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Old 01-02-2013, 04:15 AM   #16
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5amp Fuse Blowing (again)


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Originally Posted by kirwinjd View Post
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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Old 01-05-2013, 12:57 PM   #17
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5amp Fuse Blowing (again)


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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Old 01-05-2013, 03:50 PM   #18
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5amp Fuse Blowing (again)


Ohm out the 24 volt coil on the contactor.
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Old 01-05-2013, 04:41 PM   #19
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5amp Fuse Blowing (again)


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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Old 01-05-2013, 05:45 PM   #20
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5amp Fuse Blowing (again)


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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Old 01-15-2013, 04:23 PM   #21
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5amp Fuse Blowing (again)


Just to close the loop on this, after one too many days of coming home to a cold house I called an HVAC guy. Turned out to be a bad wire nut of all things, on one of the small wires in the outside unit. It would vibrate and arc inside and blow the fuse. I would have probably found it eventually, but that's a pretty obscure problem. The tech was very good about the whole thing, he had to charge me the $98 diagnosis fee but that was it. I know it's not quite the DIY outcome, but I learned some good stuff about how heat pumps work, and now I have reliable heat again.
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Old 01-15-2013, 06:19 PM   #22
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5amp Fuse Blowing (again)


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Originally Posted by JLeather View Post
Just to close the loop on this, after one too many days of coming home to a cold house I called an HVAC guy. Turned out to be a bad wire nut of all things, on one of the small wires in the outside unit. It would vibrate and arc inside and blow the fuse. I would have probably found it eventually, but that's a pretty obscure problem. The tech was very good about the whole thing, he had to charge me the $98 diagnosis fee but that was it. I know it's not quite the DIY outcome, but I learned some good stuff about how heat pumps work, and now I have reliable heat again.
Thanks for the update.....and $98 sounds like a fair price....you have to consider his travel time....and the fact that he found it....

All too often the most complicated problem has a simple fix.....

You also have to consider what your time is worth.....

For the record....it is a DIY solution....you gave it your best shot....knew when to throw in the towel and call an expert....and learned from it....I'm willing to bet that the next time you have a similar issue, you will start checking for things like that.....

Money well spent.
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Old 01-15-2013, 07:24 PM   #23
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Oh, believe me, I'm not complaining about the money. They told me up front it was $98 to get started. He could have charged more for his time, but he said it was an easy fix. I'm a little disappointed that I looked at that wiring and still didn't find the problem, but I can at least take solace in the fact that it was fairly difficult to spot.

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