while also benefiting from lower electricity costs.


Furthermore, there's an aspect of comfort that cannot be overlooked. An air conditioning unit plagued by capacitor problems may deliver inconsistent cooling performance - fluctuating between blasts of cold air and periods where it struggles ineffectively against rising temperatures indoors. Such inconsistency defeats the purpose of having an air conditioner in place altogether.


To conclude this exploration into how faulty capacitors affect air conditioning systems: prevention through regular maintenance should be prioritized over reactive measures once problems arise visibly; proactive care ensures reliability during peak usage times when having functional AC units becomes more critical than ever before! Regular inspections allow technicians not only spot potential faults early but also provide peace mind knowing everything runs smoothly behind scenes without unexpected breakdowns disrupting daily life unexpectedly!

Long-term Consequences of Ignoring Capacitor Issues

Importance of Regular HVAC Maintenance for Detecting Faulty Capacitors

Regular HVAC maintenance plays a crucial role in ensuring the efficient operation of air conditioning systems, and one of the key components that can benefit significantly from such upkeep is the capacitor. Capacitors are small yet vital parts within an HVAC system, responsible for storing and releasing electrical energy necessary to power various components, including fans and compressors. When these capacitors become faulty, the entire air conditioning system can suffer from inefficiencies or even complete breakdowns.


A faulty capacitor can lead to several issues within an air conditioning unit. One of the most noticeable effects is diminished cooling performance. Since capacitors help kickstart and maintain the operation of motors within the system, a failing capacitor can cause these motors to struggle or fail to turn on entirely. This results in inadequate airflow, leading to insufficient cooling during hot weather when it is needed most.


Moreover, a malfunctioning capacitor often causes motors to overheat as they attempt to draw more power than usual. This not only reduces the lifespan of these motors but also poses a risk of further damage throughout the system due to increased wear and tear on other components. In some cases, this strain can lead to complete motor failure, necessitating costly repairs or replacements.


Additionally, faulty capacitors can result in higher energy consumption. Because they play a key role in regulating voltage levels for efficient motor operation, any inefficiency in their function forces the system to work harder than necessary. This increased demand for electricity not only raises energy bills but also contributes unnecessarily to environmental degradation through heightened power usage.


Regular maintenance enables technicians to detect early signs of capacitor failure before they escalate into more serious problems. During routine inspections, professionals can check for physical signs such as bulging or leaking capacitors and test their electrical output with specialized equipment. By identifying and replacing these defective components promptly, homeowners can ensure that their air conditioning systems continue operating smoothly while avoiding unexpected breakdowns.


Furthermore, regular maintenance offers additional advantages beyond merely addressing capacitor issues. It provides opportunities for cleaning filters, checking refrigerant levels, inspecting ductwork for leaks, and lubricating moving parts-all essential tasks that contribute to overall HVAC efficiency and longevity.


In conclusion, regular HVAC maintenance is indispensable for detecting faulty capacitors within air conditioning systems. By addressing potential problems early on through routine inspections and timely interventions by trained professionals, homeowners not only safeguard against inconvenient malfunctions but also ensure optimal performance from their AC units over time-ultimately saving money on repairs while reducing energy consumption and promoting environmental sustainability.

Steps Involved in Inspecting and Replacing Faulty Capacitors During Annual Maintenance

During the annual maintenance of an air conditioning system, inspecting and replacing faulty capacitors is a critical task that ensures the efficient operation of the system. A capacitor, though small in size, plays a pivotal role in the functioning of an air conditioner by providing the necessary initial jolt to start the compressor and fan motors. A faulty capacitor can lead to various issues such as inefficient cooling, increased energy consumption, and even complete system failure. Therefore, understanding how to properly inspect and replace these components is essential for maintaining optimal performance.


The first step in inspecting a capacitor is to ensure safety by turning off power to the air conditioning unit. This precaution prevents any electrical accidents during maintenance. Once power is disconnected, accessing the capacitor involves removing the service panel on the exterior unit where most capacitors are located.


After locating the capacitor, visual inspection helps identify any obvious signs of damage. Bulging or leaking capacitors are clear indicators of failure and should be replaced immediately. However, some faults may not be visible externally; thus, using a multimeter to test the capacitance levels provides a more accurate assessment. The multimeter should be set to capacitance mode before testing each terminal connection on the capacitor against its rated microfarad value printed on its casing.


If testing reveals that a capacitor's readings deviate significantly from its rated values or if it shows no reading at all (indicating an open circuit), replacement becomes necessary. To replace a faulty capacitor, note down its specifications - including voltage rating and capacitance value - ensuring that you find an exact match or compatible substitute.


