# Pressure switch or control board?



## zerofill (Dec 19, 2007)

K...the exhaust fan also won't kick off ever it seems...I turned the thermostat all the way down...
Is it possible that a bad thermostat could cause all this?


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## bigMikeB (May 31, 2007)

There are three terminals on the pressure switch, "C", "NC" , and "NO" . When the fan is off the "NC" and "C" should have continuity, when the fan is on the "NO" and "C" should have continuity. If that isn't what happens the switch is shot. The inducer fan should only run when there is a call for heat to the control board on the "W" terminal. If the inducer is running when the wire is removed from that terminal, change out the board.


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## zerofill (Dec 19, 2007)

Is it common or correct for it to signal high rollout and the flame to go out and then after about 5 minutes light goes out and flame reignites? Is this a safety cool down feature?


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## zerofill (Dec 19, 2007)

bigMikeB said:


> There are three terminals on the pressure switch, "C", "NC" , and "NO" . When the fan is off the "NC" and "C" should have continuity, when the fan is on the "NO" and "C" should have continuity. If that isn't what happens the switch is shot. The inducer fan should only run when there is a call for heat to the control board on the "W" terminal. If the inducer is running when the wire is removed from that terminal, change out the board.


Ok so I feel I am mistaken on what I took for the pressure switch and what is the actual pressure switch....

I assume the switch under the exhaust motor is the pressure switch? My first assumption was that the actual switch behind the diaphram was the switch?









So I am taking it that the little micro switch or whatever on the back of the diaphram where I marked "A" is not the pressure switch. "B" is the actual pressure switch?
C - Flame sensor
and "D" that you can't see cause its cut off is the high rollout sensor that looks similar to "B" but is right above the burners.

Am I correct in all this? Or am I completely off lol


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## zerofill (Dec 19, 2007)

So been playing with it forever again...
same stuff happens if I take the hose off for about 10 seconds put it on the furnace runs...and heats...
And seems to keep doing that...but if I turn the thermostat down the furnace doesn't ignite anymore but the exhaust fan runs forever...doesn't seem it will stop.


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## zerofill (Dec 19, 2007)

I have found that the control unit is a Model 50a50-110 and is made by white rogers...They have a new style that replaces this one. The new one is called 50A55-843. What is the best way to go about troubleshooting this board to see if it is the actual thing at fault? Or is it wise to change the pressure switch first? I imagine this damn board is expensive.

Here is a picture of the control unit in it:


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## zerofill (Dec 19, 2007)

Ok...
A - is the pressure switch
B - is the rollout sensor
C - is the flame sensor
D - I have no idea what it is for...

So... The pressure switch gets voltage and there is only 2 wires on mine. Voltage is around 26v

B - I took this heat sensor out and placed it by a torch to warm it up...it seems to work fine

C - Flame sensor thing seems to be fine...I don't think the furnace has any problem detecting a flame.

D - I do not know what that little sensor is?

Also the the exhaust fan always runs continuously...

So what should I check now? Is there anyway to identify a problem with that control board with a multimeter?


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## zerofill (Dec 19, 2007)

Just for future reference if anyone else has these similar problems...it was the controller....They cost about $117

There isn't much to these furnaces...you can check about 4 things and then its all in that damn controller heh.


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## binchentx (Feb 1, 2008)

I have the exact same problem, light flashes twice, the exhaust fan runs forever. Unplug PS and plug it back will kick the Furnace to work a circle.
I switched a good pressure switch from another unit to make sure it's not the PS problem. I am going to change the controller, where did you get it for $117?


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## bigMikeB (May 31, 2007)

Before you assume it's the control, check for a blockage in the flue piping, if the pressure switch is opening up and stopping the cycle it could be blocked in some way.


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## zerofill (Dec 19, 2007)

Security Supply...
Believe they are in a lot of areas...

And on mine I did check everything prior...
I will have to grab the number for the new unit...it has changed model numbers about 3 times to newer revisions...

But really...this furnace has about 4 or 5 things you can check and after that it's in the controller....not much to these furnaces. Actually the price for the controller was $96 from there. I will try to post it later tonight...5:30am here bed time hehe.


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