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Heating, Air Conditioning, Fridge, HVAC/do I have a blown hvac transformer?


QUESTION: Is my hvac transformer bad?  My outside condensor/fan doesn't run, nor does the fan in the inside blower/control unit.  I replaced the control board, checked the thermostat, and there is juice to the outside condensor and inside blower unit.  If a transformer is bad, will the blower fan run?  Or will the condensor still run?  Thanks.

There is one transformer in the inside unit that transforms 110 voltage down to 24 volts,
If the transformer goes bad you won't have a 24  volt circuit in side or out.
A loose wire or broken wire on the 24 volt circuit will do the same thing as a blown transformer, just check the transformer and see if there is power a spark at the low voltage output side?

---------- FOLLOW-UP ----------

QUESTION: Jim, I did not have low voltage so I replaced the transformer.  Now the control board fuse blows when I have the thermostat on Cool. So far I've replaced the transformer, capacitor, and will pick up and install the contactor/coil tomorrow.

With various tests I've discovered the following:
1) I have 240 at the contactor coil unit in the condenser housing;
2) I have 120 input into the transformer, am not certain about the output because the fuse blows; however, the new transformer hums quietly, like the old one used to before it went bad and became quiet;
3) I removed the small wires at the contactor's sides, turned off the power at the condenser, kept the stat on cool, and the fuse blew when I turned the power on at the air handler.

As I understand the tests I conducted, my capacitor is probably good, as is the contactor.  I'm going to replace the contactor anyway.  Please note that I replaced the control board with another one, and I still get the same results with a blown fuse with the stat on Cool.

At this point, I don't know where to place my multitester leads on the capacitor to prove if it works, nor on the small circuit board beneath the capacitor to test it, either.  It has 4 wires, two in from the contactor, and two out to the air handler.  And I don't know how to test any non-power wires.

When I depress the spring relay contacts on the contactor, the outside fan runs.  Also, when I crossed wires on the contactor, both the outside fan and condenser ran.  So the outside fan works, as does the condenser.  Also, the air handler inside runs, just not on Cool, due to a blown fuse.

FYI, the problem surfaced when I shorted some wires, which lead from my master panel in the garage to the outside conduit leading to the condenser unit.  I fixed the short outside, as evidenced by the 240 at the condenser.

The output side of the transformer 24 volts goes to the thermostat, the thermostat is a switch that distributes the 24 volt power to different heating or cooling controls depending on it's setting, when the thermostat is set to cool and the  temperature indicator is set to where the cooling should come on the thermostat sends 24 volt power to the contactor in your outside condensing unit, when the contactor gets the 24 volt power it turns the contactor on, the 240 volts goes directly to the contactor from the breaker box and is always there live as long as the breaker is on, when the 24 volts from the thermostat turns the contactor on the 240 volts goes on to the compressor and fan motor and they both run.
your problem is in the 24 volt circuit between the transformer and the contactor.
The 240 volt circuit, capacitor etc. is not the problem.
Check the 24 volt low voltage wires for a short circuit, the 24 volt wires touching some place along the way, the wires are usually taped to the refrigeration lines that run from the inside unit to the contactor and are subject to animals and varmints chewing them outside.

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Jim Barnhart


Fifty + years conditioning the air.


Answer questions about , residential and commercial. Answer questions about sheet metal fabrication. Fifty years plus experience. No answers for oil equipment, No answers for kitchen appliances, No answers for laundry appliances, I don't specialize in one particular area of the HVAC area, I'm more of a General Practitioner, Limited in refrigeration/

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