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AIR CONDITIONING SYSTEMS
A/C COMPONENTS
CONTROLS & SWITCHES
A/C DATA TAGS
A/C TYPES, ENERGY SOURCES
RATED COOLING CAPACITY
AIR CONDITIONER BTU CHART
SEER RATINGS
OPERATING COST
SYSTEM OPERATION
OPERATING TEMPERATURES
OPERATING DEFECTS
LOST COOLING CAPACITY
COMPRESSOR CONDENSER
AIR HANDLER UNIT
AIR FILTERS for HVAC SYSTEMS
CONDENSATE HANDLING
CLEANING & Legionella BACTERIA
DUCT SYSTEM DEFECTS
A/C REFRIGERANTS
A/C DIAGNOSTIC FAQs
INSPECTION LIMITATIONS
CRITICAL DEFECTS
Air Conditioning "How To" Books

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Photograph of the outdoor safety switch for an air conditioning residential system Air Conditioning System Controls & Switches
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  • What are the basic air conditioning 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

Our site offers impartial, unbiased advice without conflicts of interest. We will block advertisements which we discover or readers inform us are associated with bad business practices, false-advertising, or junk science. Our contact info is at inspect-ny.com/appointment.htm.

This website lists and explains the 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. Example home inspection report language is provided to describe common air conditioning system defects. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

Air Conditioning & Heat Pump CONTROLS & SWITCHES

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

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 closeup 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 burnup 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 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 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
  • 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 breaker or a fuse pair.
  • Circuit breaker - air handler/blower: in the electric panel there will be a switch 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 this equipment.
  • Compressor service switch: Photograph of an air conditioning compressor/condenser service switch that is unsafe 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 this 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.
  • Thermostats for air conditioning systems:
    Photograph of the indoor thermostat for an air conditioning residential system Photograph of the outdoor safety switch for an air conditioning residential system, shown here with the cover open

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. Air conditioning thermostats are typically wall mounted in the living area. The thermostat, if it's a combination unit, may have both a temperature setting switch and a switch that can be moved from "cool" to "off", or "heat" positions. For the thermostat to call for cooling it must be in the "cool" position and the temperature set to a level below the ambient air temperature at the thermostat location. Thermostats contain an internal temperature sensor which compares the air temperature at the thermostat to the temperature called-for by the user, turning the cooling (or heating) equipment on or off as appropriate. The small barrell shaped silver device in the photo is a dessicant or dryer used to remove unwanted moisture from the A/C refrigerant.

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

  • Thermal expansion valve: Photograph of an air conditioning thermal expansion valve for a heat pump 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 thermal expansion valve is a refrigerant metering control 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). Technical note: The air conditioning system thermal expansion valve or "TEV" is a metering device which regulates the flow of refrigerant from the incoming high pressure side (from the compressor/condenser) into the low pressure side (in the cooling coil). This valve maintains the pressure difference (high and low) at the entry point to the cooling coil, thus assuring that as the high-pressure refrigerant enters the low pressure space of the cooling coil, it can "evaporate" from a refrigerant liquid to a gaseous form, thus producing the temperature drop that cools the cooling coil itself. (All cooling systems using refrigerants use some type of expansion valve, of varying complexity. Even a simple window air conditioner or a refrigerator make use of an expansion valve, in the form of a small-diameter capillary tube or "cap tube" which meters refrigerant into the cooling coil.) --[Photograph of the TEV courtesy of Alan Carson Carson Dunlop in Toronto.]
  • Air conditioner fan or blower motor overload reset buttons: Photograph of the overload reset button on an electric motor 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. When the motor has cooled sufficiently this button can usually be simply pressed back down to "reset" the switch. If the motor overload switch won't reset (stay depressed) either the motor is still too hot (wait) or there is another failure that needs diagnosis. Sometimes the reset button is present but hard to find, depending on the position in which the motor was bolted in place. If the installer rotated the motor so that the button is facing away from you and impossible to see, feel around on the bottom and back side of the motor for the button's presence. Especially if the motor's internal protection has trippped, it shoudl be easy to feel the button since it'll be sticking up about 1/2". We cheated a little in this photo: it's a photograph of an oil burner electric motor not an air conditioning blower fan motor. But this one shows the red reset button most clearly. You can see that in this picture the button is flush with the motor surface. Sometimes these buttons are hard to find but they are usually present on heating and air conditioning system electric motors for fans and blowers.

