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Photograph of the outdoor safety switch for an air conditioning residential systemComplete List of All Air Conditioning & Heat Pump System Controls & Switches

  • A/C - HEAT PUMP CONTROLS & SWITCHES - CONTENTS: complete list of air conditioning system parts, controls and switches. What is the function of each air conditioning control or air conditioning switch? What check first if your air conditioning won't start. Key switches and controls to check if your air conditioner or heat pump is not working
  • POST a QUESTION or READ FAQs about how to find, identify, & use or troubleshoot each control or switch found on air conditioners or heat pumps.
  • REFERENCES
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HVAC system controls & switches: this article explains where to find and how to use the switches and controls for air conditioning and heat pump systems.

We list and explain the function of each air conditioner or heat pump control or switch, including providing identification photographs and troubleshooting tips.

HVAC control definitions & photos are organized by where they are found: indoors or outside of the building, and at the air handler/blower assembly or the outdoor compressor/condenser unit, or perhaps in other locations such as wall thermostats or electrical panel main switches.



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Air Conditioning & Heat Pump CONTROLS & SWITCHES

A/C Compressor control board (C) D FriedmanWe include photographs to assist readers in recognizing cooling system defects. Individual HVAC control articles at this website discuss certain of these controls in greater detail

Here we list all of the controls and switches on a typical split system air conditioner with indoor and outdoor components include the items listed just below.

We explain these many electrical switches and controls that control an air conditioner or heat pump system. You'll need to check these first if your air conditioner won't start or won't keep running.

If the A/C system won't operate, before requesting a service call check that it is turned on at every control, switch, or circuit breaker, and that the thermostat is properly set.

Article Series Contents

If the A/C or heat pump system will not run check all of these control and safety switches listed here before calling your service technician. If someone or some condition has turned one of these switches off, resetting it may be all that's needed. Not all of these switches will be present on every system; fuses may be used instead of circuit breakers; fuse pullouts may be used instead of a circuit breaker or fuse at some service switches.

Watch out: Safety warning: do not put your fingers or hands inside of a heating furnace or air conditioner blower or blower compartment without making certain that all electrical power to the unit has been shut off. If the blower starts turning you can lose a finger, and there are also electrical shock hazards in these areas.

Key Air Conditioner or Heat Pump Main Electrical Control Switches, Fuses, or Circuit Breakers

List of Outside A/C or Heat Pump Service Switches, Fuses, Circuit Breakers

Photograph of the outdoor safety switch for an air conditioning residential system, shown here with the cover open Photograph of the outdoor safety switch for an air conditioning residential system

The pair of photographs just above show the outdoor air conditioner or heat pump compressor/condenser service control switch, in this case a circuit breaker, installed outside at a compressor for a ductless cooling system compressor. More photographs of a ductless or split system air conditioning system are
at A/C TYPES, ENERGY SOURCES.

Our page top photograph is an important one to study further. The air conditioning system compressor/condenser service switch for this outdoor unit was a 240V fused circuit with outdoor fuses in the box shown in the photo.

In this close up photograph you can just make out that someone has installed 1/2" copper pipes in place of the original fuses.

This might be a dangerous installation, risking fire as well as a burn up of the air conditioning equipment. But as Mark Cramer pointed out, if the circuit were properly protected by breakers or fuses at the main panel, the insertion of metal pipes in these fuse sockets just converts the device from a fuse box to a simple switch.

In any case, simply installing fuses would restore the proper safety device but it's likely that further testing and diagnosis of the electrical circuit and the compressor/condenser unit will be needed to determine why someone installed copper pipes where fuses belonged in the first place. When someone converts fuses to a switch in this location it may be because the air conditioning system was frequently blowing the fuses -- someone wanted to force the compressor/condenser to run.

Compressor Condenser Controls & Switches

Circuit breaker(s) at the electrical panel protect the circuit supplying power to the air conditioning system. Typically separate circuit breakers (or fuses) power the compressor/condenser unit and the indoor air handler/blower assembly.

