control fuse blows then breaker trips

Problems related to residential installations.

control fuse blows then breaker trips

Postby fractilian » Tue Dec 31, 2013 7:56 pm

I have a 2 stage heat pump system. About a month ago the 5 amp fuse for the 24 volt control circuit was shot. I disconnected all the wires at the thermostat, in-door, and out-door units. I check all wires for shorts to each-other and to ground and all was clear. I also ohmed out the coils for the relays. All was clear. I put everything back together and everything worked fine. Yesterday I come home to find the same thing. I checked out all the control wires as I did before and all was clear...again. I put everything back together fired up the system and it ran for about 45 minutes and then cut off. This time the breaker tripped. I reset it and tried it again. The system ran again for about 20 minutes and tripped again. The breaker feels warm to the touch. i tried it again in normal and emergency modes and the breaker tripped all but once when the fuse went. I can leave the breaker on with the thermostat in the off position and it holds.
I am not sure if the issues are related or not. I am wondering if the 24 volt transformer may be on the way out. It is the only component that is on both the high and low voltage sides. Any suggestions.
fractilian
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- control fuse blows then breaker trips

Postby Freon » Wed Jan 01, 2014 12:06 pm

Is the circuit breaker 15 amp, 120 volt that is dedicated to the heat pump system? Can you post the make and model of the heat pump and its age.

1st test would be to disconnect the 24 volt transformer primary side (120 volt AC) from the control board. Turn off all power to the heat pump before disconnecting anything. Now see if the circuit breaker pops as before. If the circuit breaker holds then turn off the power, reconnect the primary side of the transformer to the control board and now disconnect the 24 volt side of the transformer from the circuit board. Does the circuit breaker pop?

If the breaker does not pop in either situation above, then I think the problem has the highest probability of being on the 24 volt side of the circuit and not the transformer.

There is something causing the 5 amp fuse on the 24 volt side to blow. It is possible the excess load on the 24 volt side is causing the 120 volt breaker to pop meaning the transformer is okay. With the relays and solenoids on the 24 volt circuit, there are many possibilities for the load to become excessive on the 24 volt side. But the first test above will let us know if the 2 problems are connected, as you suspect.

There are two control boards with relays and solenoids. One is at the air handler and the other is at the outdoor condenser. A problem in either of these boards could be causing the 5 amp fuse to blow.I am assuming the fuse blows when you call for heat. Does/did it blow on a call for AC?
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- control fuse blows then breaker trips

Postby fractilian » Wed Jan 01, 2014 5:54 pm

Thank You for the reply. My Emergency heat is electric so the unit is fed by a 60 amp 240 volt breaker. I also have an enclosed circuit breaker in the attic as a means of disconnect. I noticed that the breaker in my main panel was tripping, not the disconnect. My though was that if the issue was with the heater the disconnect would trip first. I swapped out the breaker in my main panel and it seems to have worked.
Thanks for viewing and responding.
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- control fuse blows then breaker trips

Postby Freon » Sat Jan 04, 2014 11:40 am

Since you seem to have the circuit breaker issue resolved, you're back to the 5 amp fuse on the control board blowing. As I mentioned before, there are two circuit/control - one on the air handler and one on the condenser unit outside. By isolating the power to only the air handler control board, you should have a clue as to where to examine further.
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