Thermo pride Gas furnace

[halfplumbee]

I Have an MHA-125n power vented unit that was tied into a field control wall mount power vent with the water heater kit. The problem is the filed control comes on when either unit fires but the furnace faults with no indication on the mother board of the fault. Power down the unit with the main switch and the unit operates flawless for 1 to 2 days. I think it is the furnace pressure switch but it seems to work after reset. Does anyone have any ideas. Ron

[Freon]

Has your system worked perfectly until recently. How old is the field controlled power vent that both the furnace and hot water heater share. Did you check the flue and chimney for any kind of obstructions.

You can TEMPORARILY jumper the pressure switch and see if that helps. Usually there are two wires connected to spade connectors on the switch. Connecting those wires together will simulate a closed pressure switch. But since the furnace runs after a reset I'm not sure that switch is the problem. Check the hose connection from the pressure switch to the inducer assembly looking for condensation and or cracks in the hose itself.

[halfplumbee]

Thanks for the reply. The power vent was replaced last year in an effort to solve the problem. the hose is clear and no cracks. the switch seems to operate. this am homeowner called stating that she woke to a 50deg house with t stat on 65 ran a little water and the unit kicked. now im leaning back to the field control pressure swithch

[Freon]

By TEMPORARILY shorting the pressure switch leads at the furnace, you eliminate or confirm a pressure switch problem. If there is no change in random "no start" behavior for the furnace, then you know the pressure switch is not the problem.

You need to determine what type of safety sensor or switches are in the field control unit. I would assume there is something to tell the furnace that unit is operating correctly.

You say the power vent was replaced to solve this problem. Was there a previous power vent. If so, did the furnace work correctly with this previous *