Good morning all,

My house is 20ish years old. Biggish house, fairly upscale when it was built 20 years ago. I've got two condenser units outside, one controlled by an up stairs thermostat and one controlled by a downstairs thermostat. The AC is of an unknown vintage, I can dig to around to the home inspection report if necessary. It gets a "tune-up" service twice a year, no issues identified so far.

Over the past week or so, if I run both the upstairs and downstairs AC units at the same time, it trips a random UNLABELED 20 amp breaker after some 10 or 20 minutes. Both thermostats (Nest smart thermostats) report a no voltage/power off Error. They are 3 wire thermostats, green, red, white.

Down at the electrical panel, I have two breakers that are connected together with a metal plate, each breaker either 25 or 30 amps, and those are marked AC. Those do not get tripped.

Do we suppose that the evaporator/air handler in the attic is on a separate 20 amp circuit?

My electrical panel is original to the house. Most of the writing/labeling on the circuits have long worn off, so most of the breaker switches control unknown parts of the house. I've ordered a Klein ET310 Circuit Finder and I'll go through and try to identify each of the household circuits eventually, but that is a later project.

For the time being, do I either call an HVAC guy to come out for a tune-up (and mention the problem), or do I call him out to explicitly diagnose the problem that's causing the breaker tripping, or do I call an electrician?

The bridged breakers are for 220 power, and that is likely just for the compressors outside. The air handlers are probably 110 volt, and that is probably what is on that unlabeled breaker.

As a test, when the system is working properly, throw the unlabeled breaker and see if the upper air handler shuts down.

quote:

Originally posted by ArtieS:
The bridged breakers are for 220 power, and that is likely just for the compressors outside. The air handlers are probably 110 volt, and that is probably what is on that unlabeled breaker.

As a test, when the system is working properly, throw the unlabeled breaker and see if the upper air handler shuts down.

When that unlabeled breaker trips (i.e., same as me throwing the switch), compressors and air handler all shut off. Theoretically, the thermostat might be ordering the compressors off to protect the system if it loses power to the Air Handler.

quote:

Originally posted by Aeteocles:
Do we suppose that the evaporator/air handler in the attic is on a separate 20 amp circuit?

Almost certainly. And your thermostats are almost certainly powered by that 125V 20A circuit (there's a 24VAC transformer in the air handler).

Most of the time, the AC compressor (outside) will kick on before the fan in the air handler - if the breaker is tripping when the fan kicks on, it's probably the fan relay on the control board, or possibly the fan motor (less likely). It might even be the fan-start capacitor failed & the fan is trying to pull to much current at startup. If it's variable speed, ignore this paragraph.

There are usually error codes via LED flashing on the air handler, but attics in August suck & they might be erased when the breaker trips. Mine has a light switch & outlet attached to the air handler for easy resets & testing - but mines also in the basement.

I'd go with HVAC tech, the likely cause of breaker tripping is the HVAC unit & a competent HVAC tech can handle or at least diagnose the AC (current) side of HVAC wiring as well.

quote:

Originally posted by Aeteocles:

quote:

Originally posted by ArtieS:
The bridged breakers are for 220 power, and that is likely just for the compressors outside. The air handlers are probably 110 volt, and that is probably what is on that unlabeled breaker.

As a test, when the system is working properly, throw the unlabeled breaker and see if the upper air handler shuts down.

When that unlabeled breaker trips (i.e., same as me throwing the switch), compressors and air handler all shut off. Theoretically, the thermostat might be ordering the compressors off to protect the system if it loses power to the Air Handler.

Simpler than that - when the Tstat loses power, the AC compressor no longer sees a call to run, so it shuts off.

My house has a similar set up, two air handlers in the house, two units outside the house.

Each of the outside units has their own breaker, and each of the air handlers in the house have their own breaker.

Your unlabeled breaker is probably an air handler.

Does the breaker trip if you run one system at a time, run the downstairs, if it remains on then shut it off and run the other, see if it runs or trips the breaker.

Could be a breaker going bad or like others said one of the air handlers has a fault tripping the breaker when the load increases as both run..

I put my bet on the motor in the air handler. It's drawing too much power. I'd first check the run capacitor on the motor, then the motor itself.

