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jeremyp969 04-18-2012 11:50 AM

York furnace - blower motor works intermittently on medium speed
 
So, I'm a relative newbie but am trying to diagnose a problem I've been having for several months that I've been neglecting. I have a York furance and at the start of the winter I noticed this problem:

When the heat turns on occasionally(about 1/2 the time) the blower motor will fail to start and the gas will shutoff because of the high limit switch. The system will then try again 5 or 10 minutes later when it's cooled down enough and upon that try the blower motor usually starts just fine. Also, when the thermostat switch is in the heat position and I move the fan to "On" it has the same issue where sometimes it comes on and sometimes not. The heat is wired to the "medium" speed setting on the blower motor.

When using the A/C everything works fine. The blower motor always comes on when requested either via the Cooling request from the t-stat or if I move the Fan to "On". The Cool is wired to the "high" speed setting on the blower motor.

That tells me the problem is either in the blower motor or the control board, but how can I tell which one.

Thanks!

Doc Holliday 04-18-2012 12:02 PM

Have you tried bypassing the stat altogether and controlling the heating sequence from the furnace control board itself to see if it heat comes on each and every time? I have a feeling it's the stat.

Keep the stat in the off position. Remove the furnace doors, tape the power door switch in/shut and jump the r to the w terminal on the control board. Heating sequence should begin.

After full ignition, remove the jumper wire and let the system shut off completely. Then jump it again. Repeat the process and see of the heat comes on each time.

R to G is the fan on only, you can do the same for it.

Look for any led flash codes on the board if and when the heat does not come on while on call. There will be a legend on the inside of the service doors of the furnace explaining what the flash code means.

jeremyp969 04-18-2012 12:06 PM

Interesting, no I didn't try that. So the thermostat controls the sequence of turning on the gas and then the fan? I always thought the furnace itself did that and the thermostat only requested heat.

Doc Holliday 04-18-2012 12:11 PM

Quote:

Originally Posted by jeremyp969 (Post 901743)
Interesting, no I didn't try that. So the thermostat controls the sequence of turning on the gas and then the fan? I always thought the furnace itself did that and the thermostat only requested heat.

No, the control board does all the sequencing. The stat is simply a switch which tells the board which call to start.

Have you tried switching the cooling and heating speeds of the blower on the control board or using the LO speed tap wire for heat and see what happens?

jeremyp969 04-18-2012 12:37 PM

Quote:

Originally Posted by Doc Holliday (Post 901745)
Have you tried switching the cooling and heating speeds of the blower on the control board or using the LO speed tap wire for heat and see what happens?


Good idea, I just assumed that it would work perfectly. I will try that tonight and report back. Thanks!


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