Photograph of client using tissue to look for air conditioning air flow How to Diagnose & Repair
an Air Conditioner that Won't Start
Fix an A/C or Heat Pump that won't start
     


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What to check if your air conditioner or heat pump just won't start at all. Here we give a diagnostic sequence of things to check if the A/C is just not working. Checking these simple items, switches and controls, including some you may not know about (overflow pan switch for example) may get your system running without a costly service call.

This article forms part of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperature, or an air conditioner that won't start.

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What to Check First if the Air Conditioner or Heat Pump Won't Start

Air conditioning thermostat (C) Daniel FriedmanAir Conditioner Won't Start - Air conditioning system is not running at all:

Is there no cool air at all coming out of the supply registers?

Is the air conditioner or heat pump indoor or outdoor unit silent?

If so it's not running. Here is what to check first.

Check first that

  • the air conditioning equipment is turned on,
  • the thermostat is calling for cooling, and that
  • the blower unit or air handler is actually blowing air through the ductwork.

Here are the details of what to check in what order if your air condtioner or heat pump doesn't start at all when you set the room thermostat to call for cooling:

  1. Check the Room Thermostat Temperature Setting: Set the thermostat to at least 5 degrees below room temperature. Our elderly mom has no patience with switches and controls. She regularly calls her air conditioning service company with a service request, sometimes late at night, because she has simply failed to set the temperature on the thermostat lower than the room temperature. Don't drive your A/C like our mother.
  2. Check that the Room Thermostat is set to "Cool" not "Off" or "Heat". If the thermostat is not set to "cool" it is simply turning off your A/C. If the thermostat display is blank then it's not receiving power (for modern digital thermostats). Check that electrical power is on at the air handler and to the the low-voltage transformer that supplies power to the thermostat.

    If the thermostat has power, check that when you set the thermostat temperature down at least 5 degrees below room temperature the thermostat calls for cooling. If it doesn't then check for broken or shorted thermostat wires anywhere between the wall thermostat and the control board at the air handler.

    You can easily eliminate possible thermostat problems as a cause of failure of the air conditioner to start by simply eliminating the thermostat from the picture: disconnect the thermostat wires at the blower unit's control board and instead connect the two thermostat terminals directly together with a jumper wire. If the system starts then the problem is in the thermostat itself or in its wiring.

    If the thermostat is working but the compressor condenser unit won't start, you could skip ahead
    to COMPRESSOR CONDENSER DIAGNOSTICS but I wish you'd double check the remaining steps in this article first because there are some sneaky snafus listed below that might still be the problem.

  3. Check that electricity is on for the equipment. Check all of the electrical switches and controls that can turn electrical power off at the indoor air handler or at the outdoor compressor/condenser unit. There are more of these switches than you might guess. Here's a list of what to check:

    Electrical power switches and service switches outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel. Don't forget to check that the access covers to the equipment are properly closed and latched. Otherwise a
    BLOWER DOOR SAFETY SWITCH could be keeping the equipment from running.

    There are several other safety switches and controls, both manual and automatic that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch, an electric motor overload or overheat switches, and a condensate tray spillage detector switch.

    Some hard-to-find electrical switches on an air conditioner or heat pump could be keeping your air conditioner from starting, such as
    a FLOAT SWITCH on Condensate Tray that could
    be
    causing CONDENSATE PAN SWITCH LOCKOUT - condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.
    or
    a blower MOTOR OVERLOAD RESET SWITCH could be keeping a fan motor from starting.

    A bad or failed starter capacitor could also be leaving your system shut down, failing to start a blower, fan, or compressor motor.
    See CAPACITORS for HARD STARTING MOTORS

    Watch out: See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system.

If the air conditioning system is still not working, or if the A/C system is running but cool air is not coming out of the supply

Air conditioning thermostat (C) Daniel Friedman

Thermostat problems: Air Conditioner Thermostats:

Air conditioner won't turn on, or fan won't turn on or won't turn off.

See THERMOSTATS for a discussion of how air conditioner thermostats work and how the air conditioner blower fan controls work.

Air Conditioner Air Handler Blower motor Won't Start?

See MOTOR OVERLOAD RESET SWITCH just to be sure that the air handler blower fan motor has not shut off on thermal overload.

And if your blower fan is driven by a fan belt and an electric motor, of course check to see that the drive belt is in place and un-damaged. If the blower fan belt is broken the electric motor will run just fine (you may be able to hear it) but the blower fan assembly itself won't be turning.

Air Conditioner Won't Start - electrical problems

If the air handler or indoor blower assembly does not start in response to a call for cooling the no-start problem is probably at the indoor thermostat or at the blower assembly itself.