I bet if you run it until the breaker trips, and then go check the air handler, you'll see it has no power, and the fan motor will be hot as hell. If the run capacitor is bad, then the motor is operating entirely on the primary windings, causing the motor to pull way more power and heat up.

It's just as snidera has already said. The DC transformer to drive the TStat is usually in the air handler or furnace. When that trips, the outside compressor no longer gets a run call, as if the TStat is satisfied. Compressors operate with voltage=On, no voltage=OFF signal.

The breaker does not trip if I run one system at a time.

Thanks for the help guys. Good to know/confirm that the air handler is on its own 120v circuit. I'll have the HVAC guy look at it while he does the regular annual maintenance.

Do you have furnaces or air handlers? If they are furnaces, those will run on 15 or 20a SP circuits. If they are A/Hs they will be typically a 30/60 combo, a 30/30 combo, 60a, or possibly a single larger than 60a 2P breaker.

The difference is if you have gas you are powering only blower, circuit board, and transformer, plus probably a condensate pump on gas. For electric only, then you have coil packs typically between 5kw-20kw.

Check your panel to see if there are any breakers labeled furnace, if not it could be your furnace or less likely a condensate pump connected to the furnace.

The odd part is "when running both systems". Both systems should be completely independent of each other.

If the air handler loses power, you lose 24VAC because that is typically where the control power transformer is located. That's the power that the thermostat uses to turn on the compressor contactors... So yeah, everything shuts off.

quote:

Originally posted by Skins2881:
Do you have furnaces or air handlers? If they are furnaces, those will run on 15 or 20a SP circuits. If they are A/Hs they will be typically a 30/60 combo, a 30/30 combo, 60a, or possibly a single larger than 60a 2P breaker.

The difference is if you have gas you are powering only blower, circuit board, and transformer, plus probably a condensate pump on gas. For electric only, then you have coil packs typically between 5kw-20kw.

Check your panel to see if there are any breakers labeled furnace, if not it could be your furnace or less likely a condensate pump connected to the furnace.

The odd part is "when running both systems". Both systems should be completely independent of each other.

I pulled out our inspection report from when we bought the house.

Furnace: Carrier Model 58PAV111-20 Mfg 1999. Vertical Forced Air - Natural Gas, Approx 110,000 BTU.

AC: Carrier 38CK060300 mfg 1999

Separate Furnace and AC for each of the upstairs and downstairs. (4 serial numbers listed on the inspection).

The oddity is also that both systems go down/turn off when you flip that single 20amp breaker.

Probably because you don't want the compressors running unless there is a way to move the heat that the air handler is trying to suck out of the house....

I know I wrote that strange but that is actually what is gong on... and if you don't have the air handler performing that function... blowing cold air.. then the compressors could just run till they either burn up or freeze up with ice.

You can't move cold... you can only move heat. Cold is just the lack of heat....

quote:

...run both the upstairs and downstairs AC units at the same time, it trips a random UNLABELED 20 amp breaker ... breaker either 25 or 30 amps, and those are marked AC. Those do not get tripped.

Reality (circuits tripping) trumps labels.

Learned a couple houses ago not to trust/rely on the labeling in the panel. Some discrepancies are worse than others. My learning event was getting a mild shock when touching both the kitchen faucet and the outside metal frame of the oven (rental house with a very small kitchen). Turned off the ganged breakers labeled 'oven' - and still got the shock! Turned out the owner of the house - not an electrician - did his own electrical work.....

If you're looking to get more accurate info about what each breaker controls, you can get a cheap setup like this to help identify (for regular 120V room outlets) which breaker goes with which outlets on your own.

For big stuff, worthy of its own breaker (HVAC component, electric dryer, water heater, etc.) you can trip the breaker and see what just turned off. Lighting is easy to tell, as well.

Back-filling circuit info is tedious, and when you're done (in my experience) the results probably won't fit neatly into the tiny writing areas in the panels. Means creating some other reference (like a spreadsheet) and printing something to go into the panel area.

Then, of course, you need to keep the database up, as you have electricians change/add stuff after you've done the research.

quote:

Originally posted by Aeteocles:

The oddity is also that both systems go down/turn off when you flip that single 20amp breaker.

They should not be served by the same breakers.

If they are both gas furnaces each should.have it's own 15 or 20a breaker, the only exception is for accessories like a humidifier or a pump. Are you 100% sure it's not when only one of them is on, but to you you think it's when both are on?

