Aux Heat with Nest and Two Zones

[DT_NestReset]

Hi,

I recently set up two mirror faced Nests to control two zones on a single heat pump (Carrier 4 ton single stage). The downstairs zone is much larger than the upstairs zone, maybe by a factor of 5ish. The wiring on both Nests is identical, including an O/B wire for the Aux heat. Both Nests were originally set up to use Heat Pump Balance with Max Savings. The beginning set points for daytime heat were 73 downstairs and 71 upstairs. I'm in NC, we have a few nights each year with outdoor temps below freezing ('never' below 20), several below 40, and above that the rest of the time.

Experimenting with Aux heat found unexpected (but maybe 'smart' normal?) behavior. The downstairs (larger) zone would call for Aux if the set point was more than 2 degrees above current indoor temperature, just like the un-smart Honeywell it replaced. But the upstairs (smaller) zone did not call for Aux heat, even when the set point was as much as 13 degrees above current indoor temperature.

I was hoping to use schedules and setbacks to reduce energy usage, and I really, really do not want to use Aux heat. Before Nest I was able to avoid using Aux by not using schedules and setbacks even though they were available on the un-smart Honeywells. The heat pump alone could keep up with steady, constant, round the clock equal set points even when below freezing outside.

I suppose I could use a big setback upstairs, and just 1 - 2 degree setback downstairs, but this still leaves the option for Nest to turn on Aux in some unknow circumstance where it 'thinks' that's the right thing to do. Don't want that. And I don't think a large setback in the very small zone will result in much savings, which could easily be wiped out (and reversed) if Aux ever came on downstairs. 

Here's what I've set up to hopefully get what I want:

  - Turn off Heat Pump Balance and use an Aux outdoor temperature lockout in the high 30's on both Nests.

  - Set up setbacks and schedules so setpoints drop overnight, downstairs by 5 degrees and upstairs by 10 degrees. From a comfort point of view I'm Ok if the indoor temperature has not yet reached set point when I get up in the morning (Early On helps that anyway).

  - If the weather forecast shows overnight lows dropping into the low 40's then set a Hold temperature to keep both zones at their daytime settings overnight.

My questions:

  - Just curious - why such different behavior when Nest is controlling Aux downstairs (turned on with more than 2 degree difference) vs. upstairs (not turned on with as much as 13 degree difference)?

  - Are there any potential problems or holes in the workaround suggested above?

Thanks,

Darryl

[CoolingWizard]

Your symptoms you are seeing is not the likely thermostat but is more likely the zone controller causing this.  Some less sophisticated zone controllers require a “primary” thermostat that controls the system. This is the larger downstairs zone in your case. Can you locate the zone controller and send a picture of the make and model number?

Ken, The AC Cooling Wizard

[DT_NestReset]

The zone controller is a honeywell HZ432. The downstairs thermostat is in zone one. Not sure how to attach a picture….

Thanks!

[DT_NestReset]

This makes sense... When doing the change to the nests I took off both old thermostats and put the 1st nest on the 2nd zone upstairs. It was controlling Aux using the 2ﾟ limit at that time, I'm guessing because it was the only thermostat on the controller.

[CoolingWizard]

The Honeywell HZ432 TrueZONE is very intelligent zone controller. If your system has a variable speed blower in the air handler, this HZ432 can allow any thermostat to control the HVAC equipment and thus the Auxiliary and Emergency Heat can be  so configured.  So, either Nest Thermostat can initiate the second stage of heating. If there is both a W1/E wire and an AUX wire, the E can be manually set by either thermostat when configured for heat pump and an E wire is defined.  

If your heat pump is a 2-stage heat pump you will have Y1 and Y2 wires defined. Since it is a heat pump, Stage 1 heat is Heat Pump (Y1), and Stage 2 heat is heat pump (Y2). You might have a more capability auxiliary and emergency heating as well.  These system have 2 electric heat strips for Auxiliary Heat and 2 additional heat strips for Emergency Heat. Emergency Heat is wired with a relay and a sequencer to activate the auxiliary heating coils as well.  

