Thermal Zone furnace not working

[onefourthefun]

I have a Thermal Zone furnace with a Nordyne control board, part # 624591-D. The furnace does not operate correctly most of the time. What is confusing is about 5% of the time is does. This is what happening. I apply power to the furnace. I make sure that the thermostat is applying the 24v to the control board to start the heating cycle. The exhaust motor starts. A few seconds later the ignitor starts. A few seconds later I hear the gas relay valve click (may be the control boards sending power to the relay). Then most of the times the cycle stops here. When the cycle does continue gas comes out and lights and then the blower motor starts. When the cycle stops the red LED on the control boards flashes 3 times. The code for this is pressure switch stuck closed. This makes no sense since the furnace manual says this error is detected before the exhaust fan is started. I've also tested the switch and it does open and close correctly. Is the control board going bad? Or the gas relay valve? Or something else?

Jeff

[xpogi]

This makes no sense since the furnace manual says this error is detected before the exhaust fan is started. I've also tested the switch and it does open and close correctly.

Well it does make a little bit of because the control board first looks for an open switch before it starts the exhaust fan and would see a closed switch at that point in time as a failure. (BTW the "exhaust fan" is more properly called a inducer or combustion air blower). If the inducer is actually coming on, it probably means that the control board did initially see an open switch condition. Try this: Replace the pressure switch with a manual switch. Make sure the switch is open (off) before you start and then apply a call for heat at the thermostat. After the inducer blower comes on close the switch. Does the furnace light and continue to run normally? If so you either have a bad switch or not enough pressure. Not enough pressure can be caused by a blockage in the pressure hose or more seriously and potentially deadly a blockage in your flue or fresh air inlet (on sealed combustion furnaces) or in the heat exchanger. Proceed with caution and do not jumper the pressure switch out of the circuit except for troubleshooting.

[onefourthefun]

The pressure switch was working correctly. Tested it a couple time with a volt meter both when it should be opened and closed, no issues. Ordered the Honeywell universal replacement furnace control board. Was very easy to installed and save $500 by doing it myself. The furnace works just fine now.