Thermal device open - http://www.hvacmechanic.com/ Forums
Original message
| nufaze2000
 |  "Thermal device open"
, posted Wed 8 Oct 16:45  
I have a trane XE-78 with a white rogers 50A55-486 board that flashes four times indicating thermal devices open. so the ignitor and furnace to not fire up. Blower runs but does not initiate ignitor. where do I find the device?
Ngraf
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Replies:
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| jmar
| "Re(1):Thermal device open" , posted Fri 10 Oct 18:00
the schematic will indicate where the limits are located, its basic series circuit. All limits should read closed when ohmed out and zero volts with power through series circuit indicating they are closed. If your tripping limits though you should call a pro and suck up the loss. If the flame is rolling out or the rollout limits are tripped its a good posibilty the heat exchanger is plugged full of soot, many techs argue with me on this but gas really does not burn clean like many people think if the manifold pressure is set to high or low it will not burn properly.......... I have punched out many gas boilers that got plugged as bad as an improperly tuned oil fired furnace or boiler. If the reverse flow switch or aux or main limit switch is open check the air filters, and make sure they are clean. Dirty filters will raise the Delta T past its bonnet plate rating. IF filters are clean verify blower is running if so and the furnace still is not operating properly call a pro. The heat exchanger may be part of the issue, or from begining of install fan speed was not properly set, furnace ran at excessive temp and wore out the limit. There are a number of factors and by that model number its best you called a pro it may be on its last days....
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| Amech
| "Re(1):Thermal device open" , posted Wed 8 Oct 21:42
Could be the high limit or rollout switch. Call a pro because tripping limits are a serious issue. Are the filters clean?
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