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Matthew
Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 1994
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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My heat pump system stopped working after a power outage and

Customer Question

My heat pump system stopped working after a power outage and continues to blow the low voltage 5 amp fuse when we turn it on. We tested the transformer and it read about 21 volts. We tested the start capacitor and it is fine. We replaced the thermostat, it started once and we shut it down to replace the furnace cover. Of course the second time we started the system the fuse blew again. There's occasionally a sulpher smell in the unit if that's relevant. Can you help with any ideas? Thanks...C Evans

Thanks for the info - I disconnected the low voltage wires at the outside unit and the fuse did not blow when the AC was turned on - also the outdoor unit did not come on at all. I checked the contacts and they were pretty corroded. It was too late to get a replacement so I just filed and cleaned the contacts and replaced. The AC turned on for about 2 minutes then blew the 5 amp fuse again. New fuse - I tried just using the heat setting & the fuse did not blow - switched to AC and it blew immediately. I'm pretty sure the contactor should be ok now that it's cleaned, but I'll try to find a new one. Tomorrow I'll start looking for the defrost control board and try that unless you steer me in another direction...thanks.
Submitted: 4 years ago.
Category: HVAC
Expert:  Matthew replied 4 years ago.

Hello,
My name is XXXXX XXXXX I will do my best to help you today.

 

In order to find the source of the direct short you will have to do a process of elimination. First unhook the low voltage wires going to the outdoor unit where they hook in to the outdoor unit. Make sure they are not touching each other or anything else and turn the system to cooling like you normally would. If the fuse doesn't blow the short is in the outdoor unit either the contacts or the defrost control board. If it blows again then you move on to the thermostat.

 

hook the outdoor unit wires back up and then go to the thermostat and unhook the wires from the stat. Twist the red, yellow, orange, and green wires together and the cooling should come on. If the fuse doesn't blow the thermostat is the problem if it does blow then the problem is the furnace control board or the wires themselves going from the outdoor unit to the furnace and thermostat are bare and shorting against each other.

Customer: replied 4 years ago.
Thanks for the info Matthew - I disconnected the low voltage wires at the outside unit and the fuse did not blow when the AC was turned on - the outdoor unit did not come on at all. I checked the contacts and they were pretty corroded. It was too late to get a replacement so I just filed and cleaned the contacts and replaced. The AC turned on for about 2 minutes then again blew the 5 amp fuse again. New fuse - I tried just using the heat setting & the fuse did not blow - switched to AC and it blew immediately. I'm pretty sure the contactor should be ok now that it's cleaned, but I'll try to find a new one. Tomorrow I'll start looking for the defrost control board and try that unless you steer me in another direction...thanks.
Expert:  Matthew replied 4 years ago.
I would disconnect the wires to the outdoor unit at the furnace. This way you can determine if it is the wire or the outdoor unit itself. It could be that your low voltage wires are just shorting against each other somewhere on their way from furnace to outdoor unit.
Expert:  Matthew replied 4 years ago.
I don't know what I was thinking last night posting that. You have already determined that the wire is fine because it was still hooked to the furnace and the fuse didn't blow. The problem is indeed the outdoor unit. The 24v circuit for a heat pump is as follows.


24v enters and goes tot he defrost control board. Out of the defrost control board 24v goes to the reversing valve, the low pressure switch, and then finally to the contacts. You can unplug the two wires for the reversing valve from the defrost control board and then try it to see if the fuse blows. The low pressure switch you have to jump across the two terminals that it hooks to at the defrost board then try it. Unhook the 24 wires from the board that goes to the contacts and then try the system one side of the contacts will be from the board the other side will come directly from the thermostat wires this is the C wire. Then finally if you unhook the thermostat leads (red,yellow,orange(blue if it is a ruud/rheem) these are the three that feed 24v to the unit)from the actual board and try it you can see if it is the board itself.

Edited by Matthew on 5/6/2010 at 11:03 AM EST

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