I'd suggest determining which breakers serve the furnaces. Turn one system on and one off. Flip breaker and listen to see if the unit that is on turns off, label breaker for upstairs or downstairs unit. Then turn opposite system on and find breaker the same way and label it.

After doing that and confirming they didn't violate the code and put two furnaces on the same breaker. Run systems one at a time and confirm it's only when two furnaces are running.

Something doesn't add up here.

quote:

Originally posted by Skins2881:

quote:

Originally posted by Aeteocles:

The oddity is also that both systems go down/turn off when you flip that single 20amp breaker.

They should not be served by the same breakers.

If they are both gas furnaces each should.have it's own 15 or 20a breaker, the only exception is for accessories like a humidifier or a pump. Are you 100% sure it's not when only one of them is on, but to you you think it's when both are on?

I'd suggest determining which breakers serve the furnaces. Turn one system on and one off. Flip breaker and listen to see if the unit that is on turns off, label breaker for upstairs or downstairs unit. Then turn opposite system on and find breaker the same way and label it.

After doing that and confirming they didn't violate the code and put two furnaces on the same breaker. Run systems one at a time and confirm it's only when two furnaces are running.

Something doesn't add up here.

Would it be possible that both Thermostats are running off a single transformer? The unit with the transformer trips the breaker, both thermostats lose power, so both AC turn off--giving the appearance of everything being on a single breaker?

quote:

Would it be possible that both Thermostats are running off a single transformer? The unit with the transformer trips the breaker, both thermostats lose power, so both AC turn off--giving the appearance of everything being on a single breaker?

Possible.

You might run a non contact IR thermometer over your breakers to determine if the one popping is running hotter than the others. Several years ago my outside unit kept popping the breaker late in the afternoon. I checked it with my IR thermometer and that breaker was running about 30 degrees hotter than the other breakers. I had a electrician/AC tech come out and after checking the outside unit and determining it was fine he checked the breaker. I showed him that it was hotter than neighboring breakers and after checking connections he said the breaker may have gotten weak. He replaced it and I've had no issues since.

As I posted above the answer to me is simple, but I'm pretty simple.... if the air handlers are on their own breaker and they stop working then you don't want the compressors running.... because they are trying to extract heat from the home and can't do it and will burn themselves out.... and so there is some kind of default system to keep the compressors from working when the air handler quits. Why the 20 amp breaker is tripping, I don't know... could be a bad breaker... short in the line.. bad fan... or the shunt capacitor on one (I actually like that suggestion from someone at first)

quote:

Originally posted by Aeteocles:

quote:

Originally posted by Skins2881:

quote:

Originally posted by Aeteocles:

The oddity is also that both systems go down/turn off when you flip that single 20amp breaker.

They should not be served by the same breakers.

If they are both gas furnaces each should.have it's own 15 or 20a breaker, the only exception is for accessories like a humidifier or a pump. Are you 100% sure it's not when only one of them is on, but to you you think it's when both are on?

I'd suggest determining which breakers serve the furnaces. Turn one system on and one off. Flip breaker and listen to see if the unit that is on turns off, label breaker for upstairs or downstairs unit. Then turn opposite system on and find breaker the same way and label it.

After doing that and confirming they didn't violate the code and put two furnaces on the same breaker. Run systems one at a time and confirm it's only when two furnaces are running.

Something doesn't add up here.

Would it be possible that both Thermostats are running off a single transformer? The unit with the transformer trips the breaker, both thermostats lose power, so both AC turn off--giving the appearance of everything being on a single breaker?

I'm not an HVAC tech, just dealt with a lot of electrical problems with them, so I can't say it's impossible, but I've never seen two furnaces share one transformer and can't imagine any benefit of using that configuration if it is ever done.

Did you confirm that there were two furnace breakers and labeled them? If you turn off/on that single unlabeled breaker both thermostats turn on/off?

If they do somehow really share a single breaker I'd like to know what happens if you switch off the local disconnect at each furnace one at a time and try to run one system at a time.

At this point I'm probably in this for my own curiosity, if I was there I could narrow it down in just a few minutes to HVAC or electrical and which system by process of elimination, but it would be hard for me to convey the process over a paragraph or two and without seeing everything.