Before installation of a new capacitor, discharge any residual electricity by bridging terminals with an insulated screwdriver – this step protects against potential shocks when handling metal parts later on. Installations require careful attention: connecting wires must match their original positions to prevent wiring errors which could lead to further operational issues.


Once installed securely within its bracket or holder inside your AC unit's control box area (and fastening screws tightened appropriately), restore power supply cautiously monitoring startup sequences for smoothness without unusual noises/vibrations indicating possible misalignment/misconnections needing adjustments/re-checks promptly addressed if noticed!


In conclusion: thorough inspection coupled with timely replacement whenever needed helps mitigate risks associated caused due negligence towards upkeep practices regarding crucial elements like capacitors running HVAC equipment smoothly throughout seasons without interruptions! Regularly scheduled evaluations ensure longevity cost-effectiveness investments made into home comfort solutions today tomorrow alike!

Benefits of Preventive Maintenance for Longevity and Efficiency of AC Systems

Preventive maintenance plays a crucial role in ensuring the longevity and efficiency of air conditioning systems. Among the various components that can impact an AC system's performance, the capacitor is often overlooked yet highly significant. A faulty capacitor can lead to a host of problems, underscoring the importance of regular maintenance.


At its core, a capacitor is an electrical component that stores and releases energy, crucial for starting the compressor and fan motors in an air conditioning system.

How a Faulty Capacitor Affects an Air Conditioning System - climate control systems

  1. emergency heating repair
  2. boiler maintenance
  3. smart thermostat installation
When functioning correctly, capacitors help maintain a stable flow of electricity, allowing these motors to perform efficiently. However, when a capacitor malfunctions or fails entirely, it disrupts this flow, leading to various operational issues.


One immediate effect of a faulty capacitor is difficulty in starting the AC system. Known as "hard starting," this issue causes the unit to struggle or fail to turn on altogether. This not only frustrates users but also puts extra strain on other components as they attempt to compensate for the lack of initial power boost provided by a healthy capacitor.


Moreover, even if the system manages to start with a defective capacitor, its efficiency drastically decreases. The motors may run less effectively or overheat due to inconsistent power supply, which increases energy consumption and reduces overall cooling performance. Over time, this inefficiency leads to higher utility bills and unnecessary wear and tear on other parts of the system.


The risk doesn't stop at inefficiency; prolonged operation with a faulty capacitor can cause irreversible damage to more expensive components like compressors or fans. Replacing these parts involves significant costs compared to regularly replacing capacitors during preventive maintenance sessions.


This is where preventive maintenance becomes invaluable. Regular inspections and timely replacement of worn-out capacitors ensure that your AC system operates smoothly without interruptions. Moreover, technicians can spot early signs of deterioration in other components during routine checks, mitigating potential failures before they escalate into costly repairs.


In conclusion, while capacitors might seem like minor players within an air conditioning system's complex structure, their role is anything but trivial. Ensuring their proper function through preventive maintenance not only extends the life expectancy of your AC unit but also enhances its efficiency-resulting in better performance and reduced operational costs over time. Thus, regular checks are not just recommended; they are essential for anyone looking to preserve both their comfort and investment in an air conditioning system.

 

A digital thermostat
Honeywell's "The Round" model T87 thermostat, one of which is in the collection of the Smithsonian.
A touch screen thermostat
An electronic thermostat in a retail store

A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.

Thermostats are used in any device or system that heats or cools to a setpoint temperature. Examples include building heating, central heating, air conditioners, HVAC systems, water heaters, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators. In scientific literature, these devices are often broadly classified as thermostatically controlled loads (TCLs). Thermostatically controlled loads comprise roughly 50% of the overall electricity demand in the United States.[1]

A thermostat operates as a "closed loop" control device, as it seeks to reduce the error between the desired and measured temperatures. Sometimes a thermostat combines both the sensing and control action elements of a controlled system, such as in an automotive thermostat. The word thermostat is derived from the Greek words θερμός thermos, "hot" and στατός statos, "standing, stationary".

Overview

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A thermostat exerts control by switching heating or cooling devices on or off, or by regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. A thermostat can often be the main control unit for a heating or cooling system, in applications ranging from ambient air control to automotive coolant control. Thermostats are used in any device or system that heats or cools to a setpoint temperature. Examples include building heating, central heating, and air conditioners, kitchen equipment such as ovens and refrigerators, and medical and scientific incubators.

Construction and control

[edit]

Thermostats use different types of sensors to measure temperatures and actuate control operations. Mechanical thermostats commonly use bimetallic strips, converting a temperature change into mechanical displacement, to actuate control of the heating or cooling sources. Electronic thermostats, instead, use a thermistor or other semiconductor sensor, processing temperature change as electronic signals, to control the heating or cooling equipment.

