HVAC Technicians: Need your expert opinion

[John Young]

I’m looking for HVAC technicians to review this situation and weigh in with your professional opinions.

Last week I helped my brother-in-law install a new NEST thermostat at his lake house in North Carolina. The house has a 14-year-old heat pump for AC and heat. In the process of replacing the thermostat, I believe I shorted out the heat pump (compressor).

Before starting, I took a picture of the existing wiring to the old thermostat. I then shut off power to the air handler, but I DID NOT shut off the circuit breaker to the heat pump. (I continue to ask myself how I could have forgotten to do this ☹)

Regardless, I proceeded to swap the old thermostat for the new, matching the wiring.

With the new thermostat installed, I proceed to test. I turned the thermostat to COOL and verified that cool air coming out of the vents. I then tested HEAT. The heat came on, but it was running on AUX heat. My first thought was, “hmmm… maybe I mixed up wiring?” After verifying everything was correct, we decided to move on to other projects around the house and dig into the HVAC issue further the next day. I set the thermostat to cool to 75 and didn’t touch it again the rest of the day. It was a pretty hot day, but temps seemed to stay comfortable, and we didn't give it another thought.

The next morning, the temperature in the house was ~78*, so the AC was pretty obviously not working. I went down to the basement and looked at the electrical panel. I see that the heat pump circuit breaker is OFF, so I think “Hey, that’s likely the problem”. Flipped the breaker and it immediately trips off again. My next thought is “Fu*k”, I don’t remember shutting this off yesterday before replacing the thermostat”. If the heat pump is not working now, and if it was not working yesterday, that would explain the why it ran only "AUX heat" when I tested. That makes me think that the AC also wasn't really working at all yesterday, but rather was just running the fan, and just seemed "cool".

We called HVAC service, and they confirmed that the heat pump compressor was “shorted to ground” and needed to be replaced.

Here’s the question:
- Did I (as I believe) short out this heat pump while swapping thermostats with the heat pump power left on?
- Or was the 14-year-old compressor on its last legs anyway and the timing is purely coincidental?

My BIL says “we’ve been having issue with the compressor for the last couple of years, so it’s probably just coincidental”. I say, “My fault - and I owe you a new compressor”.

Which is it?

 

[pcmeiners]

A 14 year old system, with issues the last couple years? At it's age you could be paying off a loan for a new system with the inefficiency of the system. Next without being there with a
VOM, and without a crystal ball with me now, there could be a couple dozen reasons for it not working. Sounds like you still have a short
If you really want to reimburse your BIL, 3 dollars should do it.

"We called HVAC service, and they confirmed that the heat pump compressor was “shorted to ground” and needed to be replaced."

That is the easy answer to all hvac issues, it generally works to produce a profit for the hvac company...... the hvac company must has a crystal ball

If you have VOM, this can help check out the problem....

https://www.acservicetech.com/post/testing-if-an-hvacr-compressor-is-shorted-to-ground-open-or-the-thermal-overload-is-tripped

 

[John Young]

Thanks for your reply and I appreciate the testing tutorial!

Techs from two different companies came on-site and diagnosed the issue. Both said the compressor was shorted to ground and not repairable. The second tech showed me how he tested to verify this, and I took him at his word. I'd like to believe neither is dishonest, but I'm back in Michigan now, so can't do any further testing myself.

At this point, my BIL has already signed the contract for the replacement and will take advantage of the various rebates, tax credits, etc. offered for a higher efficiency system (they're upgrading everything).

I do get that there's no way to determine root cause at this point. More than anything, I guess I'm just looking for absolution or blame!

 

[Yankeefarmer]

I’m not an HVAC tech, but I did stay at a Holiday Inn Express a time or two. I don’t see how you could short out a heat pump compressor by changing out a thermostat. In my limited experience, the compressor only sees two low voltage control signals from the tstat: one telling the compressor to start/stop, and one to activate the reversing valve. Neither one of these should be able to create a compressor motor line voltage path to ground.

I also don’t think the aux heat indication you saw means anything. Likely you raised the set point before switching to heat mode, and the delta T was such that aux mode was activated.

I’ll be happy to be corrected when/if a real HVAC guy comes along. There’s plenty that lurk around here.

 

[brewchief]

All the low voltage power on a normal, relatively modern heat pump or furnace-A/C come from either the furnace or air handler, if it was a low voltage problem then I would say yes it's possible that swapping the stat had something to do with it, being on the high voltage side I can't come up with a scenario where you could have hurt the compressor.

 

[chinboys]

I worked extensively with NEST thermostats from their gen 1 series to the latest generation.
I used to have 9 zones of hydronic heating under NEST thermostats' control in my home.
I have also had NEST engineers come to my home to troubleshoot NEST thermostat control problems.
In short, NEST thermostats need to be installed with the RC or RH 24 V AC power turned off as installing it with the line voltage (110V AC) feeding a drop-down transformer may fry the NEST's backplane circuit board.

Most mini-splits, central AC, and Heat pump systems have a delay restart circuit that prevents the unit's compressor motor from restarting immediately (2-3 minute delay to normalize the refrigerant pressures between the high and low side).
I don't think your installation of the NEST thermostat with the system's main power on caused the system's current demise.

