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So far I've been successful in maintaining and repairing my own HVAC system in my house. First week we bought the house (we knew going into the purchase) I replaced the main board. It's been golden for three years until now.

Thermostat makes a call for heat, nothing happens and then it makes a call for emergency heat and nothing happens. The furnace is getting power. Circuit isn't tripped, getting power at the outlet attached to the furnace (for the condensation pump), and there is an electrical hum when I turn the power on.

I opened up the furnace and the LED on the board is 100% off. Per the key on the board an off LED means that either the board is dead or there is no power getting to the board. My first inclination was to check the transformer, which is a 120v to 25v transformer.

The leads to the primary are right at 120v AC, to be expected. The primary is reading 97v AC when the leads are plugged in and the power is on. Turning off power and removing the leads to primary I get 24.4 ohms. With power on, and leads removed, the secondary side is also reading 120v AC. Leads off, power off, I get an 0.L on the secondary, when testing with my ohmmeter.

So I'm pretty sure the transformer is gone. I've ordered the part, but since it will take a week to get here I thought I'd ask and see if anyone had any idea. I also wasn't sure if the 120v on the secondary side is cause for concern. Will I need to replace the board as well?

This is a Trane XE90. It was working just fine up until yesterday morning.

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Follow-up and answer:

Replacing the transformer resolved the issue of the board not getting power. Once the transformer was replaced I swapped in a new fuse and it started up. On a hunch I plugged in the old transformer again and fuse blew. So the transformer was definitely a problem.

For anyone with a Trace XE90 looking to replace the transformer powering the control board: you will need to mount the transformer in a new location as the transformer's form factor is different from what original shipped in the furnace, still the same product but a new form factor.

This did not completely resolve my problem however, as now I am blowing a fuse whenever I make a call for heat. Next step is to pull everything involved in heat from the board and add one back at a time until the fuse blows.

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