Conventional thermostats are example of "bang-bang controllers" as the controlled system either operates at full capacity once the setpoint is reached, or keeps completely off. Although it is the simplest program to implement, such control method requires to include some hysteresis in order to prevent excessively rapid cycling of the equipment around the setpoint. As a consequence, conventional thermostats cannot control temperatures very precisely. Instead, there are oscillations of a certain magnitude, usually 1-2 °C.[2] Such control is in general inaccurate, inefficient and may induce more mechanical wear; it however, allows for more cost-effective compressors compared to ones with continuously variable capacity.[3][clarification needed]

Another consideration is the time delay of the controlled system. To improve the control performance of the system, thermostats can include an "anticipator", which stops heating/cooling slightly earlier than reaching the setpoint, as the system will continue to produce heat for a short while.[4] Turning off exactly at the setpoint will cause actual temperature to exceed the desired range, known as "overshoot". Bimetallic sensors can include a physical "anticipator", which has a thin wire touched on the thermostat. When current passes the wire, a small amount of heat is generated and transferred to the bimetallic coil. Electronic thermostats have an electronic equivalent.[5]

When higher control precision is required, a PID or MPC controller is preferred. However, they are nowadays mainly adopted for industrial purposes, for example, for semiconductor manufacturing factories or museums.

Sensor types

[edit]

Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate to within a degree of temperature.

Common sensor technologies in use today include:

These may then control the heating or cooling apparatus using:

  • Direct mechanical control
  • Electrical signals
  • Pneumatic signals

History

[edit]

Possibly the earliest recorded examples of thermostatic control were built by a Dutch innovator, Cornelis Drebbel (1572–1633), about 1620 in England. He invented a mercury thermostat to regulate the temperature of a chicken incubator.[6] This is one of the first recorded feedback-controlled devices.

Modern thermostatic control was developed in the 1830s by Andrew Ure (1778–1857), a Scottish chemist. The textile mills of the time needed a constant and steady temperature to operate optimally, so Ure designed the bimetallic thermostat, which would bend as one of the metals expanded in response to the increased temperature and cut off the energy supply.[7]

Warren S. Johnson (1847–1911), of Wisconsin, patented a bi-metal room thermostat in 1883, and two years later sought a patent for the first multi-zone thermostatic control system.[8][9] Albert Butz (1849–1905) invented the electric thermostat and patented it in 1886.

One of the first industrial uses of the thermostat was in the regulation of the temperature in poultry incubators. Charles Hearson, a British engineer, designed the first modern incubator for eggs, which was taken up for use on poultry farms in 1879.[10]

Mechanical thermostats

[edit]

This covers only devices which both sense and control using purely mechanical means.

Bimetal

[edit]

Domestic water and steam based central heating systems have traditionally been controlled by bi-metallic strip thermostats, and this is dealt with later in this article. Purely mechanical control has been localised steam or hot-water radiator bi-metallic thermostats which regulated the individual flow. However, thermostatic radiator valves (TRV) are now being widely used.

Purely mechanical thermostats are used to regulate dampers in some rooftop turbine vents, reducing building heat loss in cool or cold periods.

Some automobile passenger heating systems have a thermostatically controlled valve to regulate the water flow and temperature to an adjustable level. In older vehicles the thermostat controls the application of engine vacuum to actuators that control water valves and flappers to direct the flow of air. In modern vehicles, the vacuum actuators may be operated by small solenoids under the control of a central computer.

Wax pellet

[edit]

Automotive

[edit]
Car engine thermostat

Perhaps the most common example of purely mechanical thermostat technology in use today is the internal combustion engine cooling system thermostat, used to maintain the engine near its optimum operating temperature by regulating the flow of coolant to an air-cooled radiator. This type of thermostat operates using a sealed chamber containing a wax pellet that melts and expands at a set temperature. The expansion of the chamber operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is determined by the composition of the wax. Once the operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the coolant recirculation flow and coolant flow to the radiator to maintain the engine temperature in the optimum range.

On many automobile engines, including all Chrysler Group and General Motors products, the thermostat does not restrict flow to the heater core. The passenger side tank of the radiator is used as a bypass to the thermostat, flowing through the heater core. This prevents formation of steam pockets before the thermostat opens, and allows the heater to function before the thermostat opens. Another benefit is that there is still some flow through the radiator if the thermostat fails.

Shower and other hot water controls

[edit]

A thermostatic mixing valve uses a wax pellet to control the mixing of hot and cold water. A common application is to permit operation of an electric water heater at a temperature hot enough to kill Legionella bacteria (above 60 °C, 140 °F), while the output of the valve produces water that is cool enough to not immediately scald (49 °C, 120 °F).

