Air conditioner or heat pump won't start: 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. 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. 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:

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 check the following: 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. 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. 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 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.

Electrical problems: air conditioning system won't start: it may sound silly, but is the air conditioner turned on?ac unit copper value Has the cooling thermostat been set to "cool" and the temperature set below ambient room temperature?one ton ac unit Do both the outdoor compressor/condenser unit and the indoor blower fan/evaporator coil unit have electrical power?central air conditioner outside unit fan not working 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 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 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... 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: