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oregondean 01-09-2009 12:43 PM

Trane XV80 - low speed fan not working

My VX80 with heat pump and emergency gas works fine to heat and cool. I have left the thermostat (Honeywell RTH7500D) with the fan ON for the past 10+ years to keep the heat and cooling evenly distributed in the house. When heat/cool is not in demand the blower stepped down to a low speed and all was well.

But recently, after some occasional whining by the blower fan while on low, the blower stopped working on the low speed. Heat/cool demand with the fan at higher speeds works fine.

The FAN LED is not lit on the control board. Voltage from the thermostat is 0 (measured with wire disconnected from the controller). What voltage should I see on the "G" lead from the thermostat with fan ON? 24v? I presume is it the controller that says fan ON with no heat/cool demand should power the blower at low speed - not the thermostat.

Control board only has R, Y, G, W-W2, and B connected. Thermostat is set on config 170 to 7 for multistage heat pump with auxiliary heat.

If I jump 24v to the "G" post on the control board the blower shudders a bit and seems to be trying to run (perhaps on low) for about 20-30 seconds and then the fan kicks on at high. Is this the controller telling the fan to run on low and checking with the the pressure switch to see if air in moving, rechecking, and then finally giving up and trying high speed? High speed does kick in and works fine with the jump after 30 seconds or so.

Am I looking at a control board problem, a blower motor problem, or the thermostat? What should be my next debug step?



JohnH1 01-09-2009 01:12 PM

You probly have a BK terminal also.
You should read 24v from the B to the G coming from the thermostat. Connected to the G on the board or not. If you do not read 24v the problem is not within the furnace itself. As far as jumping I assume you are jumping the R to G at the board. This will bring on air flow at 50% of heating speed high stage if you are not calling for heat or a/c and the power was disconected wile you were doing your jumping.
I hope this was clear enough.

dac122 01-12-2009 11:56 AM

If you jumper R to G and the fan doesn't come on low speed, I would next check voltage from the board to fan. On a 3 speed fan black is high speed, blue medium and red low.

JohnH1 01-12-2009 12:28 PM

The XV80 has a variable speed motor hot, ground and around a 16 lead harness.

oregondean 01-12-2009 12:46 PM

Gentlemen - thanks!

I'll be back debugging this later today as I am the only one at home.

I believed the problem to be a thermostat controller problem. I took everything apart in the thermostat (which mean I took the plastic case off of one circuit board), checked connections and put it back together.

FWIW - the fan is running at lo speed right now with no heat demand like it should (which it wasn't last week). I will check the voltages and the fan indicator LED at the furnace later today (they were zero and off - I suspect they are now 24v and on).

I also learned when the cover to the heater was off that the blower motor must be somewhere behind the sheet metal. The small blower that is visible in the front is apparently just a vent blower.



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