Analysis

[edit]

A wax pellet driven valve can be analyzed through graphing the wax pellet's hysteresis which consists of two thermal expansion curves; extension (motion) vs. temperature increase, and contraction (motion) vs. temperature decrease. The spread between the up and down curves visually illustrate the valve's hysteresis; there is always hysteresis within wax driven valves due to the phase transition or phase change between solids and liquids. Hysteresis can be controlled with specialized blended mixes of hydrocarbons; tight hysteresis is what most desire, however some applications require broader ranges. Wax pellet driven valves are used in anti scald, freeze protection, over-temp purge, solar thermal energy or solar thermal, automotive, and aerospace applications among many others.

Gas expansion

[edit]

Thermostats are sometimes used to regulate gas ovens. It consists of a gas-filled bulb connected to the control unit by a slender copper tube. The bulb is normally located at the top of the oven. The tube ends in a chamber sealed by a diaphragm. As the thermostat heats up, the gas expands applying pressure to the diaphragm which reduces the flow of gas to the burner.

Pneumatic thermostats

[edit]

A pneumatic thermostat is a thermostat that controls a heating or cooling system via a series of air-filled control tubes. This "control air" system responds to the pressure changes (due to temperature) in the control tube to activate heating or cooling when required. The control air typically is maintained on "mains" at 15-18 psi (although usually operable up to 20 psi). Pneumatic thermostats typically provide output/ branch/ post-restrictor (for single-pipe operation) pressures of 3-15 psi which is piped to the end device (valve/ damper actuator/ pneumatic-electric switch, etc.).[11]

The pneumatic thermostat was invented by Warren Johnson in 1895[12] soon after he invented the electric thermostat. In 2009, Harry Sim was awarded a patent for a pneumatic-to-digital interface[13] that allows pneumatically controlled buildings to be integrated with building automation systems to provide similar benefits as direct digital control (DDC).

Electrical and analog electronic thermostats

[edit]

Bimetallic switching thermostats

[edit]
Bimetallic thermostat for buildings.

Water and steam based central heating systems have traditionally had overall control by wall-mounted bi-metallic strip thermostats. These sense the air temperature using the differential expansion of two metals to actuate an on/off switch.[14] Typically the central system would be switched on when the temperature drops below the setpoint on the thermostat, and switched off when it rises above, with a few degrees of hysteresis to prevent excessive switching. Bi-metallic sensing is now being superseded by electronic sensors. A principal use of the bi-metallic thermostat today is in individual electric convection heaters, where control is on/off, based on the local air temperature and the setpoint desired by the user. These are also used on air-conditioners, where local control is required.

Contact configuration nomenclature

[edit]

This follows the same nomenclature as described in Relay § Terminology and Switch § Contact terminology. A thermostat is considered to be activated by thermal energy, thus “normal” refers to the state in which temperature is below the setpoint.

  • "NO" stands for "normally open". This is the same as "COR" ("close on rise"). May be used to start a fan when it is becoming hot, and to stop the fan when it has become cold enough.
  • "NC" stands for "normally closed". This is the same as "OOR" ("open on rise"). May be used to start a heater when it is becoming cold, and to stop the heater when it has become warm enough.
  • "CO" stands for "change over". This serves both as "NO" and "NC". May be used to start a fan when it is becoming hot, but also (on the opposite terminal), to start a heater when it is becoming cold.

Any leading number stands for number of contact sets, like "1NO", "1NC" for one contact set with two terminals. "1CO" will also have one contact set, even if it is a switch-over with three terminals.

Simple two wire thermostats

[edit]
Millivolt thermostat mechanism

The illustration is the interior of a common two wire heat-only household thermostat, used to regulate a gas-fired heater via an electric gas valve. Similar mechanisms may also be used to control oil furnaces, boilers, boiler zone valves, electric attic fans, electric furnaces, electric baseboard heaters, and household appliances such as refrigerators, coffee pots and hair dryers. The power through the thermostat is provided by the heating device and may range from millivolts to 240 volts in common North American construction, and is used to control the heating system either directly (electric baseboard heaters and some electric furnaces) or indirectly (all gas, oil and forced hot water systems). Due to the variety of possible voltages and currents available at the thermostat, caution must be taken when selecting a replacement device.

  1. Setpoint control lever. This is moved to the right for a higher temperature. The round indicator pin in the center of the second slot shows through a numbered slot in the outer case.
  2. Bimetallic strip wound into a coil. The center of the coil is attached to a rotating post attached to lever (1). As the coil gets colder the moving end — carrying (4) — moves clockwise.
  3. Flexible wire. The left side is connected via one wire of a pair to the heater control valve.
  4. Moving contact attached to the bimetal coil. Thence, to the heater's controller.
  5. Fixed contact screw. This is adjusted by the manufacturer. It is connected electrically by a second wire of the pair to the thermocouple and the heater's electrically operated gas valve.
  6. Magnet. This ensures a good contact when the contact closes. It also provides hysteresis to prevent short heating cycles, as the temperature must be raised several degrees before the contacts will open. As an alternative, some thermostats instead use a mercury switch on the end of the bimetal coil. The weight of the mercury on the end of the coil tends to keep it there, also preventing short heating cycles. However, this type of thermostat is banned in many countries due to its highly and permanently toxic nature if broken. When replacing these thermostats they must be regarded as chemical waste.

Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat.

Millivolt thermostats

[edit]

As illustrated in the use of the thermostat above, all of the power for the control system is provided by a thermopile which is a combination of many stacked thermocouples, heated by the pilot light. The thermopile produces sufficient electrical power to drive a low-power gas valve, which under control of one or more thermostat switches, in turn controls the input of fuel to the burner.

This type of device is generally considered obsolete as pilot lights can waste a surprising amount of gas (in the same way a dripping faucet can waste a large amount of water over an extended period), and are also no longer used on stoves, but are still to be found in many gas water heaters and gas fireplaces. Their poor efficiency is acceptable in water heaters, since most of the energy "wasted" on the pilot still represents a direct heat gain for the water tank. The Millivolt system also makes it unnecessary for a special electrical circuit to be run to the water heater or furnace; these systems are often completely self-sufficient and can run without any external electrical power supply. For tankless "on demand" water heaters, pilot ignition is preferable because it is faster than hot-surface ignition and more reliable than spark ignition.

Some programmable thermostats - those that offer simple "millivolt" or "two-wire" modes - will control these systems.

24-volt thermostats

[edit]

The majority of modern heating/cooling/heat pump thermostats operate on low voltage (typically 24 volts AC) control circuits. The source of the 24 volt AC power is a control transformer installed as part of the heating/cooling equipment. The advantage of the low voltage control system is the ability to operate multiple electromechanical switching devices such as relays, contactors, and sequencers using inherently safe voltage and current levels.[15] Built into the thermostat is a provision for enhanced temperature control using anticipation.

A heat anticipator generates a small amount of additional heat to the sensing element while the heating appliance is operating. This opens the heating contacts slightly early to prevent the space temperature from greatly overshooting the thermostat setting. A mechanical heat anticipator is generally adjustable and should be set to the current flowing in the heating control circuit when the system is operating.

A cooling anticipator generates a small amount of additional heat to the sensing element while the cooling appliance is not operating. This causes the contacts to energize the cooling equipment slightly early, preventing the space temperature from climbing excessively. Cooling anticipators are generally non-adjustable.

Electromechanical thermostats use resistance elements as anticipators. Most electronic thermostats use either thermistor devices or integrated logic elements for the anticipation function. In some electronic thermostats, the thermistor anticipator may be located outdoors, providing a variable anticipation depending on the outdoor temperature.

Thermostat enhancements include outdoor temperature display, programmability, and system fault indication. While such 24 volt thermostats are incapable of operating a furnace when the mains power fails, most such furnaces require mains power for heated air fans (and often also hot-surface or electronic spark ignition) rendering moot the functionality of the thermostat. In other circumstances such as piloted wall and "gravity" (fanless) floor and central heaters the low voltage system described previously may be capable of remaining functional when electrical power is unavailable.

There are no standards for wiring color codes, but convention has settled on the following terminal codes and colors.[16][17] In all cases, the manufacturer's instructions should be considered definitive.

Terminal code Color Description
R Red 24 volt (Return line to appliance; often strapped to Rh and Rc)
Rh Red 24 volt HEAT load (Return line Heat)
Rc Red 24 volt COOL load (Return line Cool)
C Black/Blue/Brown/Cyan 24 volt Common connection to relays
W / W1 White Heat
W2 Varies/White/Black 2nd Stage / Backup Heat
Y / Y1 Yellow Cool
Y2 Blue/Orange/Purple/Yellow/White 2nd Stage Cool
G Green Fan
O Varies/Orange/Black Reversing valve Energize to Cool (Heat Pump)
B Varies/Blue/Black/Brown/Orange Reversing valve Energize to Heat (Heat Pump) or Common
E Varies/Blue/Pink/Gray/Tan Emergency Heat (Heat Pump)
S1/S2 Brown/Black/Blue Temperature Sensor (Usually outdoors on a Heat Pump System)
T Varies/Tan/Gray Outdoor Anticipator Reset, Thermistor
X Varies/Black Emergency Heat (Heat Pump) or Common
X2 Varies 2nd stage/emergency heating or indicator lights
L Varies Service Light
U Varies User programmable (usually for humidifier)
K Yellow/Green Combined Y and G
PS Varies Pipe Sensor for two pipe heat/cool systems
V Varies Variable speed (many can function as W2)

Older, mostly deprecated designations:

