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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22517
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a Trane X90, model # TUC080C942 B2 I previously went

Customer Question

I have a Trane X90, model # TUC080C942 B2
I previously went through trouble shooting and replacing a limit. (see pic). (I would think you can see the discussion in my previous questions.) The guy at the HVAC supply store said he hasn't seen many of those fail. I installed it and it worked fine. Maybe not quite as warm as before.2 weeks in, I notice cold air blowing again in the morning. I'm pretty sure it started the cycle warm. First thing I tried was to bypass the limit I just replaced. Everything came on like a charm. I tested the limit, after taking it out, with a multimeter. no flow.As I was taking off the piece of wire that I bypassed with, it touched part of the furnace body and sparked a little.When I got the new limit, I installed it and tried to turn it on. Nothing.I went and bought real, insulated test leads this time. I tried the R and W bypass and it worked fine.What am I dealing with?
Submitted: 8 months ago.
Category: HVAC
Expert:  BillyHvac replied 8 months ago.

so at this point it only runs with r and w bypassed?

And I did not see pic of the part it a limit or a rollout?

Customer: replied 8 months ago.
only with the R W bypassed
I hope it went through this time. I believe its a limit.
Customer: replied 8 months ago.
34;it" being the jpg photo)
Expert:  BillyHvac replied 8 months ago.


May have some bad news..that is a porcelin roll out switch. It detects when heat rolls the wrong way backwards from the heat exchanger.

This is a "one and done" switch. If it trips more than once the odds are great that you have a bad or plugged heat exchanger (both are bad).

As far as running with r and w shorted together, this means the thermostat was compromised when it shorted to ground and you need a new thermostat......BUT.....I would have the exchanger checked before investing anymore money in the unit.

The roll out is the one switch we never want to see trip because it means main heat exchanger problem..

Customer: replied 8 months ago.
Is there a way I can check to see if its plugged?
Expert:  BillyHvac replied 8 months ago.

If it is plugged it is in the secondary heat can attempt to apply as much suction to the drain direct from furnace...using a shop vacuum.

Also you can tell by studying flames as they come on.....if you see them dance when the main blower kicks on or turn tips yellow it means a crack in exchanger. Even if plugged it is a heck of a job...I have had to remove the entire exchanger and flush and then reinstall...full days job..

Customer: replied 8 months ago.
How dangerous is it to run it with the rw bypassed, Briefly and watched.
Expert:  BillyHvac replied 8 months ago.

no danger at all to run with r and w just will not shut down.

The danger is if you bypass the rollout switch.....then we get into potential fire and carbon monoxide issues.

R and W is simply taking the thermostat out of the loop....Just please do not bypass the roll out or main limit.


Customer: replied 8 months ago.
No, I understand the need for the roll out switch. Thanks for the concern.
I just wanted to make sure regarding the thermostat.
Expert:  BillyHvac replied 8 months ago.

I apologize as it is much easier for me to give advice that saves money..But this is entering in to the dividing line of major repair or replacement.

I certainly wish you the best and feel free to stop back if further info is needed.

Just rate service to move off the active list.


Customer: replied 8 months ago.
Would that have anything to do with the air temp not being quite so hot?
Expert:  BillyHvac replied 8 months ago. is directly tied to the main heat transfer component. Basically this is the equivalent of a bad motor in a car...

Customer: replied 8 months ago.
Did you review my previous discussion? I wish I would have had this info then. I would have called a tech at that point.
Expert:  BillyHvac replied 8 months ago.

I just went back and reviewed and you are correct.

As soon as the roll out was identified as being the culprit, everything should have ceased at that point.

I apologize, but then again I am old and have been working on these unit s along time...

Anything after finding an open rollout was likely putting off the inevitable...