Watch out: your HVAC equipment may be run out of a sub-panel rather than the main electrical panel. Be sure you have found all of the electrical panels, sub panels, and manual control switches for the equipment both inside the building and outdoors.

Air conditioner compressor (C) Daniel Friedman

Watch out: really watch out: if you re-set a circuit breaker or replace a fuse and the breaker trips again or the fuse blows again, leave the equipment off and call a qualified service technician. You probably have an unsafe condition. Forcing electrical equipment to run can cause shock or fire.

Compressor/Condenser Unit: the "outside" portion of an air conditioner or heat pump, the compressor re-compresses refrigerant gas back into a liquid and in the process, moves heat (in the refrigerant) either from indoors to outside (air conditioning mode) or from outdoors to inside (heat pump mode) if the system is a heat pump.

See COMPRESSOR CONDENSER for details of the diagnosis and repair of compressor problems.

Photograph of an air conditioning compressor/condenser service switch that is unsafe

Compressor service switch:The air conditioning system compressor service switch is located outdoors, at the compressor/condenser unit, typically on a building wall near the outdoor unit, this switch may be a circuit breaker, a fuse block pull-out, or a simple electrical switch.

Service switches permit a service technician to work safely on the equipment by turning off electrical power to the unit without having to run back and forth through the building to the main electrical panel.

Where the compressor/condenser service switch is omitted some technicians are tempted to work on equipment while it is electrically "live", and a few of them get shocked. These switches are required by current electrical codes but may be absent on old A/C installations.

If no service switch is installed at the compressor/condenser you should have one installed at the next A/C service call or next electrical work done on the building.

In our photograph the A/C service switch is very unsafe for two reasons: first, because it is falling off of the wall it may be difficult to operate the switch. Second, ask yourself, "What happened to the screws that were used to fasten this switch to the building wall?" We've seen cases of the mounting screws falling into the switch box, causing a short circuit or even a fire. In this particular case the vinyl siding installers pulled the switch loose to install siding and didn't bother to reinstall it.

Question: replace the cover or door of an A/C cutoff box

(Sept 7, 2016) Nola said:
Can I buy just a door for a/c cut off box

Reply:

If you are referring to an indoor or outdoor electrical power switch, check with your local electrical supplier.

If you are referring to the compartment cover or door of an air handler, possibly so, if you can read the manufacturer's name and box model number, just give the manufacturer a call.

Air Conditioner/Heat Pump Contactor Relay Switch

A/C Compressor control board (C) D Friedman

A/C and heat pump systems use a contactor relay (circled at left) because the little 12-24V wall thermostat circuit and switches are not capable of handling the higher voltage used by the compressor/condenser motors.

The contactor relay is basically a low-voltage-operated switch [typically 12-14 volts] controlled by the low-voltage room thermostat) that switches a heavier-duty electrical relay to give 120V or 240V electrical power to the compressor/condenser unit.

Most A/C and heat pump contactor relays use an electromagnetic 24-volt two-pole contactor relay that is rated for 30 amps.

The "two poles" simply means that the relay switches two electrical wires simultaneously - which is what you'd expect if your heat pump motor is running on 240 Volts.

Tips for changing out or installing a new magnetic contactor relay on an air conditioner or heat pump

Match the part numbers correctly when installing a new contactor relay in an air conditioner or heat pump. If the coil that activates the relay is not the right one for the compressor motor, you can have either of these problems:

Coil strength is too low: the coil won't reliably energize the start circuit in the motor and the compressor will be hard to get started

Coil strength is too high: the contactor relay coil won't let go of the start circuit: it will keep the start circuit active in the system even after the compressor motor has started - resulting in burning out the starter circuit.

Also locate a magnetic contactor relay switch in the proper physical position (mount it like the original was mounted) don't just hang the relay by its wires (as in our photo above). Some relays may be affected by gravity, either holding points closed too long or not holding the points closed. A current-operated contactor relay (most new units) can be mounted in any position.

