Hello all: Yesterday, I tried to replace my old thermostat with a Honeywell Digital Thermostat. After I labeled the wires and connected them to the new thermostat, nothing worked so I quickly reinstalled my old mercury thermostat but nothing works there either. I accidentally touched all the wires together ( didn't know I was not supposed to do this). Is that the problem? What can I do to get the old (or the new) thermostat working?

Robert

You shorted out the transformer in the furnace. It will need to be replaced. Always turn the power off at the furnace when working on it...

Does home depot or Lowe's carry these transformers? By the way, thanks for the quick reply.

Robert

Never seen one at Lowe's or Home Depot. Probably need to go to HVAC supply house. I do think I have seen them at Radio Shack. Just be sure to replace with the right one though. If you have a volt meter and are careful work with electricity, you can verify if you have 24V coming out of the transformer. If you have a circuit board in airhandler you may have on popped the 3 amp fuse in it. I have been lucky in the past and have found that that was the only problem

My thermostat says "24 VAC Heat Pump Thermostat" so it is 24 volt transformer. I looked inside the circuit board of the handler and I could not find anything resembling a fuse. Do you know what it looks like?

If you have a circuit board, normally they are near the bottom of it. It is a 3 amp fuse, looks (and is) the mini auto fuse. If you have a volt meter verify if you have 24 V on the low voltage side of transforme.ie. You have 120 volts coming into transformer and 24V coming out. If you have an older unit you will not have a circuit board

Also be sure the power rating of a replacement transformer meets or exceeds the rating of the existing transformer. Transformers are rated in VA (volt-amps), and the transformer should be rated as a "control transformer".

My point is not just any 24 volt transformer can be used.

Can you find a wiring diagram for you unit? Either published in the manual, and/or pasted on the inside door of the unit? If you can find this, and know how to read wiring digrams, you can find the transformer, and if there is a fuse in the circuit.

tkrussell is right, it must be a control transformer. I was "***"uming that my answer was a given. Thanks for clarifing this tk

I think I found the transformer in my garage attached to the back wall. I removed it (after turning off the power). The left terminal is "10 VA" and the right terminal is "16V". The back of it says "Class 2 60 HZ" Line 120. Two black wires coming out the back.

I have a new (less than one year old) air handler outside the house. Looked again and could not find a fuse.

The air handler is the unit inside,the condensing unit is outdoors. What model and make? To test transformer, with 120 volts coming into unit is there 24 volts coming out? If you are not comfortable working with electric call a serviceman. You must respect electric and work safely. The fuse will be on the inside unit if you have a cicuit board

Not all HVAC control systems have fuses. If the transformer is built-in on the HVAC unit, it usually has a fuse. If the transformer is added by the installer, such as yours being on the wall, there usually is no fuse.

But you say the transformer is 16 volt? This may not be for the HVAC system, and may be for your door chime. Be sure this wiring connects to your HVAC system. And be sure your HVAC system can work with 16 v and does not need 24 v.

I believe I am very careful with electricity- I turn off the power going into the house before I disconnected this transformer. This transformer has two black house size wires putting out 120 V and two telephone size wires coming out (white and red) putting out 19 V ( I checked with my multimeter). Does that mean this transformer is OK? I did not see a circuit board in the air handler (inside the house) :) :) (neither did the condenser outside, by the way:D )

Hey guys... I found the problem.. it was the fuse inside the air handler (blown). It was hiding in plain sight and was staring right at my face. Anyway, I changed the 5 amp fuse and everything is working again, including the new Honeywell thermostat. Thanks for your perseverance in helping an electrically challenged guy:) . You guys probably saved me a few hundreds bucks by not calling in professional.

Robert

Usually that is the problem. Glad you got it up and going