My HVAC shorted out!

November 26, 2008 3:24 PM

What did I do to my HVAC system when I changed my thermostat?

Newbie "Green" American that I am, I attempted to change my thermostat to a Digital Programmable model. I neglected to turn off the power to the unit before I made this switch, and the powered "C" wire touched another wire. A tiny, almost unnoticable arc happened. The new thermostat, though wired correctly, will not even turn on the fan. I called the customer support line for the new thermostat. They directed me to touch the G and Rh wires together to see whether the fan came on. It did not. I was told this means I've blown a fuse in the heat pump itself. How do I find and replace this fuse?

The Furnace is a Janitrol A36-10. The AC is a Goodman CPKE36-1. Any thoughts? No comments from the "You idiot, HVAC is not a weekend project. You should have called an electrician" guy, please.

posted by jefficator to home & garden (3 comments total)

Obviously, turn off the power. ;-) Take off the furnace cover, it should slip up and off. You should find an integrated control module, a circuit board where the wires are attaching. I looked up a manual on their site and they don't have your exact furnace, but they show a fuse right in the center of the board. It will look like an old fashioned cylindrical auto fuse (most likely). If you are having trouble finding the board, just follow the 110v going in to the transformer, then power goes to the board. Good luck, Radio Shack should have a fuse you can use.
posted by lee at 4:10 PM on November 26, 2008

Somewhere down in the blower unit will be a module that actually controls the fan motor. You can find it either by tracing the wires from the thermometer wires, or by finding the blower motor and back-tracing the power wires from it. My suspicion is that the fuse will be on or near that module.

I've never taken a really hard look at mine, but my recollection is that it's a circuit board with a bunch of fairly bulky relays on it (new ones may be solid state; I have an old furnace) along with some capacitors and other junk. It has wires running to the blower motor, and the burner ignition system. I've never seen a fuse on it, but then again I've never gone looking. Even on a heat pump system there has to be something similar to control the high-voltage/current components when told to by the low-voltage/current thermostat wires.

Basically the fuse has to be somewhere between the thermometer and the motor controller, so if worse comes to worst you can trace it and eventually you'll have to run across the fuse.
posted by Kadin2048 at 10:49 AM on November 27, 2008

Thank you for your advice! I was able to locate the 24V transformer--not a fuse--and replace it. How awesome do I feel for repairing my own Furnace and saving the $150 electrician house-call?!
posted by jefficator at 11:00 PM on November 29, 2008