Air Conditioning System Controls & Switches
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Here is a full 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
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This website lists and explains the parts and controls for and function of an air conditioning system.
These components will also be present on heat pumps.
We include photographs to assist readers in
recognizing cooling system defects. Other articles at this website discuss certain of these controls in greater detail. See SYSTEM OPERATION for a discussion of thermostats, zone dampers, and circuit breakers on air conditioners. Also see THERMOSTATS and THERMOSTATIC EXPANSION VALVES.
Here at CONTROLS & SWITCHES we explain the many electrical switches and controls that control an air conditioner or heat pump system. You'll need to check these if your air conditioner won't start.
The pair of photographs just above show the 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 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.
Air Conditioning & Heat Pump Parts, Switches and Controls on a typical split system with indoor and outdoor components include the items listed just below.
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.
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:
Return air plenum: return ducts from the occupied space bring building air to this collection point for passage into the air handler
Air filter: may be located in or near the return air plenum or preferably, at return air registers. Additional air filtration equipment may be installed at the air handler on the return-air side, such as electrostatic air cleaners, UV light sterilizers, HEPA filters, or other air quality improvement devices. See AIR FILTERS for HVAC SYSTEMS for details.
Blower compartment & blower fan: housing a fan, usually a round squirrel cage fan and electric motor (to operate the fan) which moves air from the return air plenum on towards the cooling or heating coil. An access door to the blower compartment should have a blower door interlock switch, a safety device which will prevent the blower from operating if the door is opened (see below). See FAN AUTO ON Thermostat Switch for details about what turns the blower fan on and off.
Cooling coil or heating coil (also called evaporator coil) through which refrigerant is metered by a thermostatic expansion valve (see below) in order to cool and dehumidify (or in the case of heat pump mode, warm) air from the occupied space as air is blown across this coil.
Heat Pump Heat & Backup Heat Controls: if your cooling is provided by a heat pump system, during the heating season the same equipment is used to provide heating as was used to provide cooling during hot weather. Heat pumps in climates where temperatures can get lower than the heat pump can handle include a backup heat source and all heat pumps include additional controls: an outdoor thermostat that monitors outside temperature and decides when to switch the backup heating system on; a reversing valve that switches the direction of refrigerant, and an indoor thermostat which can be set to auto, heat, cool, or off modes.
Supply plenum: a compartment which receives conditioned air from the cooling (or heating) coil and delivers this air to one or more ductwork systems which in turn move air to various areas of the occupied space.
Air handler circuit breaker: in the electric panel there will be a switch controlling power to the air handler/blower circuit, perhaps
two different circuits, one for the air handler unit, and a second for the compressor/condenser unit.
Air handler service switch: at the air handler/blower unit, typically on the unit or on a building surface close to it
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.
This switch shuts off the blower fan as a safety control if the door is opened
Blower motor overload or overheat switch: a built-in overload protector used in some electric motors, especially the electric motor used to power the air handler's blower fan, may be tripped. See Blower Motor Switch for details.
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.
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 DRIP TRAY DEFECTS and also Use of float switches on air conditioning condensate overflow pans. Thanks to Lester Richter for this tip.
Circuit breaker - compressor: in the 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.
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.
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.
See DUCT SYSTEMS for details about inspecting, diagnosing, and correcting air duct problems.
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.
Thermostats for air conditioning systems or heat pumps:
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.
The left hand 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".
The right hand photo shows a remote control thermostat used indoors to control the indoor wall-mounted cooling unit of a ductless cooling system.
See THERMOSTATS for details of the operation of air conditioning, heat pump, and heating system thermostats and switches.
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.
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.
Heating and Air Conditioning Zone Dampers & Zone Controls: 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 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.
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.
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Additional technical contributors & reference sources for this article are listed below.
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Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Lester Richer, a professional home inspector, for the reminder that a bad air conditioner condensate drip tray switch can shut the whole system down.
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 system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.
CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volume
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
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