Hey guys,

I have a Lennox G12 that was installed when the house was built back in 1990. I recently had the AC stop working altogether. I then found the control transformer to be bad. I went to Home Depot and bought a multi use transformer that the electrical guy said "should" work. After installing it, the air started working but never shuts off. I replaced the thermostat and ran all new wire. I don't see a main board and I don't believe this system has one. I ran my AC for a few days without the use of my thermostat and my last bill was $430. Any ideas?

Thanks in advance -
Greg

Hey guys,

I have a Lennox G12 that was installed when the house was built back in 1990. I recently had the AC stop working altogether. I then found the control transformer to be bad. I went to Home Depot and bought a multi use transformer that the electrical guy said "should" work. After installing it, the air started working but never shuts off. I replaced the thermostat and ran all new wire. I don't see a main board and I don't believe this system has one. I ran my AC for a few days without the use of my thermostat and my last bill was $430. Any ideas?

Thanks in advance -
Greg
It has probably frozen up. Turn your unit off for about 2 hrs ( or you can flip the thermostat over to fan, this helps it to thaw much quicker.) Then check the pressures. If your unit is still cooling your home your coal my be going bad.

It has probably frozen up. Turn your unit off for about 2 hrs ( or you can flip the thermostat over to fan, this helps it to thaw much quicker.) Then check the pressures. If your unit is still cooling your home your coal my be going bad.

It's actually been off for a week in 90 degree heat. What's a coal?

Sorry it's a typo it is actually a coil. You need to have the Freon checked. It is possible that you have a leak some where. If your freon is low your air will blow hot, making your unit run cont. because it is unable to reach the temp you have your thermostat sat on. I hope this works.

If the system was cooling and working fine before the transformer blew we need to retrace your steps, something is wired wrong with the low voltage side if its not working the way it was before. Unless you had problems before the transformer was replaced. A 24 volt transformer has a rating. Is yours rated at 30 or 40 VA?

Thanks guys. Robinson, I never thought of the Freon level being low. It actually seems to take longer to cool down also so that makes sense.

T-Top, it is some type of multi purpose transformer. If it's not the proper transformer could that cause it to run constantly and not be effected by the stat?

The wrong transformer should not make the system run longer unless its not strong enough to close the blower relay and contactor. You never did say if the system was working properly before the transformer change out.

It was working fine before it quit working altogether and then I changed the transformer out with the one from Home Depot and it started working but I couldn't control it from the stat.

Remove one of the 24 volt control wires from the contactor/starter outside. If the unit still runs it is a bad contactor/starter. If it shout down and will not run with one wire removed it is in your 24 volt control wiring including the thermostat. If you remove the yellow (Y) wire off the thermostat and it stops running it is in the thermostat. If it stays running you have a 24 volt wire short in the thermostat cable or in the inside unit.
As T-TOP has said get a REAL HVAC transformer with at least the same VA as the old one or higher.

These types of problems are not hard to track down.
One last item. Did you hook up the wires from the new transformer to the Same location as the old?

What's a contactor/starter and wheres it at? My brother who is a master electrician for Delphi wired up the new transformer. He said it wasn't the exact as the old one but it should work. Should being the operative word. I figure it is something little which is why I'm trying to track it down myself. Thanks for the info.

The contactor is in the outside unit. It will have larger power wires into and out of the contactor. The low voltage wire (pick one) Are wired to the coil side which is low voltage. Turn power off outside. Remove one 24 volt control wire. (isolate so it does not touch anything) then turn the unit back on and see what it does.