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Front Panel red outline are the two and bottom 25 keeps tripping

The Google Nest was recently installed by the Pro technician that is sent out for installation. The first day everything was working normal, heating was on and warming our house to 70/71F. Around the third day the breaker kept tripping for the thermostat.

I have a Honeywell Furnace that asks for double pole breakers. L1/L2 is 60A and L3/L4 is 25A. Currently L2 is on a 60 and L3 is on a 25.

With the knowledge that I have now, I noticed the Nest is asking for 20-30V, could that be the reason the 25A breaker is tripping?

I can turn the breaker on and the thermostat will turn on, kick the furnace on and blow heat but after 5-10mins it will trip again and the furnace will start to die down.

I was able to find a unusual fix by just removing the nest base off the wall and that seemed to do the trick. I will plug it back in to set the temp and turn it on and then disconnect the nest from the wall and the breaker will not trip unless I leave it connected to the wall for more than 20mins. Any thoughts or help would be appreciated.

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    – Michael Karas ♦
    Commented Jan 7, 2021 at 21:08

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Based on your comment about the unit running fine after indoor temperature got closer to the setting. I would say that there is an issue with the auxiliary heat strips. They come on when the requested temperature you set is more than a couple degrees higher than the room temperature.

There is a high current draw to run these strips. They also come on when outdoor temperature is too low for heat pump to function properly.

Make sure the wiring feeding the heat strips is large enough to service the load they draw. Find a label that states the KW of the heat strips. Then find what size breaker is needed to provide that. It may be that the wire and breaker is undersized for the new heat strips. 10kw strips will often be fed from a 60 amp circuit.

Edit: looking at your photo I see a double on the right bank labeled heat pump. What does that feed? It looks like that is 30A for outdoor unit.

The 60a circuit should be powering the heat strips. It could be that your old thermostat was not wired to activate the strips. The nest is wired to turn them on but the 25 amp circuit is not able to handle heat strip load. Some pictures of the air handler and how it is wired would help rule in or out this hypothesis.

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    Agreed, I think the Nest has exposed a flaw with the existing heating system, or was mis-wired.
    – izzy
    Commented Jan 7, 2021 at 15:16
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    One interesting detail about the Nest thermostat specifically is that it probably handles calling for auxiliary heat a little smarter than most thermostats, which may be contributing to the problem here. The main time heat pumps need aux heat is when it's so cold outside there's not enough ambient heat to pump in, but most thermostats don't know the outside temperature, so they instead use the heuristic you describe. The Nest, being wifi connected, actually does know the outside temperature, and uses that to decide when to engage aux heat.
    – Nate S.
    Commented Jan 7, 2021 at 17:27
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I believe there could be several possibilities causing the trip.

As the transformer would probably long ago gone up in smoke if it was the cause I think you have a coincidence here if not a skinned wire. Touch the transformer and see if it is hot. It should be warm but not so hot you cannot keep your hand on it.

I would not want to mess with it two much as far as internet trouble shooting but there are 2 things I would be looking at.

a reversing valve on the outside unit. When these valves fail all kinds of things can go wrong keep the switch in heat until spring then mess with it we can do without ac but not heat. Leave the thermostat connected

The other possibility is the contactor to pull in aux heat was triggered this may have been sticky the first few times and the cause of the trip. I am not sure how the system continued heating with the thermostat removed. The fan unit should continue blowing until the heat exchanger has cooled or the aux / emergency heat coils have cooled so I would be looking at this part of the circuit if when you replace the thermostat and it happens again. Specifically the call for heat and the call for aux or emergency heat a common wire being swapped could cause this.

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It is possible to cause a 120v circuit to trip a breaker by shorting the secondary of a transformer that is being fed from it. A transformer is NOT a current limiting protection device. Continuing to do this will of course cause the transformer to overheat but a short on the transformer secondary will basically be seen as a short on the transformer primary. (technically a very high current draw on the transformer primary that is connected to the breaker, but it's the same thing) Transformers used for HVAC setups are often tremendously bulked up compared to the amount of power that is actually required to be used from them so it's not like a EL-Cheapo AC "wall wart" that will almost immediately self-destruct (thus protecting the breaker) if shorted.

You have to understand that on the low voltage secondary side of things the transformer 24v secondary is the "power source" while the relay coil inside the HVAC unit that is being fed by the switch is the "power load"

Since this was working with no tripping using the old thermostat, there is clearly NOT a problem with the relay coils inside of the furnace.

Since you have a C wire at the Nest, and a "transformer hot" at the Nest, connecting the two of them together will create a short across the transformer secondary.

The logical conclusion here is that either the Nest is broken, or the "Pro technician" didn't know what he was doing and connected something to it wrong. These techs are NOT licensed to touch 120v circuits and so (assuming the tech was any good) if the tech dove into this installation and discovered 120v powerlines anywhere in the vicinity of the thermostat he should have immediately stopped work. I think you are wasting time looking for a short on the 120v side of things.

Since the problem started 3 days ago the warranty on installation should apply and the tech should be called back. Unless you did that already and gave them a head-scratcher? Or you don't trust them any further than you can spit a rat?

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  • You must be used to seeing bigger furnace tranformers than I usually do. The ones I've typically seen are 40VA or less, and would absolutely burn up before drawing anywhere near enough current to trip a 25A breaker. And you can't rule out an internal problem with the furnace if the new thermostat was not wired identically to the old one, which we don't know for sure.
    – Nate S.
    Commented Jan 6, 2021 at 22:29
  • It's completely logical to assume shorting the secondary on a small transformer would cause it to burn up before it would trip a breaker. But if you apply that logic here then it would be virtually impossible for anything screwed up on the existing wiring of the transformer secondary to cause a breaker to trip - because the transformer would act as a protection device. But, if you throw away that single assumption and instead assume a shorted secondary COULD trip the breaker - then a whole lot of perfectly plausible error scenarios become reasonable.
    – Ted Mittelstaedt
    Commented Jan 6, 2021 at 23:10
  • Agreed; nothing on the secondary side should be able to directly trip the breaker. But that doesn't mean that stuff on the secondary side can't indirectly trip a breaker, by activating a relay that turns something on on the primary side, which I suspect is likely what's happening here.
    – Nate S.
    Commented Jan 6, 2021 at 23:14
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    It is absurd to suggest that an overload on the secondary of the 24V transformer could could trip the breaker repeatedly. Assuming the transformer is fed 120V (1 hot leg & neutral), it would be putting out 125A, or 100 times 20-30VA transformer is rated for. The secondary winding or or the thermostat wire would blow like a fuse before the breaker tripped.
    – nobody
    Commented Jan 7, 2021 at 1:33
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    The root problem is more likely something like electric backup heat being connected to the wrong breaker.
    – nobody
    Commented Jan 7, 2021 at 1:38

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