Watch out: Some of our readers report successfully replacing minor electrical components such as switches, relays, and contactors. But unless you are qualified to do so we do not recommend trying to do work on electrical systems and components in your home as there are potentially fatal electrical shock hazards. Because air conditioner compressor/condenser units include start/run capacitors even when you have turned off power you can get a nasty shock if you're not careful.

see CAPACITORS for HARD STARTING MOTORS

Also see SAFETY for ELECTRICAL INSPECTORS

and DMMs VOMs SAFE USE OF.

Air Conditioner, Heat Pump, Refrigeration System Operating Pressure Control Switches & Electrical Sensor Safety Switches

We distinguish between operating control pressure switches and safety switches that monitor against too-low or too-high refrigerant pressures in a refrigeration system. Both are described in detail
at  Pressure Controls & Safety Switches. Excerpts are just below.

Operating Pressure Control Switches for Air Conditioners, Heat Pumps, Refrigeration Equipment

Commercial refrigeration equipment pressure control, Ranco

Operating pressure control switches set the normal compressor cut-in and cut out pressures in commercial refrigeration systems including air conditioners, coolers, refrigerators freezers.

Commercial refrigeration equipment such as refrigerators (coolers) and freezers use a pressure control switch to set the cut-in and cut out pressures at which the compressor shall operate.

The pressure control switch and the thermostatic expansion valve may both need to be inspected, tested, and set to cooperate with one another, as I describe in an old field service call report described
at REFRIGERANT METERING DEVICES TEVs & EEVs

Shown at left is a Ranco™ single pressure control switch with an operating range of 12-50 psig and a differential range of 5-35 psig. Ranco produces a wide range of switches and controls including air conditioning controllers. This particular Ranco switch shown at left "opens" on low.

Available from Grainger and other refrigeration equipment suppliers.

The Ranco Type "O" single function pressure controls are very widely used on refrigeration systems and can operate either as normal operating controls or as protection devices
See Pressure Controls & Safety Switches).

Refrigeration System Pressure Safety Switches: protect against over pressure or under pressure in air conditioning or heat pumps

Electric sensor switch on commercial air conditioner (C) D Friedman Tomkiel

Some HVAC systems (air conditioners, heat pumps, refrigeration equipment) include other sensor switches that may include an electrical pressure or temperature transducer/sensor (photo at left) or an air conditioning or heat pump pressure sensing switch that detects improper (too high) or [in some systems including automotive air conditioning] too-low refrigerant pressures in the system.

Details about these safety switches are
at Pressure Controls & Safety Switches. Excerpts are just below.

For safety the air conditioning or heat pump pressure switch can shut off the system. When pressures return to normal the pressure safety switch normally auto-resets and operation can continue.

Watch out: As Ranco and others warn, both under pressure (that can damage the compressor) and overpressure (that can damage or eve blow something up) at an air conditioner or heat pump system - can be dangerous need to be guarded against.

List of Inside Air Conditioner or Heat Pump Controls, Switches, Major Parts

Control Circuit Board - Air Conditioner / Heat Pump

A/C Compressor control board (C) D Friedman

Control circuits (typically a "control board" shown in our photo at left) along with the contactor relay (discussed above) are used in the compressor/condenser to turn it off and on in response to the indoor thermostat's call for cooling.

While diagnosing a circuit or component problem within an air conditioner or heat pump control board is beyond the skill of most homeowners, a simple visual inspection might show you that the control board has been visibly burned, broken, or damaged. Of course the board may look OK and still be damaged.

See A/C - HEAT PUMP CONTROLS & SWITCHES for details.

