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Old 12-03-2012, 02:34 PM   #1
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Help troubleshooting a home gas furnace module and induced draft system


Hello,
I have an old Amana AC unit with Gas Furnace.
It has a White Rodgers 50A51-225 module.
I was about to trash and replace the entire unit but managed to get the AC working.
The problem is that the gas furnace won't turn on.
I don't want to spend the $200. on a new module, since I'm borderline about replacing the whole thing,
so I'm not sure that a new module is necessary, or if it is, that I would buy one for this.

I understand that the sequence is that the fan must come on first,
which activates the pressure (vacuum) switches, which in turn enables everything else to power up and ignite the burners.

The problem is that the induced draft fan does not turn on when the thermostat is switched to heat.
I get either two or three flashes, indicating pressure switch open or short.
The induced draft fan has two speeds and the motor runs when I give it direct 120vAC on the high and low wires.
There are two different pressure switches that tap off of the single tube.

Can somebody answer the following questions?
Why does the induced draft fan have two speeds.
What speed does it usually run at?
Why are there two pressure switches? (My suspicion is that the vac has to be enough to trip one, but not enough to trip the other..)

If the fan doesn't turn on when switched to heat....
is the module bad?
I'm thinking that there has to be a relay on the module board that does the switching....
can that relay only be replaced?

Is there a way to bench test the module?

I hope that's enough info..
Thanks in advance for any help....
thanks
tim

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Old 12-03-2012, 03:08 PM   #2
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Help troubleshooting a home gas furnace module and induced draft system


Quote:
Originally Posted by howie50 View Post
Hello,
I have an old Amana AC unit with Gas Furnace.
It has a White Rodgers 50A51-225 module.
I was about to trash and replace the entire unit but managed to get the AC working.
The problem is that the gas furnace won't turn on.
I don't want to spend the $200. on a new module, since I'm borderline about replacing the whole thing,
so I'm not sure that a new module is necessary, or if it is, that I would buy one for this.

I understand that the sequence is that the fan must come on first,
which activates the pressure (vacuum) switches, which in turn enables everything else to power up and ignite the burners.

The problem is that the induced draft fan does not turn on when the thermostat is switched to heat.
I get either two or three flashes, indicating pressure switch open or short.
The induced draft fan has two speeds and the motor runs when I give it direct 120vAC on the high and low wires.
There are two different pressure switches that tap off of the single tube.

Can somebody answer the following questions?
Why does the induced draft fan have two speeds. Because you have a 2 stage burner with hi and low fire/2 different firing rates
What speed does it usually run at?starts on low speed and when tstat calls for 2nd stage then goes to high
Why are there two pressure switches? (My suspicion is that the vac has to be enough to trip one, but not enough to trip the other..)one for each stage

If the fan doesn't turn on when switched to heat....
is the module bad? yes
I'm thinking that there has to be a relay on the module board that does the switching....yes
can that relay only be replaced?unsafe to do so

Is there a way to bench test the module?

I hope that's enough info..
Thanks in advance for any help....
thanks
tim

.....

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Old 12-03-2012, 05:46 PM   #3
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Help troubleshooting a home gas furnace module and induced draft system


Yuri,
Thank you so much for your help.......
that explains a lot.

so, if the unit has a two stage flame,
maybe you can explain how the gas valve works.
I believe that there is only a pair of wires going to it.

Does it do something like 12 volts on low and 24 on high???

thanks
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Old 12-03-2012, 06:49 PM   #4
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Help troubleshooting a home gas furnace module and induced draft system


I am not familiar with that unit but to be a 2 stage it would need 3 wires to the valve. If the control module has a W2 terminal then it is a 2 stage but without being there I don't know what you have there. Who knows, some hacker may have changed the valve and stuck a 1 stage in.
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Old 12-10-2012, 11:59 AM   #5
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Help troubleshooting a home gas furnace module and induced draft system


Yuri,
thanks again for your expertise......
My valve does have 3 wires, so it is a correctly wired 2 stage unit.
Maybe you can explain in more detail about how the 2 stage furnace works....
I thought that the wall thermostat only switches the heat on or off.
How does the controller know if the temperature is very low, (high flame)
or only kinda low (low flame)...??

thanks again...
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