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Old 01-01-2013, 09:39 AM   #1
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5amp Fuse Blowing (again)


About 2 months ago the 5amp fuse blew in my air handler. Replaced the fuse, held my breath, and never had another problem with it until last night. Came home late yesterday afternoon and found the thermostat flashing the familiar code for no voltage. Checked the air handler and sure enough the fuse was blown again. Swapped it out and the outdoor unit kicked on and heat came out of the ducts again. All was good. A couple house later I noticed the thermostat was back to flashing the error again; another blown fuse. I replaced this one and went to bed, not too confident in my chances. This morning the house was COLD and so it's back to troubleshooting.

I changed the thermostat to Aux Heat and replaced the fuse again, and for a couple hours now it's been working fine. I figure this means it's a problem with the outdoor unit. I checked and the coils aren't iced up, and I can't find any obvious shorts in the usual places (against the piping, under zipties, etc). What component failures could cause this? The outdoor unit will start and run for a while if I go back to normal heat mode. I'd estimate the fuse blows after about 1-2 hours, but I haven't had the time or patience to sit down and time it exactly. Some other searching around leads me to believe it might be the defrost board, since that's a timed function. How do I diagnose the defrost board, or is it cheap enough that if you think it's bad you just replace it and see if the problem goes away? Anything else that might cause this/

Thanks guys, atleast I've got Aux Heat for today, but that'll get expensive if I don't nail this down.

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Old 01-01-2013, 10:46 AM   #2
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5amp Fuse Blowing (again)


Ok so it looks like you have a Heat Pump with supplemental electric heat strips.
Possible short circuit in the defrost components in the outside unit. The short occurs during the defrost
cycle. I got a service call last month with the exact same problem. Check the temperature defrost termination sensor. It's clipped on the coil.

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Old 01-01-2013, 11:04 AM   #3
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5amp Fuse Blowing (again)


You have a low voltage wire hitting metal. You may even had or have a mice nest out there were the mice may had chew up wires.
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Old 01-01-2013, 11:10 AM   #4
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5amp Fuse Blowing (again)


Quote:
Originally Posted by JLeather
About 2 months ago the 5amp fuse blew in my air handler. Replaced the fuse, held my breath, and never had another problem with it until last night. Came home late yesterday afternoon and found the thermostat flashing the familiar code for no voltage. Checked the air handler and sure enough the fuse was blown again. Swapped it out and the outdoor unit kicked on and heat came out of the ducts again. All was good. A couple house later I noticed the thermostat was back to flashing the error again; another blown fuse. I replaced this one and went to bed, not too confident in my chances. This morning the house was COLD and so it's back to troubleshooting.

I changed the thermostat to Aux Heat and replaced the fuse again, and for a couple hours now it's been working fine. I figure this means it's a problem with the outdoor unit. I checked and the coils aren't iced up, and I can't find any obvious shorts in the usual places (against the piping, under zipties, etc). What component failures could cause this? The outdoor unit will start and run for a while if I go back to normal heat mode. I'd estimate the fuse blows after about 1-2 hours, but I haven't had the time or patience to sit down and time it exactly. Some other searching around leads me to believe it might be the defrost board, since that's a timed function. How do I diagnose the defrost board, or is it cheap enough that if you think it's bad you just replace it and see if the problem goes away? Anything else that might cause this/

Thanks guys, atleast I've got Aux Heat for today, but that'll get expensive if I don't nail this down.
If you can, disconnect the DTS from the board and see if that stops the fuse from blowing. Just a possibility.
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Old 01-01-2013, 11:17 AM   #5
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5amp Fuse Blowing (again)


Quote:
Originally Posted by kirwinjd View Post
If you can, disconnect the DTS from the board and see if that stops the fuse from blowing. Just a possibility.
That is only shows you have no shorts from indoor unit to outdoor unit. Contractor ,pressure switch ,or some units reversing valve is power up after defrost board. You better off forts defrost cycle if you want to know if defrost board is bad.
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Old 01-01-2013, 11:35 AM   #6
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5amp Fuse Blowing (again)


Quote:
Originally Posted by Tator1076
That is only shows you have no shorts from indoor unit to outdoor unit. Contractor ,pressure switch ,or some units reversing valve is power up after defrost board. You better off forts defrost cycle if you want to know if defrost board is bad.
If there was a short in any of those components, they would usually blow the fuse immediately. Your wording is making it difficult to follow your logic.
As I understand, the unit runs for awhile before blowing the fuse each time. Any shorting in the wiring, usually blows the fuse immediately.
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Old 01-01-2013, 11:39 AM   #7
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5amp Fuse Blowing (again)


Quote:
Originally Posted by kirwinjd

If there was a short in any of those components, they would usually blow the fuse immediately. Your wording is making it difficult to follow your logic.
As I understand, the unit runs for awhile before blowing the fuse each time. Any shorting in the wiring, usually blows the fuse immediately.
And I stress the word USUALLY. After 30 years of doing this, I've probably seen it all so anything is possible.
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Old 01-01-2013, 03:58 PM   #8
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5amp Fuse Blowing (again)


Possible short in the RV coil.
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Old 01-01-2013, 05:00 PM   #9
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5amp Fuse Blowing (again)


it could be a short anywhere.... i had a few that drove me nuts and it was the compressor contactor coil.... sometimes not easy to find.... it does not sound like a short to ground as that would usually blow the fuse immediately
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Old 01-01-2013, 08:58 PM   #10
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5amp Fuse Blowing (again)


I'll keep looking for a short, but so far I haven't found anything. It's a 5-year old system in pretty good repair. Aside from losing a start/run cap this summer for the compressor it hasn't given me any trouble. The wiring is a pretty short run from the basement to the outdoor unit, no mice nests or other critters. I'll keep digging for a short.

How can I force a defrost cycle and see if that's the culprit?
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Old 01-01-2013, 09:36 PM   #11
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5amp Fuse Blowing (again)


While unit is running, short the test pins on the defrost board.
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Old 01-01-2013, 09:44 PM   #12
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5amp Fuse Blowing (again)


Depends on the model. Some units have test points on the board that you can jump to put into defrost. Some models are time initiated, temp terminated or temp initiated, temp terminated etc.
The way I diagnosed the problem in the past was to disable the defrost and see off its still working hours later. Since I live in Santa Barbara California, I could get away with that and the unit would still work without a defrost cycle.
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Old 01-01-2013, 09:55 PM   #13
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5amp Fuse Blowing (again)


I am glad we have no heat pumps here...or very few...they seem like a bit of a pain..... hard to get heat from -40 degree air..... some have tried ....did not work out well for hem...
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Old 01-01-2013, 10:10 PM   #14
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5amp Fuse Blowing (again)


Quote:
Originally Posted by carmon
I am glad we have no heat pumps here...or very few...they seem like a bit of a pain..... hard to get heat from -40 degree air..... some have tried ....did not work out well for hem...
I agree. Unless the condenser coil is buried below the frost line, they're only effective in moderate climates. I don't know if you have oil fired furnaces in your area but I wouldn't have a clue working on them. Here in Southern California, its natural gas, propane, heat pumps or electric. (Ranked in order of efficiency)
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Old 01-01-2013, 10:20 PM   #15
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5amp Fuse Blowing (again)


It's not necessarily in the outdoor unit. I saw a situation once where the air handler blower relay had a fried coil and was drawing too much current, but it only made the low voltage fuse blow when the heat pump was operating (which meant that the low voltage had to drive the compressor contactor, the outdoor fan relay, and the reversing valve solenoid). When the system was switched to emergency heat, it ran for a while because not as much current was being drawn off of the low voltage transformer.

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