Air Conditioner or Heat Pump Controls In or On the Air Handler Unit or Blower Compartment & At the Cooling Coil

Air conditioner air handler with blower compartment open showing evaporator coil (C) Daniel Friedman

Air Handler: the air conditioner or heat pump air handler is the "indoor" portion of the cooling (or heating) system whose job is to condition air from the living space by blowing air across a cooling coil (air conditioning) or heating coil (heat pump), sending the conditioned air on through supply ducts into the occupied space. The primary parts of the air handler unit include:

Photograph of an air conditioning thermal expansion valve for a heat pump

Thermostatic expansion valves & other refrigerant metering devices:An air conditioner thermal expansion valve or "TEV" or just "expansion valve" (tan colored device in the photo) is a device located at the cooling coil and connected between the incoming refrigerant line and the refrigerant inlet to the cooling coil in the air handler.

The thermostatic expansion valve is a refrigerant metering control device, and it is not a control or switch which can be directly operated when using an air conditioning system, but it is a critical control needed for metering refrigerant into the cooling coil, so we include its description here.

The TEV shown in this photo is used on a heat pump system so it includes extra tubing so that it can permit the refrigerant to reverse its flow of direction when changing from cooling mode (move indoor heat to outdoors) to heating mode (collect and move outdoor heat to indoors).

There is a variety of refrigerant metering devices and they're not all called "TEVs":

See THERMOSTATIC EXPANSION VALVES for details about the function, inspection, and installation of thermostatic expansion valves, automatic expansion valves,

CAPILLARY TUBES
, manual and adjustable and non-adjustable expansion valves, high side and low side float valves, all of which are used to control refrigerant flow in refrigeration equipment such as heat pumps, air conditioners, refrigerators, freezers, and dehumidifiers.

A/C or Heat Pump Air Handler Blower Door Safety Switch & Blower Compartment Switches

Armstrong Blower door safety interlock switch (C) InspectAPedia available from American HVAC Parts.com

Blower compartment door switch: an air handler blower compartment access door switch is present on newer units, and can be seen as a button or switch which is depressed when the blower compartment door or cover is properly in place.

[Click to enlarge any image]

This switch shuts off the blower fan as a safety control if the door is opened.

If your air conditioner won't run and someone has been fooling with it, be double sure that the blower compartment door is properly closed and that the interlock switch sensor button or lever is properly depressed to convey that fact to the system.

Quoting Trane's typical equipment instructions:

The blower door safety switch will prevent or terminate furnace operation when the blower door is removed 

Details about air handler, furnace, or air conditioner blower compartment door safety switch installation, testing, repair or replacement are
at BLOWER DOOR SAFETY SWITCH

Other switches found inside the air handler unit

Backup Heat Controls on Heat Pumps

Condensate Overflow Pan Safety Switch

Air conditioning condensate drip tray overflow switch

Condensate overflow tray sensor switch: at the air handler, especially in an attic or closet or upper floor air handler, installers may provide an electrical switch (rather than a separate drain pipe) to detect spillage of air conditioner condensate out of its normal air conditioning condensate drain pipe.

When we cool air inside the air handler, that step causes moisture in the air to condense out as a liquid condensate that must be collected and disposed-of.

The condensate overflow switch is a sensor which turns off the air conditioning system as soon as it detects water in the condensate drip tray - thus avoiding a costly leak into building ceilings or floors and perhaps avoiding a mold contamination problem.

See CONDENSATE PAN FLOAT SWITCH in our discussion of condensate drip tray defects

and CONDENSATE PAN SWITCH LOCKOUT in our discussion of reasons why an air conditioner or heat pump won't start

Thanks to Lester Richter for this tip.

Circuit breaker - compressor: in the building main electric panel there will be a switch controlling power to the compressor/condenser unit. Typically this is a 240V circuit operated by a double pole circuit breaker or a fuse pair.

Circuit breaker - air handler/blower: in the electric panel there will be a circuit breaker or fuse controlling power to the air handler/fan unit which distributes cool air in the building. This will typically be a separate 120V circuit dedicated to protecting the circuit which supplies electric power to this equipment

Duct System Components, Controls, Switches

Duct System: the air conditioning air ducts (or ductwork) (or heat pump duct work) carry conditioned air from the air handler to various rooms in the occupied spaces of the building.