Terminal code Description
5 / V 24 volt ac supply
4 / M 24 volt HEAT load
6 / blank Not heat to close valve
F Cool fan relay or Fault light
G Heat fan relay
H Heat valve
M Heat Pump compressor
P Heat Pump defrost
R Heat pump reversing valve
VR 24 volt auxiliary heat
Y Auxiliary heat
C Clock power (usually two terminals) or Cool relay
T Transformer common
Z Fan power source for "Auto" connection

Line-voltage thermostats

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Line voltage thermostats are most commonly used for electric space heaters such as a baseboard heater or a direct-wired electric furnace. If a line voltage thermostat is used, system power (in the United States, 120 or 240 volts) is directly switched by the thermostat. With switching current often exceeding 40 amperes, using a low voltage thermostat on a line voltage circuit will result at least in the failure of the thermostat and possibly a fire. Line voltage thermostats are sometimes used in other applications, such as the control of fan-coil (fan powered from line voltage blowing through a coil of tubing which is either heated or cooled by a larger system) units in large systems using centralized boilers and chillers, or to control circulation pumps in hydronic heating applications.

Some programmable thermostats are available to control line-voltage systems. Baseboard heaters will especially benefit from a programmable thermostat which is capable of continuous control (as are at least some Honeywell models), effectively controlling the heater like a lamp dimmer, and gradually increasing and decreasing heating to ensure an extremely constant room temperature (continuous control rather than relying on the averaging effects of hysteresis). Systems which include a fan (electric furnaces, wall heaters, etc.) must typically use simple on/off controls.

Digital electronic thermostats

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Residential digital thermostat
Lux Products' Model TX9000TS Touch Screen Thermostat.
Lux Products WIN100 Heating & Cooling Programmable Outlet Thermostat shown with control door closed and open.

Newer digital thermostats have no moving parts to measure temperature and instead rely on thermistors or other semiconductor devices such as a resistance thermometer (resistance temperature detector). Typically one or more regular batteries must be installed to operate it, although some so-called "power stealing" digital thermostats (operated for energy harvesting) use the common 24-volt AC circuits as a power source, but will not operate on thermopile powered "millivolt" circuits used in some furnaces. Each has an LCD screen showing the current temperature, and the current setting. Most also have a clock, and time-of-day and even day-of-week settings for the temperature, used for comfort and energy conservation. Some advanced models have touch screens, or the ability to work with home automation or building automation systems.

Digital thermostats use either a relay or a semiconductor device such as triac to act as a switch to control the HVAC unit. Units with relays will operate millivolt systems, but often make an audible "click" noise when switching on or off.

HVAC systems with the ability to modulate their output can be combined with thermostats that have a built-in PID controller to achieve smoother operation. There are also modern thermostats featuring adaptive algorithms to further improve the inertia prone system behaviour. For instance, setting those up so that the temperature in the morning at 7 a.m. should be 21 °C (69.8 °F), makes sure that at that time the temperature will be 21 °C (69.8 °F), where a conventional thermostat would just start working at that time. The algorithms decide at what time the system should be activated in order to reach the desired temperature at the desired time.[18] Other thermostat used for process/industrial control where on/off control is not suitable the PID control can also makes sure that the temperature is very stable (for instance, by reducing overshoots by fine tuning PID constants for set value (SV)[19] or maintaining temperature in a band by deploying hysteresis control.[20])

Most digital thermostats in common residential use in North America and Europe are programmable thermostats, which will typically provide a 30% energy savings if left with their default programs; adjustments to these defaults may increase or reduce energy savings.[21] The programmable thermostat article provides basic information on the operation, selection and installation of such a thermostat.

Thermostats and HVAC operation

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Ignition sequences in modern conventional systems

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Gas
  1. Start draft inducer fan/blower (if the furnace is relatively recent) to create a column of air flowing up the chimney
  2. Heat ignitor or start spark-ignition system
  3. Open gas valve to ignite main burners
  4. Wait (if furnace is relatively recent) until the heat exchanger is at proper operating temperature before starting main blower fan or circulator pump
Oil
Similar to gas, except rather than opening a valve, the furnace will start an oil pump to inject oil into the burner
Electric
The blower fan or circulator pump will be started, and a large electromechanical relay or TRIAC will turn on the heating elements
Coal, grain or pellet
Generally rare today (though grains such as corn, wheat, and barley, or pellets made of wood, bark, or cardboard are increasing in popularity); similar to gas, except rather than opening a valve, the furnace will start a screw to drive coal/grain/pellets into the firebox

With non-zoned (typical residential, one thermostat for the whole house) systems, when the thermostat's R (or Rh) and W terminals are connected, the furnace will go through its start-up procedure and produce heat.

