How can I make my Nest thermostat activate emergency heat?
January 6, 2015 5:27 PM   Subscribe

Last summer, I replaced my thermostat with a Nest thermostat. It worked great until now that winter is here—when the temperature drops below freezing, my heat pump can't produce warm air. Tomorrow night it's forecast to be 8°F. Can you help me figure out what I've wired incorrectly?

I assume that the heat pump's "emergency heat" (or maybe "auxiliary heat"?) isn't being activated, because of a wiring problem. The Nest itself lights up red, indicating that it's aware that it should be using emergency heat. But the house just doesn't warm up until it gets above freezing, indicating that the emergency heat is never coming on. This is not the case upstairs in my house, where I'm still using one of the old thermostats. That gets heated just fine. So this is definitely a thermostat problem.

Here's an image gallery showing the wiring before and the wiring now.

My house is heated with a heat pump (not a furnace). It's entirely electric (no oil or gas), and there's no whole-house humidifier or dehumidifier. The Nest is a second-generation model.

I've tried fiddling with all kinds of wires, googled all around, read Nest's installation guides, etc. Clearly I have no idea of what I'm doing. What wire(s) do I have in the wrong place?
posted by waldo to Home & Garden (9 answers total) 2 users marked this as a favorite
Best answer: Did you try calling their support? I think they're open 24/7. 855-469-6378
posted by three_red_balloons at 5:38 PM on January 6, 2015 [2 favorites]

I can't answer your Nest specific questions, but I also have a heat pump (ground source) with emergency heaters in the ducts, though I've always kept them turned off at the breaker because I've never needed them.

But just to some basic troubleshooting thoughts:

Are you sure by disconnecting wiring you didn't trip the breaker for the emergency heater? It might have its own breaker and you might have a separate heater in the upstairs, which or course didn't get tripped.

Do you have the old thermostat that you could swap back in for the time being, or just to verify that it's an issue with the Nest? Alternately, could you swap the Nest with the upstairs thermostat, just to make sure the problem follows the Nest (and goes away when the older one is swapped in downstairs?) If you do that, first draw yourself a diagram of the original wiring.

Worst case maybe you could short out the Emergency Heat wire for a little while in order to activate it enough to get the emergency heat on. I am not an electrician, an HVAC tech, and I can think of a million reasons why that might be a bad idea but I'd probably do it in a pinch if I needed heat.
posted by bondcliff at 5:38 PM on January 6, 2015 [1 favorite]

Best answer: I've got a single stage heat pump with auxiliary heat strips and a second gen Nest (all electric house hold here as well). I'm guessing you have a two stage unit from the Y2 wire, so my advice might not apply but I'll put it out there anyways. I've got my aux heat in W1, and no wire in the * slot. Maybe try moving the wire in W2 into W1 and the wire in * to W2? In my old thermostat my E wire was shorted to my Aux wire, which doesn't appear so from your picture, but maybe they both go to the same terminal on your heat pump/air handler and you wouldn't actually need both with the Nest thermostat. So you might just try unplugging the * wire as well and just have one wire in W1. Good luck.
posted by noneuclidean at 6:45 PM on January 6, 2015

Nthing contacting Nest support. They were really helpful with my setup.
posted by cgg at 6:50 PM on January 6, 2015

Best answer: Do you have heat pump balance set up on your Nest? If not, are you using a lockout temp that's set really low so that the auxiliary heat never turns on?

I have a heat pump w/aux. heat too and use a Nest with it. It turns red when it's using just the heat pump to heat the house, and then red with the words "Aux. Heat" display when it's using auxiliary heat.

My wiring's different and I don't have a separate wire for emergency heat, but my understanding is that emergency heat is something that'd you turn on manually. Auxiliary heat uses the same electric heating elements as emergency heat, but your thermostat should kick it in automatically when the heat pump alone isn't enough to get your house up to temp.
posted by radiomayonnaise at 7:32 PM on January 6, 2015

Best answer: Definitely call Nest if you haven't. Nothing looks wrong with the wiring, but the Nest's identification of the white AUX wire as "alt heat" and not "aux heat" might be your signal something's wrong with the settings ("alt heat" usually means a second fuel source, like a backup gas burner or something). Also you're wired for two stages, but does the Nest ever actually display HEAT STAGE 2 or does it only ever say "HEAT?"

Also you can try manually enabling emergency heat.
posted by fedward at 8:30 PM on January 6, 2015

Response by poster: Yay, it's fixed!