Return air ducts bring air from the occupied space to the air handler. In some installations only a single return air register and return air duct may be installed, usually in the ceiling over a stairwell in a two-story home; where multiple return ducts are provided you will find two or more return registers in the building; If you observe that most rooms have only a supply register and duct and no individual return air ducts, keeping the room doors open will probably improve air circulation and reduce heating or cooling costs for the building.

Supply air ducts bring air from the air handler (where it has been cooled and dehumidified, or heated if a heat pump is in heating mode) back into the occupied spaces in the building. Where each supply duct enters a room through the room ceiling, wall, or floor, a finned supply register should be installed to permit control of the direction and amount of air that exits the duct at that location.

Zone dampers: HVAC ducts in some installations may include motorized zone dampers controlled by individual room thermostats or switches.

See ZONE DAMPER CONTROLS for details about these controls.

See DUCT SYSTEMS for details about inspecting, diagnosing, and correcting a broad range of HVAC air duct problems.

Duct System Filters on HVAC Systems

Electrostatic air filter on an air handler (C) Daniel Friedman

Filters: air filters provide one or more levels of air filtration should be found installed on any air conditioning or warm air heating system.

Our photo shows an electrostatic air cleaner installed on an air handler. This installation included a disposable air filter installed at a central return air inlet grille (not visible in this photo) so that the air cleaner itself needed cleaning less often.

Air filters may be installed at the return registers, at the air handler, or on occasion at other locations.

See AIR FILTERS for HVAC SYSTEMS.

Starter Capacitors and Start/Run Capacitors on HVAC System Motors

A bad starter capacitor or the need for one where none is installed can be the cause of motors that fail to start, motors that chatter or stutter, or electric motors that won't keep running (bad run capacitor).

See CAPACITORS for HARD STARTING MOTORS.

Thermostats & Controls on HVAC Systems

Photograph of the indoor thermostat for an air conditioning residential system

The photograph above shows a typical indoor thermostat used to control heating or cooling. Note that in this photo the thermostat is switched to "heat" mode. The air conditioner will not run with the switch set to "heating".

Thermostats for air conditioning systems or heat pumps:

Photograph of the outdoor safety switch for an air conditioning residential system, shown here with the cover open

The photo just above shows a remote control thermostat used indoors to control the indoor wall-mounted cooling unit of a ductless cooling system.

An air conditioning system thermostat is a switch to turn on or off the A/C equipment as indoor air temperature varies around the thermostat's set point.

See THERMOSTATS for details of the operation of air conditioning, heat pump, and heating system thermostats and switches.

More photographs of a ductless air conditioning system are at A/C TYPES, ENERGY SOURCES.

Thermostatic Expansion Valves at A/C or Heat Pump Coils

Another type of thermostat that controls heating and cooling in your HVAC or heat pump system is the TEV or thermostatic expansion valve. This device monitors temperatures at the evaporator coil in the air handler and meters refrigerant into the coil accordingly. TEV devices can fail in several ways, Often involving a clogged sensor tube.

While it's possible to fool around with some TEV's, tap on them, adjust their metering rate, etc. this task should be left to your service technician since without proper training and test equipment you will have little idea where the adjustment has been set (if it can be set).

Photograph of an air conditioning thermal expansion valve for a heat pump

Thermostatic expansion valve:An air conditioner thermal expansion valve or "TEV" or just "expansion valve" (tan colored device in the photo) is a device located at the cooling coil and connected between the incoming refrigerant line and the refrigerant inlet to the cooling coil in the air handler.

The thermostatic expansion valve is a refrigerant metering control device, and it is not a control or switch which can be directly operated when using an air conditioning system, but it is a critical control needed for metering refrigerant into the cooling coil, so we include its description here.

The TEV shown in this photo is used on a heat pump system so it includes extra tubing so that it can permit the refrigerant to reverse its flow of direction when changing from cooling mode (move indoor heat to outdoors) to heating mode (collect and move outdoor heat to indoors).

