Thermostat worked great all winter until one hot day I tried to switch it to AC then I noticed the thermostat went dead. We have two units, one upstairs and one downstairs. The one upstairs works fine and ac running great. Please provide any suggestions before we call service man.

Have you checked the circuit breaker for the downstairs air handler unit? When the thermostat goes blank it's usually due to loss of the power feeding it. You also could have a blown fuse on your 24 volt control circuit. You should also check to see if you have a overflow switch in your air handler pan, normally this is wired to shut off the outside unit only but some service techs. wire it to shut down everything.

I am really a dummy with all this. The suggestions you made sound all very valid. I have at least already checked the circuit breaker which resides in the garage where the units has all the circuit breakers for everything else.

A blown fuse. Where should I locate this to determine the fuse is blown? I have a crawl space that is 5' high. Should it be down there since the upstairs unit is on the attic.

Can you also help locate where the air handler pan is? What should I check with the overflow switch once I locate it?

Thankyou for all these tips. You might just save me money not have to call someone out.

The fuse is usually inside the air handler unit. It could be a glass fuse or a plastic fuse like you would find in a car. The float switch (if you have one) is usually attached to the pan underneath your unit. Some of the newer installations now have them in the drain line itself. The switch will probably only be a problem if the pan is full of water. When the drain line backs up water may overflow into the pan which in turn shuts off the unit. Be sure to shut off the power before removing any covers.