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I have a Trane XL-80 with White Rogers control board 50a51-405,

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these are original since the...
I have a Trane XL-80 with White Rogers control board 50a51-405, these are original since the house was built in 1993. last week, the furnace could not get ignited, the light on the board shows one slower single flashing first, I think it means requestin heat, then it goes to a fast double blinks after failing in ignition, I think it means system lock-out, then the fan kicks on, the light is static on. The fan seems never shuts off.

I check the vent, it is all clean. I had a guy came in from "First Call" a service company, he measured the voltage coming out the board to "R" terminal is only 13v. it is supposed to be around 24v. he also verified that the transformer works fine, it does generates the 24v going into the board, then board should take the 24v from transformer and sending a 24v signal to "R" terminal which in my case it is only 13v. So he suggested to change the whole board which will cost me $500. Is this the root cause? I made it ignited after turn off/on thermometer.
Submitted: 7 years ago.Category: HVAC
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1/4/2010
HVAC Technician: Douglas, HVAC Technician replied 7 years ago
Douglas
Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3,089
Experience: Manufacture Rep for Major Brand, Technical Trainer, NATE
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without actually using the meter, it's difficult to say but I will say this..

IF the volatge was measured without a load, the reading may be incorrect because solid state will allow voltage through but no current. The way to test the 24v in solid state is to have a known load such as a relay. Measure the voltgage with the relay.

If the measurement was made while the furnace was locked out, the current to the R terminal may have been interupted by the control.. This is how it stops the unit from running.

IF I were there, I would test the circuit by removing the thermostat and thermostat wiring from the equasion and simply jump R-W. If it fired, I would repeat the process several times. If there is a safety locking it out such as flame failure, I could go from there to determine if the flame actually came on, and then back off or could begin troublshooting from there.

My bet, is it would work with a jumper, the flame would ignite and go back out telling me that the flame sensor simply needs cleaned.

Now, I said all that because I dont know what his steps were. I am not saying he is wrong because there is no way from here to tell, but 13v sounds like an unloaded solid state type reading.
Douglas
Douglas, HVAC Technician
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Customer reply replied 7 years ago

 

Doug,

 

Thanks for your info, I saw the service guy removed the wire connection at R terminal during the measurement, and he acutally tried to feed the output from the transformer into "R" terminal directly, but no flame, maybe the thermostat is off at the time, because he wants me to have thermostat is off for safety. I guess try to approve the voltage problem which is too low.

HVAC Technician: Douglas, HVAC Technician replied 7 years ago
Certaily there is room for being skeptical on a bill that large but my hope is he backs up his claim and when the board doesnt do the job, he doesnt bill you for it. It's not for me to question his ability, he may very well have nailed it.
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