See THERMOSTATIC EXPANSION VALVES for details about the function, inspection, and installation of thermostatic expansion valves.

Motors in HVAC Systems: Electric Motor Overload or Overheat Reset Switches

There are at least three electric motors in a conventional air conditioning or heat pump system, plus sometimes a fourth:

  1. An indoor blower fan motor - causing air to move across the evaporator (cooling) coil [or heating coil in heat pump mode], and through the duct work into occupied spaces.
  2. An outdoor compressor motor - compressing refrigerant gas back into a liquid
  3. An outdoor compressor unit fan motor - draws air across the condensing coils to condense hot refrigerant gas back to liquid form.
  4. Condensate pump motor: me systems also use a condensate pump or condensate drain pump to help remove condensate from the air handler to a building drain. A failure here that leads to lost cooling can be subtle: if the condensate removal system fails (bad pump, clogged drain) and if condensate spills into an overflow pan that uses a sensor switch to detect this condition, the failed condensate motor can actually stop an air conditioner from running.

Any of these motors can fail to start or may be "off on reset" due to a motor overload, or motor overheating.

Photograph of the overload reset button on an electric motor

Air conditioner fan or blower motor overload reset buttons:

An air conditioner fan or blower motor overload reset button may be present on the blower motor in the air handler/blower compartment.

Look for a red or yellow button which is normally flat with the motor surface but which will pop up to show that the motor has been shut off by its internal overload protection circuit.

Some electric motors (such as submersible well pumps and some A/C or heat pump compressors) have an internal thermal reset switch that will reset automatically when the motor cools down. Others such as that shown in our photograph have to be reset manually by pushing the button in when the motor has cooled.

See ELECTRIC MOTOR OVERLOAD RESET SWITCH for details of how to find and how to reset this switch when necessary.

Zone Dampers & Zone Controls on Heating and Air Conditioning Systems

Some heating and air conditioning systems use manual or automatic zone dampers to control the flow of conditioned air to different building areas. A manual damper is just a lever somewhere in the duct work that opens or closes a baffle inside the duct to limit air flow through that duct section.

An automatic zone damper may be controlled by an individual thermostat for the area served by that zone. So if the heating or cooling system is not running, or if it is running but not sending air to an area in the building, look for and check out any manual and automatic zone damper controls.

Details about zone dampers used on ductwork: warm air zones and air conditioning zones and zone dampers are
at ZONE DAMPER CONTROLS.

If the A/C system will not run check all of these control and safety switches before calling your service technician. If someone has turned one of these switches off, resetting it may be all that's needed. Not all of these switches will be present on every system; fuses may be used instead of circuit breakers; fuse pullouts may be used instead of a circuit breaker or fuse at some service switches.

Watch out: Safety warning: do not put your fingers or hands inside of a heating furnace or air conditioner blower or blower compartment without making certain that all electrical power to the unit has been shut off. If the blower starts turning you can lose a finger, and there are also electrical shock hazards in these areas.

How to diagnose and fix an air conditioning system that is not working

Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below.

To return to our air conditioning and refrigeration home page go
to AIR CONDITIONING & HEAT PUMP SYSTEMS.

If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.

...


Continue reading at CAPILLARY TUBES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES.

Or for heating equipment controls see BOILER CONTROLS & SWITCHES (hot water or steam heat) or see FURNACE CONTROLS & SWITCHES (forced air heating)

Or see ELECTRICAL POWER SWITCH FOR HEAT

Or see A/C HEAT PUMP CONTROL / SWITCH FAQs

The main operating controls for HVAC systems and their functions are are discussed
at SYSTEM OPERATION where you will find a discussion of thermostats, zone dampers, and circuit breakers on air conditioners and heat pump systems.

Other air conditioning and heat pump diagnostic procedures are
at LOST COOLING CAPACITY.

Suggested citation for this web page

CONTROLS & SWITCHES, A/C - HEAT PUMP at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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