Furnace fan not kicking on until you smack the door.

[swiftymcvay8]

so its 90 degrees outside. My AC unit outside will kick on with the AC fan working. Inside the house the furnace fan will not kick on to circulate the cold air in the house. the AC line leading into the furnace will actually freeze up due to compressor running and no where for it to go. If you go downstairs and just give the lower furnace door a nice tap the fan will kick on. I can not figure it out. I have tested the door switch which shuts furnace off when door is not on. I have unplugged almost all connections on the board to make sure they are clean and getting good contact. I am pretty mechanically inclined but this has me stumped. Sometimes the system works just fine and other times it will not. In the last 2 days we have had to hit the door 5 times to kick it on. Is there a solenoid somewhere that I am not seeing going bad? Is the motor itself going bad and intermittently needing a small bump to get started? Also the furnace looks to be from 2007 and is a york furnace. Any help would be greatly appreciated! thanks in advance.

 

[Stuff]

Yes, could be relay on control board. Sometimes there is a connector on the motor itself that could be bad. See if you can get a multimeter wired up to the motor leads to check if it has voltage when not starting. Remote possibility that it is the start cap - check the connections on it.

 

[Sponsor]

 

[Reach4]

Instead of smacking the door, try removing the door very gently, and activating the door switch to the service position if there is a service position. Otherwise activate the switch as best you can.

Then prod around gently, then increasingly less gently, until the fan starts. The thing you last tapped or prodded would likely be the thing to fix.

 

[swiftymcvay8]

sounds good. I will try both next time it wont kick on. ( it has not acted up at all today..... sigh. It will wait till only the wife is home.)

 

[Jadnashua]

Turn the power off, and check to see how easily the fan motor blades turn. Try to detect if it's stiff at all. The bearings could be going. They don't always make noises until they are really bad. On most, the fan should move easily and keep going for awhile.

Many of the fans are direct drive these days, but if it's belt driven, make sure that the belt is both tight and that the motor might be actually going.

 

[WorthFlorida]

It's one of three items. The contactor (fan relay) for the fan motor is my first choice of the problem, the start capacitor (usually there is one), or the motor itself.

On the control board there should be a green wire from the thermostat and near by the fan relay which is usually a black sealed box soldered to the board. Below shows a typical board and there may be more than one relay.

To test it, place the thermostat in the fan "on" mode. This brings 24v's ac to the green wire connector. If the fan is not working, first wiggle the wire to be sure the connection is not bad. If that doesn't work look for the relay and tap it, the fan turns on its the problem. If the fan does not turn feel if the relay is operating. It's a about 1" square relay cover you can usually hear or feel the relay operating. You pull the green wire off and on at the connector and check if the relay is operating.

If the relay seems to be working and the fan does not, with the fan in the on mode carefully give the fan squirrel cage a little push. If it turns it's the motor or start capacitor. On line there are plenty of you tubes on how to test a start capacitor. If it tests good then its the motor but from your description, doubtful.

I do not know of any cabinet that would have a door safety switch. If there is one that would be the first item to check.

 

[swiftymcvay8]

Got home today and it was not working again so I actually got a chance to trouble shoot it. There is a door panel switch but I already bypassed that. I tapped on the different relays and when I got to a specif one I just touched it and bam it started running. Traced it on the wiring diagram and it is indeed the blower motor relay. Going to try to track down the relay as I can solder but no such luck yet. Hopefully a new board isn't that spendy. Thank you everyone!

 

[Reach4]

Going to try to track down the relay as I can solder but no such luck yet. Hopefully a new board isn't that spendy.

The problem could be a cracked trace. A strand of wire soldered across the crack could fix it.

 

[WorthFlorida]

Glad you found it and thanks for letting the forum know.

I’ve google search on numbers printed on these relays or micro switches and you might find it.

In the meantime at the thermostat change the fan setting to “on”. This will let the fan run continuously and keep the relay closed. The fan doesn’t use that much power and cheaper than the compressor running with no air going over the coils. If puts a lot of stress on the compressor.