Depending on where you are located, heat pumps default to either cooling or heating modes provided you made the proper wire connections on the thermostat (the reversing valve will default to cooling or heating) while the compressor doesn't care as its function is to compress or bring the lower pressure refrigerant vapor to that of a higher pressure vapor during call for cooling or heating.

Most heat pumps or their compressors die as a result of overheating problems... the returning refrigerant vapor on the suction line cools the compressor before it is compressed to a higher-pressure vapor. The thermal overload protection circuit on the compressor could have gone bad thus allowing the heat buildup to eventually short the compressor's run windings.

Only an autopsy performed on the compressor will definitively show the problem.

 

[Dagny]

You are not guilty.

 

[fitter30]

Only one R wire at t stat transformer would be in the air handler. Assuming the t stat you programmed and made sure the reversing valve was in the right position. Heat only ran in aux electric heat, compressor never started. 5 minute compressor timer program on? Don't think you caused the compressor to go to ground.

 

[isb cornbinder]

I’m looking for HVAC technicians to review this situation and weigh in with your professional opinions.

Last week I helped my brother-in-law install a new NEST thermostat at his lake house in North Carolina. The house has a 14-year-old heat pump for AC and heat. In the process of replacing the thermostat, I believe I shorted out the heat pump (compressor).

Before starting, I took a picture of the existing wiring to the old thermostat. I then shut off power to the air handler, but I DID NOT shut off the circuit breaker to the heat pump. (I continue to ask myself how I could have forgotten to do this ☹)

Regardless, I proceeded to swap the old thermostat for the new, matching the wiring.

With the new thermostat installed, I proceed to test. I turned the thermostat to COOL and verified that cool air coming out of the vents. I then tested HEAT. The heat came on, but it was running on AUX heat. My first thought was, “hmmm… maybe I mixed up wiring?” After verifying everything was correct, we decided to move on to other projects around the house and dig into the HVAC issue further the next day. I set the thermostat to cool to 75 and didn’t touch it again the rest of the day. It was a pretty hot day, but temps seemed to stay comfortable, and we didn't give it another thought.

The next morning, the temperature in the house was ~78*, so the AC was pretty obviously not working. I went down to the basement and looked at the electrical panel. I see that the heat pump circuit breaker is OFF, so I think “Hey, that’s likely the problem”. Flipped the breaker and it immediately trips off again. My next thought is “Fu*k”, I don’t remember shutting this off yesterday before replacing the thermostat”. If the heat pump is not working now, and if it was not working yesterday, that would explain the why it ran only "AUX heat" when I tested. That makes me think that the AC also wasn't really working at all yesterday, but rather was just running the fan, and just seemed "cool".

We called HVAC service, and they confirmed that the heat pump compressor was “shorted to ground” and needed to be replaced.

Here’s the question:
- Did I (as I believe) short out this heat pump while swapping thermostats with the heat pump power left on?
- Or was the 14-year-old compressor on its last legs anyway and the timing is purely coincidental?

My BIL says “we’ve been having issue with the compressor for the last couple of years, so it’s probably just coincidental”. I say, “My fault - and I owe you a new compressor”.

Which is it?

I went to trade school for sheetmetal and HVAC in the early 1960s. I have installed hundreds of systems, I have never seen a system that connects to the high voltage to a thermostat. My first guess is the motor had reached the end of its service life.
The low voltage for a thermostat is 12-24 volts DC. Your HVAC compressor could run 220 volts AC, like the system in my home.

 

[P0234]

I can't see how even twisting all the wires together at the thermostat could have caused the compressor to fail. Maybe the HVAC control board , but not the actual compressor. The compressor motor has two wires that go to it, and neither is connected to the low voltage circuit that controls everything.

 

[OccupantRJ]

The only thing I have ever seen happen in that case of shorting a thermostat was to short out the control transformer. A control board could possibly die in that case also.

 

[teknikfrog]

- Did I (as I believe) short out this heat pump while swapping thermostats with the heat pump power left on?

Bullllllllllllllllllllllllllllllllllllllllllllllllllllllllshit. I really wish state AGs would start running stings on these scam outfits attempting FELONY FRAUD.

BTW you can ohm out a compressor with 5 minutes and a harbor freight multimeter, with the power off so this is as risk-free as it gets to double check. Or heck just (carefully) push in the contactor with power on and hear the compressor spin up.

 

[bonneyman]

A 14 year old system, with issues the last couple years? At it's age you could be paying off a loan for a new system with the inefficiency of the system. Next without being there with a
VOM, and without a crystal ball with me now, there could be a couple dozen reasons for it not working. Sounds like you still have a short
If you really want to reimburse your BIL, 3 dollars should do it.

"We called HVAC service, and they confirmed that the heat pump compressor was “shorted to ground” and needed to be replaced."

That is the easy answer to all hvac issues, it generally works to produce a profit for the hvac company...... the hvac company must has a crystal ball

If you have VOM, this can help check out the problem....

https://www.acservicetech.com/post/testing-if-an-hvacr-compressor-is-shorted-to-ground-open-or-the-thermal-overload-is-tripped

Good advice and great link, sir! I haven't seen an article on compressors that detailed in a long time.

 

[PoorUB]

I was in HVAC service for many years. There is no way messing with the low voltage shorted out the compressor.