For starters, I'll say that it simply never occurred to me to call Nest. I assumed that their support would be awful and that they'd either waste my time or they'd tell me to go hire an electrician. But, having heard good things here, I was going to call Nest today if I couldn't get it fixed. But then—

noneuclidean , radiomayonnaise, and fedward's comments led me to the solution. I simply had no idea that "aux" and "alt" heat were different, and I figured that "emergency" heat was probably another term for the same thing. Different terminology for different manufacturers, right? Nope. I had no "heat pump balance" option on my Nest, which is how I knew that my Nest probably wasn't configured correctly (with confidence that the wiring was OK, thanks to fedward). Retracing the configuration steps, I realized that I'd set up the * connection for "emergency heat" instead of "stage 3" heating. This morning, the thermostat still unable to heat first floor above 62°, I changed that configuration. The heat pump soon started to draw 12 kW, and the house was toasty within half an hour.

Of course, if I'd called Nest, they probably would have told me the same thing. :) But by stepping through this debugging process, I've actually managed to learn a few things about how HVACs work. Thanks, folks!
posted by waldo at 7:22 AM on January 7, 2015 [1 favorite]

The terminology is sort of confusing and I think you still want the * to be Emergency Heat, not stage 3. I suspect your problem is still in the identification of the white AUX wire. Assuming the Nest knows it's connected to a heat pump (which it can detect by the presence of the O/B wire) it should still know the difference between the different sources. Here's a summary:
  • Stage N heat: one or more compressor stages, all the same technology (that is, a heat pump compressor), which are designed to kick in as necessary to allow the system to operate efficiently and be comfortable*.
  • Alt heat: heat from a different power source than the heat pump. Sometimes it's a gas burner in the air handler, sometimes it's … something else (you should know if you actually have a second heat source, and if you do, you probably wanted a Pro installation to make sure the Nest was configured properly).
  • Aux heat: resistance heating coils located in the condenser unit outside the house (the thing that looks like an air conditioner). Its primary purpose is to keep the condenser from freezing solid in cold weather. Aux heat should be controlled by a simple control panel inside the condenser unit connected to a thermocouple, and should kick on automatically based on the outside air temperature and the temperature of the condensing unit. It can (optionally) be controlled by your thermostat inside, via the AUX wire, but that control shouldn't strictly be necessary. The Nest has the advantage of knowing the weather forecast and not just the temperature, so it can make the system run a little bit more efficiently by kicking auxiliary heat on itself.
  • Emergency heat: in a heat pump, emergency heat is generally nothing more than a manual override switch that turns the auxiliary heat coils on for as long as you have the switch set. The idea is that when the efficient system fails (coolant leak, compressor failure, etc), you can still flip the emergency heat switch and not freeze, just with a really high electric bill at the end of the month.
* While heat pumps are efficient they generally have to run a really long time to heat or cool a large space at maximum efficiency, and when heating they don't put out hot air (like a furnace), they just put out warm air (like, 80 degrees or something). A heat pump that is too big won't run for enough time before cycling off, which both increases wear on the system and fails to reduce humidity enough in the summer. A heat pump that is too small won't ever get your house cool in the summer and/or will use too much aux heat in the winter, wiping out any cost savings. There's a real goldilocks sort of sweet spot, and a multi-stage heat pump basically allows you to have a system that operates at highest efficiency within the sweet spot most of the year, and still operates mostly efficiently when more oomph is needed.

So on the one hand it shouldn't matter that your Nest thinks the AUX wire is connected to Alt Heat, not Aux Heat, since the heat pump should be turning auxiliary heat on by itself anyway. But on the other hand, the Nest has a few settings which should make its use of auxiliary heat more optimal (both for your comfort and for maximum efficiency) and if it's not able to raise the temperature enough, something's not right. You should make sure the W2/Aux wire is identified as Aux Heat, not Alt Heat, and see if the system works right that way. Note that when you get this set, you should then have the Heat Pump Balance option available (the fact you don't have it now is also an indication of a configuration issue).

If you get this configuration straightened out and continue to have problems getting your house warm, contact an HVAC tech since you may have had a system failure.

I hope that's all clear. I configured our Nest for a heat pump in July 2013 (when we bought our house) but then when that heat pump failed a few months later (unrelated to the Nest) we had a gas furnace installed instead, so I can't just go check our settings.
posted by fedward at 9:20 AM on January 7, 2015 [1 favorite]

Response by poster: Thanks for that great explanation, fedward! The Heat Pump Balance option hasn't shown up yet, but since Nest says it can take a few days, I've been checking back a couple of times each day. This will be looked at professionally soon—I'm having solar panels installed at the house in a couple of weeks, with some of the crew being home-heating technicians , so I'm just holding out until they show up. :)

FWIW, it was 9°F at 8 AM this morning, but 67° in the house. So while you're likely right that everything might not be operating quite right, I'm glad that we're not freezing.
posted by waldo at 8:13 AM on January 8, 2015

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