With zoned systems (some residential, many commercial systems — several thermostats controlling different "zones" in the building), the thermostat will cause small electric motors to open valves or dampers and start the furnace or boiler if it is not already running.

Most programmable thermostats will control these systems.

Combination heating/cooling regulation

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Depending on what is being controlled, a forced-air air conditioning thermostat generally has an external switch for heat/off/cool, and another on/auto to turn the blower fan on constantly or only when heating and cooling are running. Four wires come to the centrally-located thermostat from the main heating/cooling unit (usually located in a closet, basement, or occasionally in the attic): One wire, usually red, supplies 24 volts AC power to the thermostat, while the other three supply control signals from the thermostat, usually white for heat, yellow for cooling, and green to turn on the blower fan. The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).

A thermostat, when set to "cool", will only turn on when the ambient temperature of the surrounding room is above the set temperature. Thus, if the controlled space has a temperature normally above the desired setting when the heating/cooling system is off, it would be wise to keep the thermostat set to "cool", despite what the temperature is outside. On the other hand, if the temperature of the controlled area falls below the desired degree, then it is advisable to turn the thermostat to "heat".

Heat pump regulation

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Thermostat design

The heat pump is a refrigeration based appliance which reverses refrigerant flow between the indoor and outdoor coils. This is done by energizing a reversing valve (also known as a "4-way" or "change-over" valve). During cooling, the indoor coil is an evaporator removing heat from the indoor air and transferring it to the outdoor coil where it is rejected to the outdoor air. During heating, the outdoor coil becomes the evaporator and heat is removed from the outdoor air and transferred to the indoor air through the indoor coil. The reversing valve, controlled by the thermostat, causes the change-over from heat to cool. Residential heat pump thermostats generally have an "O" terminal to energize the reversing valve in cooling. Some residential and many commercial heat pump thermostats use a "B" terminal to energize the reversing valve in heating. The heating capacity of a heat pump decreases as outdoor temperatures fall. At some outdoor temperature (called the balance point) the ability of the refrigeration system to transfer heat into the building falls below the heating needs of the building. A typical heat pump is fitted with electric heating elements to supplement the refrigeration heat when the outdoor temperature is below this balance point. Operation of the supplemental heat is controlled by a second stage heating contact in the heat pump thermostat. During heating, the outdoor coil is operating at a temperature below the outdoor temperature and condensation on the coil may take place. This condensation may then freeze onto the coil, reducing its heat transfer capacity. Heat pumps therefore have a provision for occasional defrost of the outdoor coil. This is done by reversing the cycle to the cooling mode, shutting off the outdoor fan, and energizing the electric heating elements. The electric heat in defrost mode is needed to keep the system from blowing cold air inside the building. The elements are then used in the "reheat" function. Although the thermostat may indicate the system is in defrost and electric heat is activated, the defrost function is not controlled by the thermostat. Since the heat pump has electric heat elements for supplemental and reheats, the heat pump thermostat provides for use of the electric heat elements should the refrigeration system fail. This function is normally activated by an "E" terminal on the thermostat. When in emergency heat, the thermostat makes no attempt to operate the compressor or outdoor fan.

Thermostat location

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The thermostat should not be located on an outside wall or where it could be exposed to direct sunlight at any time during the day. It should be located away from the room's cooling or heating vents or device, yet exposed to general airflow from the room(s) to be regulated.[22] An open hallway may be most appropriate for a single zone system, where living rooms and bedrooms are operated as a single zone. If the hallway may be closed by doors from the regulated spaces then these should be left open when the system is in use. If the thermostat is too close to the source controlled then the system will tend to "short a cycle", and numerous starts and stops can be annoying and in some cases shorten equipment life. A multiple zoned system can save considerable energy by regulating individual spaces, allowing unused rooms to vary in temperature by turning off the heating and cooling.

Setback temperature

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HVAC systems take a long time, usually one to several hours, to cool down or warm up the space from near outdoor conditions in summer or winter. Thus, it is a common practice to set setback temperatures when the space is not occupied (night and/or holidays). On the one hand, compared with maintaining at the original setpoint, substantial energy consumption can be saved.[23] On the other hand, compared with turning off the system completely, it avoids room temperature drifting too much from the comfort zone, thus reducing the time of possible discomfort when the space is again occupied. New thermostats are mostly programmable and include an internal clock that allows this setback feature to be easily incorporated.

