I've been having an intermittant problem where the 3-amp fuse on the circuit board keeps burning out. Lately, the problem has occurred in the late afternoon (3-7pm) such that when I replace the fuse, it will last for 3-4 minutes before blowing again. If I wait until late in the evening to turn on the air conditioning (say 10:30pm), it is likely to run all night without issue.

I have had someone out numerous times. He has completely changed the circuit board. After taking a second look at the exterior unit, we found that the contacter was going bad. The exterior unit was about 20 years old, and the compressor was also starting to go, and I have a 8.5 months pregnant wife -- so I gave him the go-ahead to replace the A/C unit and the coils inside the furnace. I just wanted it to work at this point!

He installed in Monday by 3pm, and it ran until about 5pm the next day, when once again the 3-amp fuse blew. I replaced it twice and same result, blows in five minutes or so.

So I've replaced the circuit board, the entire exterior unit. What else could it be? Could it be the 24V transformer or the thermostat? Or a wiring issue? I've run the blower without the A/C for long stretches without an issue during this mess, too. If it is a wiring issue, do I need a licensed electrician to look at it?

Any thoughts would be appreciated -- I'll pass them on to the technician who will come today or tomorrow. :)

Other than a failure in the fuse, a fuse will blow if there is a surge in electrical current.

Faulty fuses may happen every now and then, but certainly not with the frequency you're suggesting.

Without knowing the specifics of your installation:

A fuse will blow if there is a "faulty" device after the fuse. So you need to establish what devices the fuse protects. I would be inclined to isolate each component and fuse each with an inline 3A fuse. (Keeps the safety regs) and change your main fuse for a 5A. Now when a fuse blows at least you have identified which device is cauing the problem.

If there are too many devices, isolate only 1 or 2 at a time.

Repair or replace the fault device.

Hope this helps

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. It may also provide power to a control board. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit.

I am sure the 3 amp fuse has nothing to do with the outside unit other than the contactor. Likely it only protects the circuit board and transformer. If the transformer is separate from the board, not replaced with the board, my first guess is that it is going bad and shorting out when hot, blowing the fuse. However, if your blower only test included the peak afternoon heat, I would look at the A/C contactor, and the wire from the thermostat to it. If the contactor was replaced, that mostly leaves the wire.

Next time the fuse blows, shut the air off at the thermostat, and jumper the R terminal at the furnace to the Y terminal. That bypasses the thermostat and its wiring. The A/C should come on and stay one. If so, the wiring to the thermostat may be bad. If the fuse blows again, likely the pair of wires going out to the contactor is bad. Checking the wiring can be a pain where it runs through inaccessible places. You might start anywhere you have done any other work that may have damaged hidden wires. Also check ones that are assessable to any animals. I have had mine damaged by both my dog and chipmunks.

The only need for a licensed electrician would be for his skill in tracing wires and fishing new ones through walls and other places.

Thank you both for replying.

I have discovered the problem, I believe. It is on the wire which runs from the board to the contacter, it was crushed -- literally -- between a hot water pipe and a joist. The casing was destroyed and wire squashed flat. By tugging on the wire, I could blow the fuse (and just to make sure, blew it twice).

Replaced the wire this morning... and so far so good. I'm going to have to remember this board, and try to answer some questions other people have -- it's a fantastic place to get ideas.

Cheers!

The $&$@#$*&^%$#$^ chipmunks cost me 2 unfused transformers when they ate insulation off my thermostat wires.

All right I had the same thing. 3 amp fuse keeps blowing. I don`t have an ac hooked to this like a lot of these posts everywhere, just a furnace and a thermostat for heat. I`ve been working on this for 2 days and I can say after all this I found that I had an intermittent problem but didn`t know it. I`d check something and then 20 minutes later I`d check it and it was different. Make sure things you already checked are the same 20 minutes later. Overall, I found thermostat wire wore through were it enters the furnace and shorting to ground. I changed the board and thermostat, but as it turns out it was the short and no need for that stuff. The best way to fig this out is, disconect the thermostat at the board see if fuse blows, if so short inside furnace. If not, short out thermostat at board and while leaving thermostat wire DISCONNECTED. If blown then maybe board, or short, keep looking. If it doesn't short it's the thermostat wire somewhere on its way to thermostat.

Success! This forum was right on! After replacing the contactor (it needed it), the problem of the 3 amp circuit board fuse blowing still existed. I disconnected the 24V wire from the contactor and the circuit board and tested for continuity. It was shorting somewhere. I traced the wire (we have an attic unit, not fun to work on when it's hot)and found the problem. The wire was being pinched in one of the hanger bracket metal straps. Fixed the short with shrink tubing, reconnected everything and all is good now.

Learned a lot here. Thanks everyone! :)

You might also try using a different pair of wires. Usually the thermostat wiring has at least five wires. Use another wire or wires to see if it isolates the problem. You only need 2 wires from the furnace to the outdoor unit. Usually red and yellow. This will save you from running new wires. Just remember that you changed them.