01-12-2023 06:35 AM
It’s a pretty common setup in the cold climate around here to have a central heat pump feeding a hot air furnace with electric coils for Aux Heat - and for the heat pump to control the switch to Aux Heat below a certain temperature (typically-10C or so), rather than the thermostat. So the thermostat calls for primary heat, the heat pump tells the furnace sorry, can’t do, and the furnace turns on the Aux Heat. The thermostat has no clue this has happened.
The question is : how does a Nest thermostat deal with that, and more specifically with the house heating faster at lower temperatures than at higher ones? Does that mess up the Early-on estimates?
01-12-2023 12:10 PM
Usually if you have a gas furnace and a heat pump then it is called a dual fuel system and you define a lockout temperature that when outdoor temp hits that temp it shuts off the heat pump and switches over to the gas furnace only.
If you have electric heat coils they are called Aux heat and run at the same time as the heat pump to help it along in cold outdoor temps.
You set the system up differently for both setups so you have to know what kind of system you have.
Do you have a gas furnace and a heat pump or do you have a heat pump with electric coils aux heat?
Or do you have both in your system?
Read this info about heat pump balance settings for electric coils, but then note it says if you have a gas furnace you cannot set up balance you have to set up a lockout temp,
01-12-2023 12:24 PM
Do you have a heat pump with a gas furnace that kicks on when very cold?
Or do you have electric heat coils that assist the heat pump (aux heat)?
Or both in the same system?
The gas furnace and heat pump is called a dual fuel system and needs a breakpoint temp to switch over to the gas furnace.
Read this info
01-12-2023 12:39 PM
As indicated in my initial post, it's electric coils aux heat. Those can be controlled by the thermostat (through the white/W2/Aux wire), but the heat pump can also control it. The thermostat calls up Aux Heat during scheduled temp increases (as set by the balance setting), but the heat pump can also request that Aux Heat be activated. This setup allows for dumb thermostats that are unaware of outside air temp.
01-12-2023 01:47 PM
The Nest would not know if the heat pump turned on the coils it only controls the output of the W wire that kicks on the aux heat. Do you have a W1 and W2 wire?
Other than that you would need to read the installation setup manual for the heat pump and see what it says about using different thermostats. Some have proprietary thermostats that talk on a data bus to the heat pump, Usually they show wiring for communicating tstats and wiring for standard thermostats in the installation manual for the heat pump. There may even be little dip switches that have to be changed to use a smart tstat to let a Nest be in control of the aux heat.
01-12-2023 03:31 PM
Some heat pumps when paired with the proper matching indoor air handler with electric heat strips have a W2 wire from the defrost control board to the air handler control board W2. This W2, when energized by the outdoor unit will initiate the auxiliary heating and change the blower fan down low speed. The outdoor unit does this when it enters the defrost cycle. Typically, this feature is not wired as Auxiliary Heat, it typically is set up as Emergency Heat and can be initiated at the thermostat by the homeowner. In this approach if the heat pump outdoor unit should fail, the homeowner still has a to get some heat. Thermostats that were programmable in such a way that if the heat pump has been running, and the temperature continues to fall in the home, it can initiate the emergency heat as a booster. In those systems you typically have an L wire that when energized, illuminates a LED to indicate emergency heat is on. Those thermostats had a switch for emergency heat to manually turn it on.
The AC Cooling Wizard
01-12-2023 03:51 PM
Yes, my setup has this feature (heat pump activating aux heat during defrost cycles). The initial installer didn’t do this, and I can vouch for how essential it is - a defrost cycle at -10C is both frequent and very uncomfortable without aux heat. And since the aux heat coils are upstream of the heat pump exchanger in my installation, the defrost cycle is very efficient :).
This said - my setup works. The Nest calls up aux heat when it thinks it needs to (schedule temp increase in the morning or when returning home), but aux heat also gets turned on without the thermostat being aware of it (anytime we’re below -10C or so, which is frequent here; there can be weeks without the heat pump ever turning on).
01-12-2023 04:46 PM
Heat pump manufactures use a couple different models for defrost cycle checking. Basically if a heat pump has been running continuously for 30 minutes, and the outdoor temperature is 35° or less 35, the system will automatically go into defrost cycle, but when the temperature sensor on the outdoor coil reaches 75°, it terminates the defrost cycle. Thermostats on the other hand, if the system has been running for 30 minutes, and the temperature continues to drop, thermostats will automatically, in the background turn on the auxiliary heat if it’s been defined. For this to happen, the system has to be properly sized for your home and the proper airflow that is, all vents open filter is relatively new.
Ken, The AC Cooling Wizard
01-13-2023 05:42 PM
Checking in — we hope you've got the answer you're looking for. Let us know if you have more questions in mind.
I appreciate the help, Houptee and CoolingWizard.
01-18-2023 09:29 PM
I just wanted to follow up to see if you still need our help. Please let us know as we would be happy to answer any questions you may have.
01-25-2023 12:21 PM
Actually, nobody did reply to the actual question - which was related to how the Nest algos react to a heating system which activates aux heat by itself (and therefore largely changes the time-to-heat characteristics).
01-25-2023 02:43 PM
Your specific question was answered by both me and Houptee. We explain to you how a heat pump controls, the exhilarate heat, the emergency heat, and how the nest can control the second fuel source when using in combination with a heat pump.
The AC Cooling Wizard
01-25-2023 03:40 PM
This means I probably misstated my question. It wasn’t about how does the heat pump control aux heat - that’s pretty clear to me. It was about how do the Nest algorithms react to a system that ends up at a certain point heating faster at a lower outside air temperature than at a higher one.
But it’s really a moot point - whether the thermostat ends up starting heating a bit too soon or a bit too late does not change a lot in anyone’s life. 🙂
01-25-2023 03:58 PM
I'm not sure about the algorithm but I assume the nest learning model over time as you use it "learns" how long the system takes to reach setpoint.
But you raise a good question as to how does it take aux heat into account in the calculation.
Guess you would need to speak to the programming team.
01-31-2023 02:41 PM
02-03-2023 03:53 PM
It's me again. I wanted to check back in to see if you have other questions and concerns. Feel free to let me know if you do.
02-05-2023 10:51 AM
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