In the attic you will need to find the furnace for the upstairs unit. Easy to find, just follow the duct work from one of the upstairs rooms. Turn the power off to the unit at the breaker and check voltage at the unit to see whether it is dead. Because you have two units, you have to be certain that you have the correct circuit. Once that is confirmed, remove the cover from the side that accesses the control wiring. You will see the power wires entering the same area. You are looking for the control transformer. If you don't know what it looks like, then I would suggest consult a friend that does. Find the secondary wires (low voltage ones) at the transformer and attach your meter to them in order to get a voltage reading. Once that is done, restore power to the unit and check the output voltage of the transformer.
If there is 24 volts there (or in the area of it) then you have a wiring problem. You must then turn the power back off and trace out all the t-stat wiring to locate the loose connection. Repair it and restore power.
If there is not power there then you must do the same thing. Only this time look for a short, as in two wires touching that shouldn't be. One thing to check is the back yard. If you have a dog, they seem drawn to chewing on these wires and shorting them out. If you can, put your meter on the highest setting and disconnect the transformer's secondary wires. Connect the test leads to the house side of the wires and see what the reading is. If you have a reading in the range of 0 to 5 ohms, you have something shorting out that burned out your transformer. You have to locate this short in order to keep from frying the new transformer when you install it.
Putting in the new transformer will be a simple reverse removal, being sure that the wiring matches the diagram that comes with the new one. Oh yeah, Go to Lowes and pick you up a new ones. There right there on the shelf. You need a 40va one. Not the 25va for doorbells. The 25va is not enough for the system and will just burn out shortly after installing it.
Another cause of transformer failure that you will find with a system resistant test, is one of the coils in a contactor or control relay has gone bad. This will overload the transformer a burn it out also.
If none of the above problems are found then just put in the new transformer and write it off to just a faulty transformer. It happens, but I have seen it rarely. Hook up your new one and test the voltage on it as soon as you turn on the power. If it is very low then kill the circuit and find the short. There is no magical spot it could be, especially if you have mice. good luck. Any questions, email me.
I Am The A/C Man
· 9 years ago