Personally, I would have installed your heat pump system was no auxiliary heat at the thermostat, only Emergency Heat. The outdoor units defrost control board would be wired to operate the auxiliary heat for defrost cycle. If you live in the northern states where the winters are brutal, then the Auxiliary and Emergency Heat approach would be required.  There is no way to save energy with a heat pump and electric heat strips unfortunately.  The best you can hope for to save energy is not have auxiliary heat configured in the thermostat and let the Heat Pump do all the work.  

Ken, The AC Cooling Wizard

[DT_NestReset]

Thank you CoolingWizard, for your thorough and detailed information. I'm in NC, not really bad winters here. I don't think I have a variable speed blower and I'm sure the heat pump is single stage, and the extra needed wires are not at the thermostats. Looks like I've done as much as I can to minimize Aux heat at the thermostats (manual, low aux lockout temperature) and save a little energy (schedules with setbacks down to mid 30's, then just hold at Comfort). I can manually turn on Emergency heat at the Nests if needed, and the heat pump will use Aux for the defrost as needed.

Thanks again, very helpful!

[DT_NestReset]

One last question - I haven't found a way, maybe someone knows how, for Nest to send a notification if Auxiliary heat gets turned on?

Thanks for this great forum!

[CoolingWizard]

Smart thermostats as a general rule do not notify you when normal system operations are underway. When the exhilarate heat is turned on, it’s typically turned on through a heat pump defrost control board on the outside unit. Smart thermostat can be programmed to apply auxiliary heat when the temperature is outside or so cold that the heat pump is struggling to maintain temperature. this only occurs in extremely cold areas of the country so for the most part, you don’t typically use axillary heat manually. What you put in is a emergency heat service so that the homeowner can activate the auxiliary heat in a “emergency” time such as a failed outdoor unit.

when we install heat pump systems, we always wire in an emergency heat as well as auxiliary control.

Ken, The AC Cooling Wizard

[DT_NestReset]

Interesting thing happened this morning - it was cold, Google said the outdoor temp was 31F. I was hanging out with the air handler for a few minutes, the heat was on and the Discharge Air Temperature Sensor (DATS) was showing 94F. Then I heard what sounded like a high pressure wheeze, and the DATS started dropping. It went down to 44F, and was there for a short time. Then there was another high pressure wheeze and the DATS started climbing, went back up to 92F.  During the time DATS was low the system (heat pump and air handler fan) stayed on, and coolish air was coming out of the room vents (no surprise). The zone controller did not show auxiliary heat turning on during this time, I think if it had DATS wouldn't drop so far. I'm guessing this was a heat pump defrost cycle done by just reversing the heat pump, no auxiliary needed? The Nest auxiliary lockout was set to 30F.

[Jake]

Hey there,

Thanks for the help CoolingWizard.

I wanted to check in and see if you managed to see CoolingWizard's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.

Best regards,
Jake

[DT_NestReset]

All set, very helpful!

[Markjosephp]

Hello DT_NestReset,

I'm glad to hear you've got the answer you're looking for. If you're still in need of assistance, feel free to let us know. Otherwise, this thread will be locked within 24 hours.

Thanks,

Mark

[CoolingWizard]

Some manufacturers put an algorithm into their defrost control system that when it goes into defrost system, that it doesn’t necessarily turn on auxiliary heat when the timer is on the first step, which means it will only be in a defrost cycle for a very short period of time. On the other hand, if the defrost cycle timer expires, then it goes to a longer cycle and it therefore turn on the auxiliary heat. Some manufactures will actually send a signal to the intelligent type air handler‘s to go into low speed. It all depends on the manufacture, and how they want to approach. But you are correct you heard the reversing valve switch and the change the flow in the air handler. 

The AC Cooling Wizard