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Discussion Starter · #1 ·
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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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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HVAC Tech/ Browns Fan
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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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JLeather said:
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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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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Tator1076 said:
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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kirwinjd said:
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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Possible short in the RV coil.
 

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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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Discussion Starter · #10 ·
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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While unit is running, short the test pins on the defrost board.
 

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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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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...:eek::eek::eek::eek::eek:
 

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carmon said:
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...:eek::eek::eek::eek::eek:
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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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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Here in Southern California, its natural gas, propane, heat pumps or electric. (Ranked in order of efficiency)
You posted cost of operation for your area.

In order of efficiency, it would be heat pump, electric resistance, natural gas, then propane.

Efficiency, and cost of operation are not the same.
 

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Discussion Starter · #17 ·
Ok, sorry there haven't been any updates for a few days. I don't get home from work until after dark, and I didn't want to try tracing this problem down in the darkness. Since Tuesday the heat pump has been operating fine on Aux Heat. The fuse hasn't blown once and the house has stayed warm. Today I had some time off so I dug into the outdoor unit again. I have a Defrost Control Board that just runs based on time and coil temp (defrost thermostat). I switched the thermostat in the house back to normal heat mode and waited for the outdoor unit to kick on. I let it run a few minutes and then forced a full defrost cycle by jumping the test leads on the DCB. The unit went through a full defrost cycle and a couple minutes later went back to heating the house. The fuse did not blow. I gave it another half hour and then forced another full defrost cycle, and the unit again went back to normal after a couple minutes. It's been running for an hour now on normal heat mode without any fuse problems. The DCB is set for 90 minutes so in another half hour or so it'll run through its first standard defrost cycle. We'll see how that goes.

Not really sure where to go from here. Without any visible damage to any of the DC control wires, and without any reliable way to make the unit blow the 5amp fuse, how can I continue tracing the problem?

On a side note, it occurs to me that both times I've blown this fuse now it was on a particularly cold night/evening. Is there anything that could be causing this problem only when the system is working hard? Today it's not very cold, maybe 45*F, so it's not working hard. I have the heat turned way up to the mid 70's to make sure it keeps running for the whole 90 minutes before defrost.
 

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Ohm out the 24 volt coil on the contactor.
 

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At HVAC supply houses they sell these test items called tattletales that you put in the different circuits in the units.

It's 2 wires with a little bulb/flash type fuse in it rated at different ampereages.
What you would do is put one one the safety circuit, put one on the defrost circuit, put one on the compressor circuit, etc wherever you can put one. Then when you find the circuit that's causing the failure you just start eliminating compo nets from that circuit.
Could be a while before finding the problem but it works for troubleshooting.
 

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Do you have the right 5a fuse. There are two basic types....fast blow and slow blow. Typically you use a fast blow for non inductive loads.....i.e., just electronics.....but if you have an inductive load (motors, relays, etc) you typically want a slow blow.

Is it possible your unit requires a slow blow and you have a fast blow in it?
 
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