Dummy thermostats

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It has been reported that many thermostats in office buildings are non-functional dummy devices, installed to give tenants' employees an illusion of control.[24][25] These dummy thermostats are in effect a type of placebo button. However, these thermostats are often used to detect the temperature in the zone, even though their controls are disabled. This function is often referred to as "lockout".[26]

See also

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Notes and references

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  1. ^ Energy Information Administration, Residential energy consumption survey, U.S. Dept. Energy, Washington, DC, Tech. Rep., 2001.
  2. ^ thermostathub (June 26, 2023). "Easy Home Heating: Get Started with the Danfoss Wireless Thermostat". Thermostat Hub. Retrieved October 23, 2023.
  3. ^ Homod, Raad Z.; Gaeid, Khalaf S.; Dawood, Suroor M.; Hatami, Alireza; Sahari, Khairul S. (August 2020). "Evaluation of energy-saving potential for optimal time response of HVAC control system in smart buildings". Applied Energy. 271: 115255. Bibcode:2020ApEn..27115255H. doi:10.1016/j.apenergy.2020.115255. S2CID 219769422.
  4. ^ Roots, W. K. (1962). "An introduction to the assessment of line-voltage thermostat performance for electric heating applications". Transactions of the American Institute of Electrical Engineers, Part II: Applications and Industry. 81 (3): 176–183. doi:10.1109/TAI.1962.6371813. ISSN 0097-2185. S2CID 51647958.
  5. ^ James E. Brumbaugh, AudelHVAC Fundamentals: Volume 2: Heating System Components, Gas and Oil Burners, and Automatic Controls, John Wiley & Sons, 2004 ISBN 0764542079 pp. 109-119
  6. ^ "Tierie, Gerrit. Cornelis Drebbel. Amsterdam: HJ Paris, 1932" (PDF). Retrieved May 3, 2013.
  7. ^ "An Early History Of Comfort Heating". The NEWS Magazine. Troy, Michigan: BNP Media. November 6, 2001. Retrieved November 2, 2014.
  8. ^ "Thermostat Maker Deploys Climate Control Against Climate Change". America.gov. Archived from the original on April 18, 2009. Retrieved October 3, 2009.
  9. ^ "Johnson Controls Inc. | History". Johnsoncontrols.com. November 7, 2007. Retrieved October 3, 2009.
  10. ^ Falk, Cynthia G. (2012). Barns of New York: Rural Architecture of the Empire State (paperback) (First ed.). Ithaca, New York: Cornell University Press (published May 1, 2012). ISBN 978-0-8014-7780-5. Retrieved November 2, 2014.
  11. ^ "Dr-Fix-It Explains a Common Pneumatic Comfort Control Circuit". dr-fix-it.com. RTWEB. 2005. Archived from the original on December 6, 2017. Retrieved November 2, 2014.
  12. ^ Fehring, T.H., ed., Mechanical Engineering: A Century of Progress, NorCENergy Consultants, LLC, October 10, 1980 - Technology & Engineering, p. 22
  13. ^ "Pneumatic-to-digital devices, systems and methods" (PDF).
  14. ^ Salazar, Diet (October 21, 2019). "Thermostats: Everything You Need to Know". Engineer Warehouse. Retrieved March 12, 2021.
  15. ^ Electrical potentials at and below 24 volts are classed as "Safety Extra-Low Voltage" under most electrical codes when supplied through an isolation transformer.
  16. ^ Sawyer, Doc. "Thermostat Wire Color Codes". dr-fix-it.com. Archived from the original on September 23, 2015. Retrieved March 7, 2015.[1]
  17. ^ Transtronics, Inc. "Thermostat signals and wiring". wiki.xtronics.com. Retrieved March 7, 2015.
  18. ^ Honeywell smart response technology
  19. ^ "Smart PID temperature control". smartpid.com. September 19, 2016. Retrieved October 10, 2018.
  20. ^ "Temperature Controllers Using Hysteresis". panasonic.com. October 18, 2011. Retrieved October 10, 2018.
  21. ^ "Summary of Research Findings From the Programmable Thermostat Market" (PDF). Energy Star. Retrieved March 12, 2021.
  22. ^ KMC Controls. "Room Sensor and Thermostat: Mounting and Maintenance Application Guide" (PDF). Retrieved April 12, 2021.
  23. ^ Moon, Jin Woo; Han, Seung-Hoon (February 1, 2011). "Thermostat strategies impact on energy consumption in residential buildings". Energy and Buildings. 43 (2): 338–346. Bibcode:2011EneBu..43..338M. doi:10.1016/j.enbuild.2010.09.024. ISSN 0378-7788.
  24. ^ Sandberg, Jared (January 15, 2003). "Employees Only Think They Control Thermostat". The Wall Street Journal. Retrieved September 2, 2009.
  25. ^ Katrina C. Arabe (April 11, 2003). ""Dummy" Thermostats Cool Down Tempers, Not Temperatures". Retrieved February 13, 2010.
  26. ^ Example datasheet of current art thermostat, exhibiting lockout functionality : http://cgproducts.johnsoncontrols.com/MET_PDF/12011079.pdf
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