If the indoor air handler blower runs but the outdoor compressor/condenser unit never starts then the problem is more likely there. If the indoor air handler runs but the outdoor compressor/condenser does not,
see COMPRESSOR CONDENSER DIAGNOSTICS

1. Check that electricity is on for the equipment, everywhere. Check all switches and controls, including service switches, including outside by the compressor, inside at the air handler, and fuses or circuit breakers in the electrical panel.
See KEY HVAC SWITCHES service switches, circuit breakers, fuses, power

2. Check all of the safety and interlock switches: For example, condensate spilling into an overflow pan that uses a sensor switch can be enough to shut down your air conditioner.

There are several switches and controls, both manual and automatic that can leave an air conditioner or heat pump turned "off" such as a blower compartment door interlock safety switch

  • BLOWER DOOR SAFETY SWITCH - an open blower door will shut down the air handler unit
  • CONDENSATE PAN SWITCH LOCKOUT - a condensate tray spillage detector switch (water in the overflow tray shuts down the air handler) Below in this article we give more details about condensate drip tray or overflow tray safety switch problems.
  • RESET BUTTON, ELECTRIC MOTOR - electric motor overload or overheat switches - a motor overload switch will keep fan motors from running

See A/C - HEAT PUMP CONTROLS & SWITCHES to be sure you have found and checked every manual or automatic electrical switch on the system both at the outdoor compressor/condenser unit and indoors at the air handler and duct system.

Damaged air conditioner electrical wire (C) D Friedman T HemmElectrical problems: air conditioning system won't start: it may sound silly, but is the air conditioner turned on?

Has the cooling thermostat been set to "cool" and the temperature set below ambient room temperature?

Do both the outdoor compressor/condenser unit and the indoor blower fan/evaporator coil unit have electrical power?

Has the air conditioning electrical wiring been physically damaged or cut? Photo courtesy of Tim Hemm.

Are the power switches on at these units, are the fuses good, are the circuit breakers in the "on" position, and is the thermostat set correctly?

Air conditioner compressor problems, including compressor noises, hard starting, and burned-out compressors, are explained in detail beginning
at COMPRESSOR & CONDENSING COIL and including topics such as

3. Check for a blown fuse locally inside the air handler, such as on or near a control board.

4. Check for a bad contactor or start relay inside the air handler or outside at the compressor/condenser unit

5. Check (or your service tech will check) for a bad or failed starter capacitor for the fan motor in the blower assembly or outdoors at the compressor/condenser unit could also be leaving your system shut down, failing to start a blower motor fan indoors or outside the compressor condenser unit's fan, or compressor motor.

If an electric motor hums but won't start or won't keep running the problem could be a bad start/run capacitor. Of course a frozen bearing or burned-up compressor or motor or even low voltage can also mean a motor hums but won't start - we give links to those diagnostics in step 2 above.

See CAPACITORS for HARD STARTING MOTORS

Air Conditioner Won't Start or Stays Off Due to Condensate Pan Switch - details

Condensate tray lockout switch (C) Daniel Friedman

Condensate pan switch lockout: an attic or other air conditioner air handler condensate drip tray or drip pan located under an air handler is installed to catch air handler condensate leaks if the normal condensate drain system fails.

Some condensate pans have their own separate overflow drain (a proper installation) or share their drain with the normal condensate drain (an improper installation).

But on some air conditioning air handlers the installer may provide a condensate overflow pan switch rather than a separate pan drain line.

In this installation the switch is designed to turn off the air conditioning system if it finds condensate water in the overflow pan. The idea is to shut down the air conditioner before there is a more costly leak into the building insulation or ceiling.

Condensate leaks into the condensate pan can shut down the air conditioner.

In tracking down an air conditioner condensate leak, I found that the the condensate pump drain line, a small-diameter PVC pipe, was clogged with water mold.

That was why my air conditioner wasn't kicking on. It would of ended up costing at least a hundred dollars just for a service tech to fix something as simple as that. I had to take the pump apart and clean it. I took off the PVC drain line coming out of inside air conditioner and blow and clean it all out too. - Jacob Behrends, FL

(July 24, 2014) Randy said:

The central a/c in my Florida home (in late July) would not turn on and the temp in the house was well above the preset temp of 78. Fearing a complete failure (the system is 17 years old) I googled my problem and eventually found this very clear and concise Inspectapedia site.

The past few days I could hear water dripping in the condensate drip pan and new something was up as this never happened before. I climbed my ladder at 11:30p and sure enough the condensate float switch was parallel to the tray meaning it tripped and turned the a/c system off. Grabbed my shopvac and within minutes the immediate problem was solved - Now to clear the drain...Excellent DIY website!

So if your air conditioning system seems to be normal in all other respects but it simply won't turn on, check for a flooded condensate pan or a defective condensate switch. See these condensate overflow or drip tray fix-it articles:

 

Continue reading at ELECTRIC MOTOR DIAGNOSTIC GUIDE or select a topic from the More Reading links shown below.

Or see CONTROLS & SWITCHES, A/C - HEAT PUMP

Or see COMPRESSOR CONDENSER DIAGNOSTICS

More Reading

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