Capacitors are crucial components in air conditioning (AC) units, playing a vital role in the system's efficiency and performance. However, like all electronic components, they are not immune to failure. Understanding the common causes of capacitor failures can help in diagnosing issues early and maintaining optimal AC system performance.
One of the most frequent causes of capacitor failure is overheating.
Voltage fluctuations also pose a significant threat to capacitors. AC units are designed to operate within specific voltage ranges, but power surges or drops can stress capacitors beyond their limits. These fluctuations can be caused by electrical storms, grid instability, or even issues within the building's own wiring. Over time, this stress leads to wear and tear on the capacitor's internal structure, reducing its lifespan.
Another common cause of capacitor failure is aging. Over prolonged periods, capacitors naturally lose their ability to hold charge due to chemical changes in their materials. This degradation process is inevitable but can be accelerated by external factors like excessive cycling - turning the unit on and off frequently - which subjects capacitors to repeated charging and discharging cycles.
Environmental factors also play a role. Moisture intrusion into an AC unit can lead to corrosion of capacitor terminals or other internal components. Similarly, exposure to contaminants such as dust or pollutants can degrade connections and affect performance.
The impact of capacitor failures on AC systems cannot be overstated. A failing or failed capacitor may prevent an air conditioner from starting up properly or running efficiently. The compressor might struggle to kick-start without sufficient boost from a healthy capacitor, leading to increased energy consumption and higher utility bills. In severe cases, it may result in complete system shutdowns or damage other components due to overloading as they compensate for reduced performance.
In conclusion, understanding these common causes of capacitor failures-overheating, voltage fluctuations, aging, and environmental factors-is essential for maintaining an efficient AC system. Regular maintenance checks that include inspecting capacitors' condition can help identify potential issues before they escalate into major problems affecting overall system performance. By proactively addressing these vulnerabilities through timely repairs or replacements when needed, homeowners can ensure their air conditioning systems remain reliable throughout hot seasons while minimizing unexpected disruptions and costs associated with emergency repairs.
Capacitors are vital components in alternating current (AC) systems, playing a crucial role in energy storage and voltage regulation. When they begin to fail, the performance of the entire AC system can be compromised. Recognizing the symptoms and signs of a failing capacitor is essential for maintaining optimal functionality and preventing costly downtime.
One of the primary signs of a failing capacitor is an unusual noise emanating from the AC unit. This could manifest as a humming or clicking sound that occurs when the system is running. These sounds often indicate that the capacitor is struggling to hold its charge or discharge energy properly, resulting in operational inefficiencies.
Another common symptom of capacitor failure is a noticeable reduction in system performance. For instance, you might observe that your AC unit is no longer cooling effectively or that it takes longer to reach desired temperatures. This reduced efficiency can be attributed to the capacitor's inability to provide sufficient power boost during motor startup, which affects overall system performance.
Physical inspection can also reveal signs of capacitor failure. Bulging or leaking capacitors are clear indicators that something is amiss. Over time, capacitors may swell due to internal pressure buildup caused by overheating or electrical stress. Leaking oil or other fluids are additional red flags indicating potential failure.
Frequent cycling on and off of an AC unit can also signal capacitor issues. A healthy capacitor facilitates smooth transitions between cycles; however, if it begins to falter, you may notice more frequent restarts as the system struggles to maintain consistent operation.
In some cases, tripped circuit breakers can be traced back to capacitor problems. When a capacitor fails, it may cause power surges that overload circuits, leading them to trip as a preventive measure against electrical damage.
Understanding these symptoms allows for timely intervention before complete failure occurs. Regular maintenance checks and prompt attention to these warning signs can mitigate risks associated with faulty capacitors and ensure sustained AC system efficiency. In conclusion, being vigilant about changes in sound, performance fluctuations, physical anomalies, cycling patterns, and circuit behavior will help identify failing capacitors early and safeguard your AC system's integrity.
Capacitors are integral components within air conditioning (AC) systems, playing a pivotal role in their performance and efficiency. These devices store and release electrical energy, providing the necessary jolt to start the AC motors that power compressors, fans, and blowers. When a capacitor fails, it can significantly affect the cooling efficiency of an AC system, leading to a cascade of issues that compromise comfort and increase operational costs.
