My Central air unit froze up today. Now my blower will not turn on...?

Question

The furnace unit inside the house is older and needs replaced but we need just a few more months out of it. We have a problem where every year after a hot week (or a few days) the unit will freeze up. Normally I clean the coils and let it thaw and it works again. The problem is this time the blower will not power up. I am able to get the compressor up and running outside but it has been about 5 hours and still the fan will not power up. I would like to check the relay and capacitor but have a few questions first. The capacitor is removable and I have a DMM (digital multimeter) to test it. Question is: Is this the same as any capacitor I would test in an electronics circuit just larger? Also the relay.... How is that tested? Thank you for all of your help

OK... Thank you for your help... Did what you said and everything looks good. Now, when I turn on the Furnace the gas burners light and the blower will turn on. When I turn the t-stat from auto to on the blower will not power up, and when I turn on the AC the condenser outside will turn on and it will get the coils cold, but the blower will not power up. Is it my t-stat or is it in the AC circuits?

Answer 1

The inside blower runs at a higher speed for air conditioning than for the furnace. That means it is a different winding connection on the blower motor. If the blower runs for the furnace but not for air conditioning, it may be that there is a problem with the connection or the relay controlling it. You've just got to get in there with your meter and check it out. If need be, you could disconnect the higher speed winding and jumper over to the furnace winding. The blower motor may also have several unused windings at speeds in between that you could use.

Answer 2

If you are comfortable with electric, take the black lead from the fan and hook it to a known hot. If the fan powers up, you know your fan works. Don't run it for long, just to see if it tuns over.

The capacitor should have a microfarad rating in .mfd or a greek lowercase Mu, 7.5, 5, 6 somewhere in that range. Your DMM needs to have a microfarad test onboard, otherwise a resistance test will only tell you if it is still connected. If you have a mfd test, place one probe on one terminal, and the other probe on the other terminal. Let the meter reach peak, and the mfd should be within 6% of the rating. If you don't have a mfd test, put your DMM on resistance and read the resistance of your capacitor from pole to pole, and from pole to ground. If you have pole to ground continuity, replace.

To test a relay, power up the unit, switch your t-stat to fan on, and set your DMM to AC volts. Trace the line from your fan motor. The lead from the fan should have 120v to ground. The coil side of the relay should read 24v +/- 3v to ground, and it may read 6v to 12v across the coil. Power off your air handler at the breaker. Verify you have no power, and remove the coil leads from your relay. Ohm out the relay. infinite ohms = open relay, won't close the circuit.

Unless you're eating Ramen soup with catfood, spend a buck and hire a pro.