furnace intermittently blowing 3 amp fuse

charliechicago

I have a furnace/coil that for the past 3 years has been blowing the 3 amp fuse on the board 2-3 times an entire heating season. (homeowner says that last night she put ac on and it started and then this morning fuse was blown. That's first time is blew in ac mode) I have checked voltages, amp of gas valve, shorts to ground, and cannot find any issue. The board was changed last year for other reasons but it had been shorting before that. Does anyone have any suggestions of what to check next? your help would be much appreciated. Michael.

JUGHNE

Are you sure it does not require a 5 amp fuse. Most are 3 sometimes there were 5 amp fuse.

Indoor blower is common use for AC and heat. To me the fan switching relay, wiring etc would be a possible suspect.

There is the "Little Popper" circuit breaker available in 3 & 5 amp.
Using the circuit breaker would let the HO reset it and perhaps catch the culprit in the act. Or you could troubleshoot easier with the wiggle and probing and actually hear the CB click.

Otherwise I have mice chew thru insulation shorting ac 24 vac line........coils with intermittent problems.....bare insulation on wire that shorts out with vibration from fan......R & C touching at T-stat...sometimes.

aircooled81

What model furnace is it?
If something like the gas valve is heating up its coil and causing the issue, its something to consider once we know what beast you are dealing with.
Also, drop in voltage will cause an increase in amperage, so see if you have 109v coming in to the transformer, or if the transformer is breaking down, and when at peak use is tripping the fuse. I think this test could be done with everything running (fan, mgv) throw an amp clamp on the 24v leaving side of the transformer and see how close your getting to 3 amps.
Is there a honeywell fan switch in there? Those are fairly common to bug out after many years of use, give that a look.
If it is tripping when your gone, then finding a short wile the unit is off is not likely. I think you have a relay or tansformer pooping out while in use for extended amount of time.

aircooled81

One wild idea, see if the furnace circuit is shared with something like a garbage disposal, or other load that might be used when the furnace is running causing the voltage drop.
I know its a reach, but it sounds like your open to some out of the box stuff by now.

unclejohn

Check the condensing unit low voltage wiring. Often have wires to low or high pressure switch rub through. If it's a hybrid system I can almost guaranty it's outside at the heat pump.

JUGHNE

I recall now another odd short from older Rheen/Ruud gas furnaces. They used an insert hi limit into the air stream between burner cells. This was a limit only, not fan/limit combo. The thermo disc button was on horizontal extensions about 3" long. The foil insulation or the metal edges of the opening would short one side of the limit to ground. It was an intermittent issue caused by air flow fluttering the foil insulation....or expansion of the metal on the too small opening....or just vibration. Not enough clearance. And no fuses at that time, it would just take out the transformer. There even may have been a service bulletin for the problem. FWIW

charliechicago

Thank you all for responding.
I will respond to everything that was said and maybe this will shed light on the issue.
Its a York GM8S060A12UH11B. I'm sure its a 3 amp. I don't think its the relay because like I said the board(which has all the relays) was replaced last year and I know that it had the issue before the board was changed. I've looked for bare wires, etc. I changed the tstat and the wiring going to it. The line/low voltage is fine. Amp draw is good too at the transformer. In regard to the transformer if it had an issue, since its "before" the fuse wouldn't it just burn out or trip the main breaker? I don't see that its attacked to anything else in the house. The condenser has nit been running so I cant see it being that. Thanks again and please let me know if you have any other ideas.

JUGHNE

Did you look at the high limit? There may be more limits on the blower fan housing.

charliechicago

i did check two other limits on the blower. I visually inspected them and also tested to ground.

JUGHNE

Condensate pump overflow switch wired wrong?? Pretty hard to screw that up but it is something seldom activated and could reset itself unnoticed......WAG.

Ironman

JUGHNE said:

I recall now another odd short from older Rheen/Ruud gas furnaces. They used an insert hi limit into the air stream between burner cells. This was a limit only, not fan/limit combo. The thermo disc button was on horizontal extensions about 3" long. The foil insulation or the metal edges of the opening would short one side of the limit to ground. It was an intermittent issue caused by air flow fluttering the foil insulation....or expansion of the metal on the too small opening....or just vibration. Not enough clearance. And no fuses at that time, it would just take out the transformer. There even may have been a service bulletin for the problem. FWIW

I had this same issue with Nordyne furnace when I moved in to my house. The guys that installed it had been unsuccessful for years trying to find it. It took me a couple of times and I finally pulled out my megometer and started isolating each component/wire one at a time.

MikeJ

Had a Nordyne furnace blowing fuse, found the metal foil that cover the insulation on the tube sheet came lose and was hitting the high limit switch once in awhile, blowing the fuse.

hdb3z4x

Hi I know this is an old post but I just wanted to say thank you, I have been chasing a fuse blowing issue for several days now and the Mr Heater tech support was not able to help. This forum helped me solve my problem, thank you so much.,

charliechicago

hdb3z4x
What was the problem, anything interesting?!!?