At the core of an AC unit's functionality is its ability to maintain a consistent and efficient cooling cycle. Capacitors assist in this process by ensuring motors operate smoothly with optimal power supply. A failed capacitor can disrupt this balance, causing motors to struggle or fail altogether. This results in reduced airflow or inadequate refrigerant compression, both of which directly impair cooling effectiveness. The immediate symptom might be that rooms take longer to cool down or never reach the desired temperature, leaving occupants uncomfortable during hot weather.
Moreover, when capacitors malfunction, they force other components within the system to work harder than usual to compensate for lost efficiency. This increased workload not only accelerates wear and tear on these parts but also leads to higher energy consumption. Consequently, homeowners may notice a spike in electricity bills without experiencing any improvement in cooling performance-essentially paying more for less comfort.
The impact of capacitor failures extends beyond just inefficiency; it can lead to complete system failure if left unaddressed. Motors reliant on capacitors may overheat due to insufficient starting torque or continuous operation under duress. Over time, this can cause motor burnout or damage other critical components like compressors-a scenario that often demands costly repairs or entire system replacements.
Preventative maintenance is key in mitigating these risks associated with capacitor failures. Regular inspections by qualified HVAC technicians can identify early signs of wear before they develop into major problems. Technicians typically check for bulging capacitors-a visual indicator of potential failure-and measure electrical charge retention capacity using specialized tools.
In conclusion, capacitors are small yet vital elements whose failures have outsized impacts on AC system performance and overall cooling efficiency. By understanding their function and recognizing symptoms of potential issues early on, homeowners can ensure their air conditioning systems operate smoothly throughout their lifespan while avoiding unnecessary discomfort and expenses. Investing in routine maintenance offers peace of mind by safeguarding against sudden breakdowns during peak usage periods when reliable cooling is most crucial.
Capacitors play a critical role in alternating current (AC) systems by improving power quality, enhancing efficiency, and reducing energy costs.
First, it's essential to understand the function of capacitors in an AC system. Capacitors store and release electrical energy, which helps to maintain voltage levels and improve power factor. A good power factor means that less reactive power is drawn from the grid, which translates into lower electricity bills since utilities often charge extra for low power factors. When capacitors are functioning correctly, they reduce the amount of apparent power needed from the grid, thereby lowering demand charges on electricity bills.
However, when a capacitor fails—whether due to wear and tear, electrical overloads, or manufacturing defects—the repercussions can ripple through the entire system. One immediate effect is a drop in the power factor. As the system's efficiency decreases without functional capacitors to offset reactive loads, more current is required to deliver the same amount of real power. This increase in current flow leads to higher line losses due to increased heat dissipation across resistive elements in the circuit.
Moreover, failed capacitors can cause voltage instability within the system. Capacitors help stabilize voltage by compensating for inductive loads such as motors and transformers; without them, voltage sags or surges may occur more frequently. These fluctuations not only disrupt sensitive equipment but also force other components within the system to work harder than necessary—leading to premature wear and potential failures elsewhere in the infrastructure.
Additionally, capacitor failures can result in increased operational costs beyond just higher energy consumption. For instance, without proper compensation for reactive power through functioning capacitors, businesses might face penalties from utility companies for poor power quality. Moreover, there may be added expenses related to maintenance and repairs as other parts of the system experience additional stress.
In conclusion, while capacitors are often overlooked components of AC systems due their relatively small size compared with other infrastructure components like transformers or generators—they perform vital functions that significantly impact electrical consumption and energy costs. Their failure can lead both directly and indirectly to higher operational expenses by decreasing overall efficiency and increasing susceptibility to costly penalties or repairs. Therefore, regular monitoring and maintenance of these components are crucial for ensuring optimal performance of AC systems while keeping energy costs under control.
Capacitors play a crucial role in the performance of air conditioning (AC) systems, acting as essential components that store and release electrical energy to help motors start and run efficiently. However, when capacitors fail, the repercussions can ripple throughout the entire AC system, leading to potential damage to other components and affecting overall performance.
At the heart of an AC system lies a delicate balance between various components working in harmony. Capacitors ensure this balance by providing the necessary jolt of energy required for motors to initiate movement and maintain operation. When a capacitor fails-whether due to age, electrical surges, or environmental factors-this equilibrium is disrupted.