If the A/C system will not run check these 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.







AIR CONDITIONING SYSTEMS
A/C COMPONENTS
CONTROLS & SWITCHES
A/C DATA TAGS
A/C TYPES, ENERGY SOURCES
RATED COOLING CAPACITY
AIR CONDITIONER BTU CHART
SEER RATINGS
OPERATING COST
SYSTEM OPERATION
OPERATING TEMPERATURES
OPERATING DEFECTS
LOST COOLING CAPACITY
COMPRESSOR CONDENSER
AIR HANDLER UNIT
AIR FILTERS for HVAC SYSTEMS
CONDENSATE HANDLING
CLEANING & Legionella BACTERIA
DUCT SYSTEM DEFECTS
A/C REFRIGERANTS
A/C DIAGNOSTIC FAQs
INSPECTION LIMITATIONS
CRITICAL DEFECTS
Air Conditioning "How To" Books
More Information

InspectAPedia TM Home & Site Map
Air Conditioning
Contact Us

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

  1. AIR CONDITIONING SYSTEMS
  2. A/C COMPONENTS
  3. CONTROLS & SWITCHES
  4. A/C DATA TAGS
  5. A/C TYPES, ENERGY SOURCES
  6. RATED COOLING CAPACITY
  7. SEER RATINGS
  8. A/C OPERATING COST
  9. SYSTEM OPERATION
  10. OPERATING TEMPERATURES
  11. OPERATING DEFECTS
  12. LOST COOLING CAPACITY
  13. COMPRESSOR CONDENSER
  14. AIR HANDLER UNIT
  15. AIR FILTERS for HVAC SYSTEMS
  16. CONDENSATE HANDLING
  17. CLEANING A/C EQUIPMENT
  18. DUCT SYSTEM DEFECTS
  19. A/C REFRIGERANTS
  20. INSPECTION LIMITATIONS
  21. CRITICAL DEFECTS

Technical Reviewers

  • Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
  • Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
  • Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
  • Thanks to Scott at SJM Inspect for suggesting this EPA document and for technical editing remarks regarding our air conditioning website, SJM Inspection Service LLC, serves the entire state of CT, sjminspect.com 203-543-0447 or 203-877-4774 5/16/07
  • Reference: Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
  • Reference: Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979

AIR CONDITIONING SYSTEMS
A/C COMPONENTS
CONTROLS & SWITCHES
A/C DATA TAGS
A/C TYPES, ENERGY SOURCES
RATED COOLING CAPACITY
AIR CONDITIONER BTU CHART
SEER RATINGS
OPERATING COST
SYSTEM OPERATION
OPERATING TEMPERATURES
OPERATING DEFECTS
LOST COOLING CAPACITY
COMPRESSOR CONDENSER
AIR HANDLER UNIT
AIR FILTERS for HVAC SYSTEMS
CONDENSATE HANDLING
CLEANING & Legionella BACTERIA
DUCT SYSTEM DEFECTS
A/C REFRIGERANTS
A/C DIAGNOSTIC FAQs
INSPECTION LIMITATIONS
CRITICAL DEFECTS
More Information

InspectAPedia TM Home & Site Map
Air Conditioning
InspectAPedia Bookstore
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Environment
Exteriors
Heating
Home Inspection
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08/18/07 - 01/31/2006 - Created 3/28/95 www.inspect-ny.com/aircond/aircond07.htm - Web page design & content © 2007 Daniel Friedman All Rights Reserved