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Old 06-19-2012, 05:12 PM   #1
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Furnace circuit trips -- sometimes -- when AC cycles on


Hi all,

I have been struggling with this problem ever since I had AC installed in my house in around 2002. The dedicated furnace/blower circuit breaker (15 amps) trips about 5% or 10% of the time when the AC cycles on. The circuit never trips in the winter when it's just the gas-fired furnace and fan running, only in the summer when the AC, which is on a dedicated circuit of its own (which has never tripped), comes on. I've had the original AC installer and another electrician try to troubleshoot the problem, and their only suggestion is to change the furnace filter. But I'm certain that's not the problem, because I replace the filter regularly. I just put a new filter in yesterday morning, and the furnace/blower circuit tripped again today.

The electrician's notes (from 2004) say "Checked unit, checked fan motor and blades for dirt buildup. Checked all wiring in furnace. No shorts found. All connections are tight. In cool mode, amp draw on indoor fan motor is 6.3. Max amps for system is 10.4. No fuse in system, breaker is 15 amp single pole. Found dirty filter in unit. May be causing motor to overheat."

I've been living with this glitch for years, but it's a mystery that I'd like to solve -- it's shaping up to be a beastly summer here in Colorado. The furnace is a standard, builder-grade Carrier (12 years old), and the AC compressor is a Trane XL13i, 2-ton as I recall.

I know that circuit breakers rarely fail, but I'm thinking of swapping out the 15 amp breaker for another, new 15-amp breaker. Any other suggestions?

Thanks,
Jill

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Old 06-19-2012, 05:42 PM   #2
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Furnace circuit trips -- sometimes -- when AC cycles on


your outside condenser should have its own circuit breaker and disconnect outside with fuses also if required.check the breaker for two different breakers one for furnace one for condenser...the start on that compressor outside is tripping it as it should too small on the rating and not to code.

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Old 06-19-2012, 05:51 PM   #3
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Furnace circuit trips -- sometimes -- when AC cycles on


Thanks for replying to my post. The outside AC compressor is definitely on its own circuit, separate from the furnace -- and the AC circuit breaker never trips. Well, according to the labels on the circuit box, the AC and furnace are on separate circuits -- I've never opened up the box to verify. But I believe the installer and the electrician both checked the wiring when I had them back to troubleshoot.
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Old 06-19-2012, 06:47 PM   #4
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Furnace circuit trips -- sometimes -- when AC cycles on


What else is sharing this "dedicated" 15 amp circuit for the furnace? Is there a humidifier also in the loop? Who did the install of the electrical circuit for the furnace, or was it an existing circuit? I would try running some romex from the breaker panel to the furnace as a test, and hook the furnace to that new run, to see if it also exhibits the same symptoms on that.

If it doesn't, then you know something is amiss somewhere between the furnace to the breaker panel. Also, the switch on the side of the furnace, the person that installed the unit, did not per chance "back-stab" the wires into the switch, did they?
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Old 06-19-2012, 07:04 PM   #5
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Furnace circuit trips -- sometimes -- when AC cycles on


Ah, good questions. The furnace is on a circuit by itself. No humidifier or anything like that. The wiring was all done by the builder's contractor, and I haven't had any other problems with any of the wiring. The AC compressor (NOT the furnace) is on the same 40-amp circuit as the cooktop -- the AC installer did that because I didn't have any unused circuits to put the AC on (I really need a subpanel), and the cooktop is gas, so the only electrical draw on that circuit, other that the AC compressor, would be the igniter. The AC + cooktop circuit has never, ever tripped.

The Romex test is a great idea. I have some 12-2 with ground wire left over from another project, but it won't be enough to make the run. Is 12-gauge ok, or do I need heavier duty wire for a 15-amp circuit?

Is it odd that the furnace never trips in the winter when the blower comes on?
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Old 06-19-2012, 07:08 PM   #6
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Furnace circuit trips -- sometimes -- when AC cycles on


Yes, 12 is fine for a 15amp circuit.
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Old 06-19-2012, 07:12 PM   #7
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Furnace circuit trips -- sometimes -- when AC cycles on


Quote:
Originally Posted by Outcrop View Post
The AC compressor (NOT the furnace) is on the same 40-amp circuit as the cooktop -- the AC installer did that because I didn't have any unused circuits to put the AC on (I really need a subpanel), and the cooktop is gas, so the only electrical draw on that circuit, other that the AC compressor, would be the igniter. The AC + cooktop circuit has never, ever tripped.
Ding, ding, we have a winner. Suggest finding that idiot, and having them bend over while you kick them in their arse. You need to get a sub-panel, and fix the problem, which is most likely why your breaker is tripping for your furnace. Somehow it is seeing a load surge somehow and tripping the breaker. I bet you will find out once you start looking over the panel, that there are a few other problems, and will find why this is happening.