One immediate consequence of capacitor failure is increased strain on motors, particularly the compressor motor. Without the extra boost from a functioning capacitor, motors may struggle to start or run inefficiently. This inefficiency not only compromises cooling performance but also generates excessive heat within the motor windings.
Moreover, a defective capacitor can cause voltage fluctuations that reverberate through the entire electrical system of the AC unit. These fluctuations may damage sensitive electronic controls and sensors designed to regulate temperature and humidity accurately. As these components become impaired, the system's ability to maintain desired comfort levels diminishes significantly.
Fan motors are also susceptible when capacitors fail. Fans circulate air over evaporator coils for efficient heat exchange; thus, their optimal performance is critical. A failing capacitor may reduce fan speed or prevent it from starting altogether, leading to inadequate airflow across coils and potentially causing them to freeze-a condition that exacerbates pressure imbalances within refrigerant lines.
The domino effect of these failures inevitably leads back to increased operational costs. With compromised efficiency comes higher electricity consumption as systems labor harder under suboptimal conditions. This not only inflates utility bills but accelerates wear-and-tear on all interconnected parts due to prolonged operation times.
Regular maintenance checks can mitigate these risks by identifying early signs of capacitor deterioration before they escalate into larger issues affecting multiple components within an AC system. Proactive replacement of aging capacitors ensures consistent performance while safeguarding against broader damage that could necessitate extensive-and expensive-repairs down the line.
In conclusion, while often overlooked due to their seemingly minor role compared with more prominent parts like compressors or condensers-capacitors hold significant sway over how effectively an AC system operates overall when functioning correctly versus when they fail ultimately highlighting why understanding their impact is vital for maintaining reliable cooling solutions year-round without encountering avoidable damages elsewhere across units themselves beyond just individual elements therein alone whenever possible instead too!
Capacitors play a crucial role in the performance of air conditioning (AC) systems, acting as vital components that store and release electrical energy to power the compressor, fans, and other essential parts. When capacitors fail, the effects on AC system performance can be significant, leading to inefficient operation or complete system shutdowns. Understanding how these failures occur and their impact is critical for effective troubleshooting and diagnosing capacitor issues.
At their core, capacitors are designed to provide the necessary voltage boost required by the motor during startup, as well as maintaining a steady flow of electricity once the motor is running. A failing capacitor can manifest in several ways: it may bulge or leak due to overheating, or it might lose its charge capacity over time due to wear and tear. These physical signs are often indicative of underlying issues such as electrical surges or prolonged exposure to high temperatures, both of which can degrade capacitor performance.
The most immediate effect of capacitor failure on an AC system is poor motor function. Without the initial jolt of electricity from a functioning capacitor, motors may struggle to start or fail altogether. This not only affects cooling capability but also places additional strain on other components within the system. In some cases, a faulty capacitor can cause motors to run intermittently or at reduced efficiency levels, leading to increased energy consumption and higher utility bills.
Furthermore, repeated attempts by a struggling motor to start without adequate support from the capacitor can lead to overheating and eventual burnout. This scenario not only compromises comfort by disrupting consistent cooling but can also result in costly repairs or replacements for both capacitors and motors. Additionally, an underperforming AC system creates ripple effects throughout its operational lifecycle—potentially shortening its overall lifespan while reducing reliability during peak demand periods.
Diagnosing capacitor-related issues requires careful observation and testing. Technicians typically employ multimeters to measure capacitance levels against manufacturer specifications; deviations from expected values often signal malfunctioning units that need replacement. It's equally important for maintenance professionals to inspect peripheral signs like unusual noises during startup phases or frequent tripping of circuit breakers—both could indicate underlying electrical problems linked back to failing capacitors.
To mitigate these risks proactively, regular maintenance schedules should include thorough inspections of all capacitors within HVAC systems before peak usage seasons begin. Ensuring clean environments free from dust accumulation around units helps prevent overheating scenarios known for exacerbating wear-and-tear processes on electronic components like capacitors themselves.
In conclusion, understanding how capacitor failures affect AC system performance underscores their integral role in ensuring efficient operation across residential and commercial settings alike. Through diligent troubleshooting practices combined with strategic preventative measures focused on early detection—and subsequent rectification—of emerging issues surrounding this critical component class within HVAC infrastructures today ensures optimal outcomes tomorrow when addressing indoor climate control challenges head-on amidst ever-evolving environmental conditions worldwide now more than ever before!
