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When the air handler blower of an air conditioning unit continues to run, it indicates a problem that needs to be resolved. When the HVAC is in the heat mode, it can cause a cool draft when the heat is not in the demand mode. When the unit is in the cooling mode, the draft is not so undesirable, but it wastes energy.
Check the thermostat. The fan should normally be set on auto. Make sure the temperature is in the satisfied condition.
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Pull the cover off the thermostat, then remove the mounting screws holding the thermostat to the sub-plate, and pull the thermostat off. If the fan stops running, the thermostat is the problem and should be replaced. If the fan continues to run, make sure the wires are secure and not touching. Replace the thermostat and its cover.
Remove the access panel from the air handler, to the controls. This is usually going to be the top panel. Remove the screws, then lift and pull the panel off.
If the heat is electric, the fan is probably controlled by a sequencer. When the thermostat calls for heat, the low voltage activates the sequencer, which in turn sends high voltage to the electric heat elements. After a predetermined time, to allow the heat elements to warm up, the sequencer sends high voltage to the blower. It is a common problem for the fan contacts to stick, causing the fan to run continuously.
Check the low voltage at the bottom terminals on sequencer. If the thermostat is not calling for heat, there should be no voltage reading. Check the high voltage contact terminals between line voltage and the fan. If there is not a voltage reading, this means the contacts are stuck closed, and the sequencer needs to be replaced.
Check the fan limit switch if the heater is gas or oil. Many fan limit switches have a push-pull switch for continuous or automatic operation. Make sure the switch is pushed in. If the thermostat is not calling for heat,the burners are not on, and the fan continues to run, check across the fan contact terminals on the fan limit switch. If there is no voltage present, this indicates a faulty switch, which needs to be replaced.
If there is an air conditioner, remove the screws to the control panel and pull it off. Locate the fan relay. With a voltmeter, check across the low-voltage coil. If the thermostat is not calling for cooling, there will be no voltage present. Check across the fan terminals. This will be a set of contacts that are normally open. One terminal will be connected to line voltage, while the other will go to the fan. If there is not a voltage reading, this indicates the contacts are stuck, and the relay needs to be replaced.
If you tap on a relay that has stuck contacts, they will normally break loose and the blower will shut off. The problem is that they usually fail to come back on. It's best to leave the blower running until you have the replacement part.
When replacing the part, be sure to turn the power off and tag it with a note, so no one will turn the power back on while you're working on it.