Can you post some pictures of the breaker panel with the door off, showing all of the wiring, because now you have raised the curiosity gene in me to see how bad this could be.
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Old 06-20-2012, 04:16 PM   #8
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Furnace circuit trips -- sometimes -- when AC cycles on


I thought you had solved the mystery, but after opening up the circuit box, I don't think that's it. Only the two A/C unit wires are going to the 40 amp circuit that I thought the cooktop was on. Just to verify, I flipped the switch on the AC circuit to kill it, came inside and turned on the stove. The stove worked fine, so it's on a different circuit, and now I'm back to thinking that the AC unit is on a circuit all by itself -- it's the uppermost circuit on the right side of the box in the photos -- hopefully the labels and arrows come through).

Now I think I'll try swapping out the furnace breaker, and if it still trips, then I'll try the Romex direct-wire experiment. Let me know if you have any other thoughts after perusing the wiring (photos below, if this works.) Thank you!!

Furnace circuit trips -- sometimes -- when AC cycles on-circuit-box-guts.jpg Furnace circuit trips -- sometimes -- when AC cycles on-circuit-box-outside.jpg
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Old 06-20-2012, 04:28 PM   #9
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Furnace circuit trips -- sometimes -- when AC cycles on


you keep saying the AC circuit which is that 40A for the condenser outside and nothing else.the other one on the left you marked if for the furnace/air handler which is the one that is tripping...right.the speed of the motor in the cooling season usually is HI compared to LO/Med for heat and amps aren't still within the 15A breaker range could be a weak breaker ..do you see any bubbled insulation at the connections within that panel.... question i see 6 breakers on the left with black and red wires into them totaling 12 wires are each of those breakers taking 2 wires each?
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Old 06-20-2012, 04:49 PM   #10
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Furnace circuit trips -- sometimes -- when AC cycles on


Right. No bubbling or other evidence of heat damage. The furnace circuit doesn't feel hot (or warmer than any other circuits) when the AC and furnace blower are running, either.

I'm thinking weak breaker too, since the electrician who checked out the problem a few years ago noted 'In cool mode, amp draw on indoor fan motor is 6.3.' Shouldn't be too difficult to swap that out.

Here's a photo of that side of the panel. Thanks for your continued help and patience. --Jill

Furnace circuit trips -- sometimes -- when AC cycles on-left-side-circuits.jpg
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Old 06-20-2012, 04:55 PM   #11
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Furnace circuit trips -- sometimes -- when AC cycles on


just run the FAN ON off the stat then shut the stat off if you can touch the frame body of it and its hot might be the HI speed winding is going...
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Old 06-20-2012, 05:25 PM   #12
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Furnace circuit trips -- sometimes -- when AC cycles on


I would hand over hand check that wire from the circuit breaker all the way to the heater/air hander just to be sure that's the only thing on that circuit. I have a feeling it's not alone especially with that loaded panel.
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Old 06-20-2012, 05:53 PM   #13
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Furnace circuit trips -- sometimes -- when AC cycles on


What is with the Blue wirenut on the Right side? The wires appear to be #12, but they are using a wirenut for 5-6 #12, 4-5 #10, or 2-3#8.
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Old 06-20-2012, 05:56 PM   #14
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Furnace circuit trips -- sometimes -- when AC cycles on


Good suggestion -- will do. I just finished swapping out the circuit breaker with a new one from Home Depot. It's going to get hot again in the next few days, so will post back this weekend about whether the new breaker trips.

Thank you all again. Subpanel is moving up to the top of my to-do (rather, have someone else do) list.
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Old 08-30-2012, 10:58 PM   #15
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Furnace circuit trips -- sometimes -- when AC cycles on


Just a brief follow up: it was a faulty circuit breaker. I replaced the circuit breaker with an identical new one, and it hasn't tripped since. Thanks, guys, for all your help.

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