Why your

heat pump or air conditioner

compressor won't run or shut off.

   If your air conditioner or heat pump compressor won't shut off unless you pull the fuses or turn off the circuit breaker. You probably have a stuck contactor. The contactor is the name we give to the big relay that controls your air conditioner, heat pump or other large electrical device.

    If the indoor blower fan will not shut off either then the problem is NOT the contactor in the outdoor unit but a problem with the thermostat or the wiring to it either at the subbase or the furnace. Before replacing your thermostat you should isolate the cause of the problem or it will persist and drive you nuts. Look for wires touching or cut too long.

    A contactor or relay is an electrical device that takes a control signal usually 12 volts DC, 24, 120, 208-230 volts ac and creates a magnetic field to pull in a set of contacts that controls another device that may or may not get its electrical power from the same circuit. Note: 99% of most control circuits are 24 volts AC.

    The contactor has steel contacts that are plated with silver. When the silver wears away you have just plain steel and the steel will weld itself together. Sometimes if you whack the contactor with a screw driver you may free it for a while but the problem will re occur. This problem is especially a problem when a piece of equipment has not been run for a period of time.

    It is normal for the contactor to wear out over time like brake pads on a car. If the contactor shows any signs of wear it should be replaced as a preventative measure or as part of maintenance.

    If you suspect a stuck contactor a 100% accurate test is to turn off the power to the indoor section which removes control voltage to the outdoor section and it (outdoor section) keeps running. Do not keep messing with it, your contactor it is bad and you will destroy your compressor if you let it run!!!

    You can also have problems with spiders building nests in the contactor; the silk is an insulator and will prevent electricity from flowing.

    Replacing a contactor is a simple matter and they are readily available. We will ship you one if you ask.
 Costs vary from about $40 installed to several hundred dollars depending on the size and current carrying capacity.

    If your compressor does not shut off when it needs to and the indoor fan does you will destroy your compressor. The high and low pressure cut-outs won't work either.

Some heat pump systems use two run capacitors (total of 3 if you include the fan) instead of a crankcase heater for the compressor (GE, Rheem/Ruud and others). It is possible for the contactor to fail where the compressor or the fan will run but not the other. When replacing a contactor on a system like this be extremely careful to get the wiring correct or the compressor may be damaged.

    Contactors are also used in motor starters. Brand names include Allen Bradley, Cutler Hammer, Telemecanique, Furnas, Honeywell, General Electric (Mars), Robertshaw, White Rodgers, Steveco, ITT and others.

    Used on the following brands: American Standard, Amana, Arco, Arco-Air, Bryant, Carrier, Coleman Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Fraser Johnson, Goodman, General Electric, Hotpoint, Heil, Intertherm, Janitrol, Kenmore, Lennox (Armstrong, Johnson Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud, Sears, Stewart Warner, Trane, Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg Warner) and others.

    To send us a question about your unit please use our cooling Fill Out Form.
    For general heating questions please use this form

Image of 1819 Bulbs avaliable at Radio Shack used to test for 24 volts.
Images of 1819 lamps and sockets that should be used to test a contactor for 24 volts. For technical reasons and the presence of the "Cooling Anticipator" you should avoid using a volt meter to check for 24 volts at the coil of a contactor that you suspect has an open coil or you will read a phantom 24 volts.

Image of Sperry Clamp on Ammeter on two pole contactor.
Image of 2 pole contactor and Sperry Digisnap Clamp on ammeter, available at Lowes for about $80.
The "non contact volts feature" is worth the price.

Images of one pole contactors. 3 phase contactors can have as many as three or four poles plus auxilliary contacts.
Image of 1 pole contactor (Honeywell and another brand). Contactors can have as may as 4 poles. Most single phase contactors have 2 poles and three phase contactors will have 3 poles plus some auxiliary contacts. These are common items that can be ordered from any supply house. Do not try to order one by part number or an exact replacement. In most cases a 30 amp two pole contactor with a 24 volt coil will be correct. 

Final Warning:
    Do not try to clean or repair a contactor, if you have then stop and turn the power off to the equipment. Once the silver erodes all you have is plain steel and it will weld closed destroying your equipment and possibly starting a fire!!!. Unless you have an industrial contactor that costs hundreds of dollars that can be repaired, the only solution it to replace the contactor.

Related pages:
    Supply house list where you can buy these contactors.
    Thermostat page.
    Wiring page.
    Heat pump problem solving page.
    Heat pump page.
    Blower fan control page.
    Electric furnace page.
    Page with other contactor images and a clamp on ammeter.
    Solar hot water system with a fancy control panel.

  1. Blower: usually a squirrel cage centrifugal air moving device. Will move large volumes of air relatively quiet. Will use less energy with more back pressure.
  2. Fan: a paddle type air moving device used where noise is not a major consideration. Will use more energy with more back pressure.
  3. Combustion blower: A blower used on high efficiency gas furnaces or oil burners to move combustion air. usually 1/20 to 1/6  horsepower.
  4. OEM: Original equipment manufacturer.
  5. Horsepower: 746 watts
  6. RPM: revolutions per minute.
  7. Service factor: the extent to which a motor can be safely overloaded beyond its name plate ratting without over heating.
  8. Air over horsepower: The rating of a motor assuming air flow through the windings usually as a result of the air moving device.
  9. SAE: Society of Automotive Engineers.
  10. High efficiency furnace: Furnace that uses over 85% of the energy in the gas.
  11. Condensing furnace: Gas furnace that uses over 92% of the energy in the gas and condenses the gas into liquid condensate and hot air. If your furnace has PVC pipe venting it then you have a condensing furnace.
  12. Evaporator. The part of an air conditioner or refrigeration system that gets cold due to evaporating refrigerant.
  13. Condenser. The part of an air conditioner or refrigeration system that gives off heat by condensing the refrigerant.
  14. Indoor coil or unit. On a heat pump unit we call the inside section "indoor" so as not to confuse it with the evaporator on an air conditioning system.
  15. Outdoor coil or unit. On a hat pump we call the outside section "outdoor" so as not to confuse it with the condenser on an air conditioner.
    This page will be updated soon
 Good Luck Scott
To identify the components of your gas furnace and gas valve system click here.
If your heat pump forms ice outside in the heat mode click here.
If your air conditioner or heat pump ices up in the cooling mode click here.
If you have water leaking problems click here to solve it.
For other heat pump or air conditioning problems click here.
For other heating system problems click here.
 Any other questions feel free to contact us by any of the means below. good luck Scott.
    If you were looking for Ice Machine repairs click here.
Written By:  Scott Meenen N3SJH of:
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation. Anything having to do with Heat and Energy.
Serving MD, DC, and Northern VA.

Contact us

Email us at: jsmeenen@toad.net

                 This text written by: Scott Meenen * G & S Mechanical

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