I have dual transformers, separate heating and AC.

I've recently installed a nest learning thermostat (3rd gen). This is my wiring. wiring The c wire comes from the the transformer on the AC side (pairs with Rc). The system works and is able to call for both heat and cooling.

However the problem I noticed is that when AC is off, the outside condenser unit has a chattering noise. I removed the side panel on the unit and found that it's the contactor trying to actuate.

I actually have another nest thermostat E but it also causes the same chattering.

I tried my old thermostat - no chattering.

It's like the nest is leaking a small voltage to the Y1 wire even if it's not calling for cooling.

After a long chat with the nest support, they asked me to remove the c wire and it did stop the chattering and they are asking me to go without the c wire. However based on what I've researched, not having a c wire could cause a lot of potential issues down the road.

I've read a reddit post that suggests adding an isolation relay (though the post added the relay on the heating side). I have not purchased a relay yet but did sketch the wiring that I think should work relay wiring.

I'm wondering if the relay is going to work or there is other solutions. If the underlying issue is indeed leaked voltage, then will having a relay simply hide the problem, e.g. the chattering may occur at the relay?

Any suggestion is greatly appreciated! Thank you!

  • Very nice for a first question! (Extra bonus for the drawing on whatever scrap of paper was handy.:) While you're waiting for an answer, take the tour and browse the help center so you can learn how to get the best out of the site.
    – FreeMan
    Jun 4, 2020 at 16:58

3 Answers 3


This is a hack, but....

Try replacing the relay with a much heavier duty one. Specifically one with a coil that uses more power. 24v is a very common contactor voltage.

Here's one option from Grainger, but really any 24Vac coil voltage relay would work. The contact rating must be equal or greater to what you currently have. Aiming for very high voltage/current contact ratings, or relays with more than 2 poles will likely get you a higher-wattage coil, and that's what you want to reduce chatter.

The heavier duty coil will pass more current through without moving. In fact you don't need to remove your current relay. maybe just connect another in parallel with it so its powered on at the same time. Leave a note for why you did that, or the next hvac tech you have out might yank it and tell you how crazy the last guy must have been to put that there.

Or for that matter, you might be able to put any sort of load in parallel with it. A suitably rated resistor for example.

I've used a nest for about 6 years myself in two different houses and what's happening to you is not normal. It sounds to me like the 'support' you were talking to were grasping at straws. I will agree with the other posters that Nest is junk, and you'd be better served returning it to the store. Google will be discontinuing them soon, and you will lose control of your thermostat overnight against your will when they shut down the cloud component of it.

But since it's already installed, if you like it, there's how to make it not clatter.

There is also a small, but non-zero chance that the base plate of your nest is defective, or one of the connector pins on it is busted, and the c wire energy isn't making it to the nest itself.

Check the nest app or the thermostat's advanced/installer interface itself to see if it is AWARE of the c-wire's presence. It should show you a photo of the wiring it detects.

If the nest thermostat itself (the piece that comes off the wall) does not detect the c-wire power, then it probably will "fall back" to stealing power from the other wires.

If both of your Nests have baseplates and you experience the same problem with both baseplates, then this is probably not the cause. My first thought when I started reading your post was the c-wire was broken. But you ruled that out with your voltage measurement. Good on ya!

  • The thermostat is detecting the c wire and the voc/vin/lin readings are normal for having a c-wire according to nest support. The voltage at the thermostat is all normal: 28v from Rc to c and y and g. But at the compressor, the voltage between the common and yellow fluctuates rapidly between 0 and about 2v (> 2v for a few moments) when the thermostat is on the backplate, and chattering occurs. If the thermostat is removed or with the old honeywell, 0 voltage. I think the thermostat is faulty and tries to steal power even with c wire.
    – David Li
    Jun 6, 2020 at 15:05
  • I'm going back to my honeywell for now. The resistor idea may just hide the nest's problem of leaking voltage even if it can stop the chattering, still thank you very much!
    – David Li
    Jun 6, 2020 at 15:05
  • Something else I just thought of. There do exist 'time delay relays'. With them the coil has to be energized for a number of seconds continuously before the contacts close. I have a hunch that modern fancy ones just use silicon, so they wouldn't work at all because they wouldn't bridge much of a load. But if you found an electromechanical one it might do the job. Then again it's really an ugly bug in the nest thermostat. Speaking of bugs, have you made sure it's on the latest firmware? (I do realize how insane it is to think of firmware versions on a thermostat....) Jun 6, 2020 at 19:56

The problems with these "smart" tstats is they try to cheat by "stealing power" from the actual control circuits rather than a dedicated power source. This is done to charge the internal batteries when the HVAC system ISN'T calling for heating or cooling. There is a small power drain across the control circuits to do this. Unfortunately, some of the controls are sensitive enough to try and actuate and then you get issues like the one you are having. The fix is to provide real power from the HVAC equipment (probably air handler or furnace) via the R and C wires. But lots of times the installed wiring doesn't have enough conductors to accomplish this, esp. in older installations and replacing that wiring is usually difficult. So the "smart" tstat manufacture's in order to sell more of their products have designed them to steal a tiny bit of power off the actual control circuits which doesn't play well with all HVAC equipment. Google "power stealing thermostats" and you'll see a myriad of complaints and problems. Again, the correct fix is to get real power to the new Tstat.

  • 1
    True it was not a problem with incandescent lights so manufacturers started doing this on lighting circuits but LED’s don’t tolerate this either but they want to sell the fancy WiFi connected stats buyer beware on multiple fronts.+
    – Ed Beal
    Jun 4, 2020 at 20:05
  • 1
    Thank you for the response! I do understand the 'power stealing' problem. I have a common wire, which is why I thought nest would be OK and bought one. My home is a new construction and I can trace the common wire to the transformer in my air handler. I also measured the voltage between the Y and C at the thermostat, which is around 27V. The nest support said it might be the C wire providing too much power and it's feeding back to the system. Another suggestion I got is add some resistor.
    – David Li
    Jun 4, 2020 at 20:12
  • 1
    The thing is, the Nest shouldn't be power stealing, given that it has a C-wire. I don't think the Nest support people are anywhere close to explaining what's going on here... Jun 4, 2020 at 23:38
  • Is it just me but it seems like there are a LOT of questions/issues/problems with these new "smart" thermostats? Who knows even if the nest tstat is using real power if available. Buggy Chinese products. Jun 4, 2020 at 23:43

Anytime the old thermostat works just fine and the Nest does not, it only means one of two things (assuming you have a good C wire):

  1. You connected the Nest incorrectly (wiring) or setup the Nest incorrectly (setup questions); or
  2. The Nest is defective.

Nest is not nearly as reliable as non-smart thermostats and can fail requiring replacement (happened to me twice). Switching to an Ecobee or Honeywell.

Note: Thermostat Wire Test with multimeter set to A/C:

R and C Using Multimeter = 24 to 28 volts

Warning: Do NOT touch R and C wires together or you'll cause a short and blow the 3 Amp fuse in the air handler or worse.

R + Y = 24 to 28 volts (AC compressor On)

R + G = 24 to 28 volts (Air Handler Fan On)

R + W = 24 to 28 volts (Heat On)

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