Capacitors play a crucial role in the efficient functioning of alternating current (AC) systems, serving as vital components that help regulate voltage and improve power quality. However, like any electronic component, capacitors are susceptible to failure, which can significantly affect AC system performance. Understanding preventive measures and maintenance tips is essential for ensuring the longevity and reliability of these systems.
One of the primary ways capacitor failures impact AC system performance is through voltage instability. Capacitors are designed to store and release electrical energy, helping to smooth out fluctuations in voltage levels. When a capacitor fails, this stabilizing function is compromised, leading to potential overvoltage or undervoltage conditions that can damage sensitive equipment connected to the system.
Moreover, failed capacitors can cause an increase in harmonic distortion within the AC system. Harmonics are unwanted frequencies that distort the waveform of electrical signals, leading to inefficiencies and potential overheating of equipment. This not only reduces the overall efficiency of power delivery but also shortens the lifespan of other components within the system due to increased thermal stress.
To prevent such detrimental effects on AC systems, regular maintenance and careful monitoring of capacitors are imperative. One effective preventive measure is routine inspection for physical signs of wear or damage. Capacitors should be checked for bulging cases, leaks, or corrosion-common indicators that they may be nearing failure.
Another critical maintenance tip involves testing capacitance levels regularly. Over time, capacitors can lose their ability to hold a charge effectively; thus, measuring their capacitance can provide insights into their health status. If a significant drop in capacitance is detected compared to nominal values, it may be time for replacement before complete failure occurs.
Environmental factors also play a role in capacitor longevity. Ensuring that capacitors operate within their specified temperature ranges is vital since excessive heat can accelerate degradation processes inside them. Proper ventilation around capacitors helps dissipate heat buildup during operation.
Additionally, selecting high-quality capacitors from reputable manufacturers during initial installation serves as a proactive step toward enhancing system reliability over time. While cost considerations often influence purchasing decisions initially; opting for quality products with proven track records minimizes risks associated with premature failures down line significantly impacting overall operational costs negatively long term if overlooked initially when setting up infrastructure investments wisely beforehand instead ensures future-proofing assets better upfront rather than regretting later once issues arise unexpectedly catching stakeholders off guard entirely without prior warning signs noticed earlier had precautions been taken accordingly beforehand already prepared adequately planned ahead strategically smarter wiser choices made proactively avoiding pitfalls altogether successfully achieved ultimately achieving desired outcomes effectively efficiently seamlessly integrated smoothly executed flawlessly consistently delivering optimal results continuously reliably sustainably indefinitely perpetually enduringly lasting long-term beneficial impacts experienced widely appreciated universally recognized highly valued esteemed greatly respected admired cherished treasured held dear regarded favorably positively embraced celebrated acknowledged commended praised lauded extolled exalted revered honored distinguished remembered fondly forevermore eternally immortalized enshrined glorified immortal legacy left behind testament testimony tribute homage owed paid respect gratitude expressed thanks given appreciation shown acknowledgment accorded credit attributed recognition conferred accolades awarded distinctions granted merits earned rewards received honors bestowed privileges afforded opportunities presented possibilities explored potentials realized dreams fulfilled aspirations achieved goals reached milestones accomplished objectives attained visions manifested realities actualized fantasies materialized wishes granted desires satisfied ambitions met plans implemented projects executed initiatives undertaken endeavors pursued ventures launched undertakings commenced enterprises begun businesses started companies formed partnerships established collaborations forged alliances created networks built communities engaged societies united movements initiated causes championed campaigns waged battles fought wars won victories secured triumphs celebrated successes enjoyed achievements savored satisfaction derived delight relished joy
Redirect to:
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 θερμÃÂÂÂÂÅ’ς thermos, "hot" and στατÃÂÂÂÂÅ’ς statos, "standing, stationary".
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.
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.
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:
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]
This covers only devices which both sense and control using purely mechanical means.
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.
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.
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).
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.
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.
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).
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.
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.
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.
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.
Not shown in the illustration is a separate bimetal thermometer on the outer case to show the actual temperature at the thermostat.
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.
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.
Older, mostly deprecated designations:
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.
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.
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.
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".
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.
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.
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.
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]