We have a 4 year old Carrier furnace and separate Carrier a/c unit outside. We have a programmable Lux thermostat. When we switch the thermostat to "on", absolutely nothing happens... like it's not connected to the systems. No fan, no blowing, no clicking. Nothing. Here's what we've done:

- changed batteries in thermostat
- tightened up 5 wire connections within thermostat
- reset thermostat
- tightened furnace door
- checked fuse inside house - no problem
- made sure the furnace switch was in the "on" position
- checked fuse outside, wired to a/c unit... not sure what I'm looking for there, but the handle
Thing came out easily (my neighbor told me that was a good thing)

I know NOTHING about heating and cooling and have no idea what else to do. Help! It's 90 degrees in my house and I have two kiddos. I don't think we can wait until our service appointment on Friday! Also, I'd rather not spend hundreds of dollars if it's something I can do myself. Thanks for any responses!!

Go to your main Panel and check all breakers, especially the 240 breakers, Air Handler, and Compressor. Disconnect outside is a pull out, Air Handler may have a breaker, instead of a disconnect, be sure it is on.
May have blown a fuse or transformer for the 24 volts that supplies power to the Thermostat.
Any Clicks at all, with thermostat?

The thermostat does "click", but that's it. Nothing else happens.

Husband checked LED diagnostic. It says "secondary voltage fuse is open".

Thanks for responding, by the way! :)

Sounds like you have a blown 3 amp fuse on your control board downstairs.

We thought the same thing about the 3 amp fuse... as soon as my husband plugged them in, they blew again. We have gone through 3 already and only have 2 left. :( Any ideas?

Does it blow with no heat or cool set, when set to cool or heat?
If fuse blows with the Unit off, There is a short in the control wiring from Air Handler to thermostat, or on board.
Identifying and removing wires from board will determine on board short or control wiring short. Then careful connect wires one by one, when fuse blows, that is the shorted wire.
Need to know, when does it blow, If when set to cool, could be a short on 2 wires that go to compressor contactor from Air Handler

If it blows when you turn the ac on odds are you need a new contactor outside.

The tstat was set to "cool" when he put the fuses in. Blew immediately. My husband is hesitant to get into any wiring. Is it difficult or rather easy?

Thanks to both of you!! :)

You need to start with the thermostat OFF, then try heat next time, then cool, I think if you put more fuses in while set to cool, the fuse will blow. Next would be remove wires at unit to thermostat to determine if On board, or the thermostat wires(identify wires and note where they go, pencil and paper, not memory.

We set tstat to off, put fuse in. set to heat... everything was fine (set for 62, reading 87). Turned to off. Set to cool. The fuse blew. :( Can you further explain what you mean by:

Next would be remove wires at unit to thermostat to determine if On board, or the thermostat wires(identify wires and note where they go, pencil and paper, not memory.

http://www.chinacontactor.net/Products/Contactor/MBC-1N-AC-Contactor.jpg

In you outdoor unit. This is most likely your problem.

I was thinking you had an Electronic board in air handler instead of just relays. Disregard.
When you set to cool it puts 24 volts to Compressor contactor. I would now go to Compressor Contactor, remove and momentarily tape control wiring connector ends. Try again set to cool, if it blows, it is control wiring to compressor contactor, if it doesn't blow, it may be a defective contactor coil that has shorted. Replacing contactor should fix, if this is the case.