# Bad Control Board?



## Bullage (Nov 14, 2009)

Have an old 1975 Payne up flow gas furnace model #394GAW000075 
that the blower wont turn off.

I have unplugged T stat... no change. (old mercury type 2 wire red and white) 
I have unplugged the limit switch... no change. (brown wire. no adjustment on the switch)
Anything other test procedures I can preform to trouble shoot and diagnose the problem? 

Or do I have a bad control board?

Thanks in advance!


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## Bullage (Nov 14, 2009)

...ttt


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## mrairflow (Oct 25, 2009)

at 35 yrs old time for a new furnace , have you made sure the heat exchanger does not have a hole in it before doing any thing else


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## Bullage (Nov 14, 2009)

Well I know a new furnace would solve the problem but at a bit higher of a price.
The furnace is old but worked well for our house so if I can fix it for a reasonable price then I would like to give it a shot.
I don't think I have a heat exchanger as it is a forced air NG furnace. Are you referring to the burners? I looked around and saw no visible holes or damage.


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## Marty S. (Oct 31, 2009)

Gas furnaces have heat exchangers and they usually last 15-20 years before cracking or rusting out. At that age you really should have it checked yearly or at least make sure the fire and life insurance is paid up.


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## Bullage (Nov 14, 2009)

what I call burners or heat exchangers were replaced a few years ago.
no other damage or problems besides the blower staying on. getting back to the original question above if everything else is ok and based on my trouble shooting should I replace the control board?


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## Houston204 (Oct 18, 2009)

The burners heat your heat exchanger. Your high limit has probably tripped due to low air flow or a hole in your heat exchanger.
I would not be able to sleep with that 1975 furnace heating my house unless it was professionally inspected.


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## Bullage (Nov 14, 2009)

Thanks for the diagram... pictures clarify a thousand terminologies.
Is there another way to test the limit switch besides just unplugging it?
If I can I would like to isolate the problem so I can decide which route to take. And say these parts check out is the control board my problem?


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## Houston204 (Oct 18, 2009)

Limits usually have 24 volts to and from them at all times. When they trip you only have 24 volts on one side.


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## Flashheatingand (Sep 7, 2009)

Based on what your post, just replacing the control board is not wise unless you don't mind gambling about $150.00. The problem might be a board, but, your problem could be something as little as a loose wire, and then what? If you want better advise, perhaps you could give some symptoms as there are some bright guys here who are "regulars". Otherwise, perhaps you can find a local guy to give you a hand. At this point, I wouldn't run out and get a control board.

Good luck


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## Bullage (Nov 14, 2009)

The original problem came about when my wife went to turn the the T stat off and it continued to blow air with no heat. Still will heat like normal if you turn the T stat up. Its an old T stat with just a dial for temperature, no fan control. Only 2 wires R and W.
The only way to get the blower to shut off is to unplug the unit or open the blower door tripping the blower door switch.
unhooked the R wire on the T stat... blower still runs
unhooked the W wire on the T stat... blower still runs
unhooked both wires on the T stat... blower still runs
I have 24vac on the power leg of the limit switch (red wire) and 24vac post the fusible link on the other side of the switch (red wire) So can I conclude that the switch is good and that I can assume that the heat exchanger is ok?
I push the blower door switch on and off and can not hear, feel or see the HFR relay closing.
All my trouble shooting and to the best of my knowledge(which could be 
flawed) and research is pointing to a bad control board??
Can anyone confirm this?? 
Thanks!


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## Flashheatingand (Sep 7, 2009)

Could it be your control board? yeah, but without a look at your wiring diagram, it's hard to say with any certainty. If the blower motor is running constantly, it's likely that somewhere in your heating circuit, you have an open switch. Generally, the circuit runs in series from the control board through the limit switches to the gas valve. Off hand with the limited amount of info, I don't think the board is the culprit. I have been wrong before. Can you post a wiring diagram?


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## Bullage (Nov 14, 2009)




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## Houston204 (Oct 18, 2009)

If the heat still works, it sounds like a board failure.
Does temporarily jumping Lim1 (red) to Lim2 (brown) turn off the fan? I doubt it.
(Never bypass safety switches for anything other than testing purposes since the result can be fatal)


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## Flashheatingand (Sep 7, 2009)

Voltage between lim1&lim2should read 0 if you get 24v between those connections, that is where your problem lies. If 0 volts there, and 24v between W&c, you should read 24v between gas1 and gas2. If you aren't getting 24v between gas1&gas2, and your limits are closed, then the board is the likely culprit.

But, make sure all wires are securely connected.


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## Bullage (Nov 14, 2009)

Thanks! I will run through those procedures when I get home this evening
and see if I can isolate the problem.


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## Bullage (Nov 14, 2009)

Continuity between the limit switch terminals.
0 vac between GAS-1 and GAS-2.


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