My central heat and air stopped working. The thermostat is a american standard electronic programmable one and the screen was blank. I replaced thermostat and ac unit would still not come on. Next I checked to see if voltage was getting to thermostat and I was getting no power. There is a transformer on the outdoor unit and one on the inside blower, could one or both of those be bad? Is there a way to test them before buying new one?
First you need to make sure you have the right thermostat. It should be a five wire thermostat.
Rh and W or O or B will be heating transformer
in the furnace. To check the transformer in the furnace you have a primary side,across those two wires you should have 115 volts, if you have a gas furnace or 230 volts if you have a air handler with electric heat, then you have a secondary side, which those two wires should be 24 volts. If you have 115 or 230 volts to the primary and nothing at the secondary then that transformer is bad.
Rc and G and Y to the outdoor transformer.
Same principle to the outdoor transformer,
If you have 230 volts to the primary and no volts to the secondary which should be 24 volts, then that transformer is bad.
You have to make sure you are wired correctly because if you get the two transformers together, you will burn one or both of them up.
You need a serviceman. No really, you do! You are in over your head and you are replacing parts just because you can see them and they are not the likely culprit. I am betting it is the contacters in your condensing unit.
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