When the set temperature is reached the outside condenser shuts off but the air handler keeps running (for hours). The only way I can turn it off is by throwing the circuit breaker. The problem started a few weeks ago. I replaced the batteries in the thermostat and everything was working fine. Now the problem is reoccurring.
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Re: the air handler won't shut off
As long as the t-stat fan switch is in the auto position the problem sounds like fan relay is stuck. It looks like a little black box about 2 inches square. Has 5 positions on it two of them are for 24 volts. Be sure you turn off power before pulling it out. Make note of the arrangement of the wires maybe label as you remove them. ken
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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:
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.
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 DIAGNOSTICSbut 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.
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.
Check the electrical supply voltage. Even if electricity is on, if the supply voltage has fallen too far below the operating voltage range of your air conditioner it's likely that the system will not operate, particlarly, you may note that the compressor motor won't start.
See VOLTS MEASUREMENT METHODS
There is probably a stuck "contactor" on the condenser. Where the line voltage wires from your house connect to the unit there is a contactor. That part has a small set of contacts/points that open & close upon demand allowing voltage to feed through the unit. Those points are welded/stick together. A 2 pole 40amp contactor should fox it.
Sounds like the contactor in the condensing unit is stuck. That is why it will not turn off when the thermostat is satisfied. If the inside air handler is not blowing and the outside condensing unit is running, then this will cause the inside coils to freeze up rather quickly.
The contactor is located in the outside condensing uni in the electrical compartment.
If it is sticking it will need to be replaced.
I have a honeywell TH8320U thermostat and American standard Air handler with heating unit put in. THE thermostat works fine with the ac and follows the programs I have set. However the heat is a different story. The thermo clicks on to follow program but the unit does not run. (auto settings) If I place the thermo in heat and turn it to on I get heat?
There is a built in delay on all HVAC systems to evacuate conditioned air (hot or cold) from the ducts -- it's just more efficient that way... but if it's staying on for more that 4-5 minutes at most then there may be a problem with the fan relay... or possibly a limit switch.
This is due to thermal overload,Which can be caused by low oil in the compressor,the fan has stopped running on the unit outside,Dirty coils,or just a worn out compressor.Although this is thermal overload,It has nothing to do with outside temperature,But rather the internal temperature of the compressor itself.
the air handler is in the attic or in a closet right? most likely the drain switch mentioned is in a pan located underneath the air handler, the switch is actuated when the condensation drain tube is clogged, ( when it cannot drain the water out of the pan), the float switch is actuated and the unit shuts off to prevent any more condensation, thus preventing overflow into your house/apt. the drain line can be sucked out by using a simple wet/dry vacuum of any kind from the end of the drain line ( usually a 3/4 inch PVC pipe located somewhere outside ).