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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a trane XV90 gas furnace that blows circ air but no

Customer Question

I have a trane XV90 gas furnace that blows circ air but no heat.
Submitted: 1 year ago.
Category: HVAC
Expert:  Phil replied 1 year ago.

Hello, tell me a lot about your skill sets with electrical things, so I can coach you most effectively please.

Customer: replied 1 year ago.
I can use a multimeter and have wired many houses.
Customer: replied 1 year ago.
Hello?
Customer: replied 1 year ago.
Relist: Other.
Never recieved an answer.
Expert:  Phil replied 1 year ago.

I am back, there is trouble on the internet today that is cause our JA server problems and I think more broadly as well, what I type is often erased before I can send it, so I will send short snippets, that should work

Expert:  Phil replied 1 year ago.

Send me a photo of the thermostat wire connections inside the furnace blower compartment... there should be small red, white, yellow, green, and blue wires, probably more, I can mark that up with some initial trouble shooting directions

Customer: replied 1 year ago.
Photo
Expert:  Phil replied 1 year ago.

Next I will probably need a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to be taken from straight on, in very good light.

You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here.

If there is no paper clip-icon, look for the '+attachfiles'link at left of your send button.

I can mark it up with trouble shooting instructions.

We can go from there.

Customer: replied 1 year ago.
Update. My furnace now has heat. I didnt do anything. The thermostat now shows "Aux Heat On". I have a heat pump also but I pull the power to it in the winter. Is there something I should do electronically so that the furnace doesn't try the Heat pump
Expert:  Phil replied 1 year ago.

Thanks, ***** ***** in my 51 year career and thousands of repairs this is the first time I have seen 4 wires attached to most of the terminals... This could be something that will not be amenable to diagnosis on the web.

I suspect a bad printed circuit card however,

Expert:  Phil replied 1 year ago.

We can do a heat stress test on it

Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has an invisibly cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.

No change however does not prove the card is good however... but a change proves the card is definitely bad.

Expert:  Phil replied 1 year ago.

We can do a heat stress test on it

Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has an invisibly cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.

No change however does not prove the card is good however... but a change proves the card is definitely bad.

Customer: replied 1 year ago.
That's probably because of a dual zone system and two thermostats.
Customer: replied 1 year ago.
Still no error codes
Expert:  Phil replied 1 year ago.

That is not how dual systems are traditionally wired at all, your furnace could be an exception, but it would be a first in 50 year experience. get me the wiring diagram from the inside of the blower service panel I will look it over. and do the heat stress test,

We can go from there

Expert:  Phil replied 1 year ago.

No error codes is why I suspect a bad printed circuit card... those are useful about half the time, the rest of the time they are entirely misleading or otherwise bogus

Customer: replied 1 year ago.
Panel
Customer: replied 1 year ago.
Panel 2