Nest Thermostat Heating Issue With Heat Pump and Aux Heat

[paul_cullum]

I hope that I'm not asking a previously answered question, but I did quickly search for an answer and did not find one.

How can I configure the Nest Thermostat so that my heat pump and aux heat (electric forced air furnance) can not run at the same time. I can play around with the settings to change the likelihood of them running at the same time but I can find a way to ensure that they don't.

One possibility is to set lockout temperatures such that they can't run in the same temperature ranges but that isn't really what I want.

Having both running causes my heat pump to trigger its high pressure overload switch. Then it requires some manual steps to reset my system before it can run again.  There must be an easy way to just configure it so that can't both run at the same time.

Ecobee has a setting called "aux heat simultaneous operation" which can be explicitly disabled.

[CoolingWizard]

Your system configuration does not seem to be correct.  Your air handler heating should be set up as Auxiliary Heat. Your primary heat is the heat pump. What wires do you have connected in the nest backplate? When your heat pumps goes into defrost mode, it will then activate the auxiliary heat. Depending on your furnace control board, it should receive a signal when in defrost and activate the auxiliary heat. 

please provide the make and model number of the air handler furnace, and outdoor Heat Pump.

Ken, The AC Cooling Wizard

[paul_cullum]

Maybe I'm not explaining the situation clearly. Most of the time, the behavior is entirely correct.

1.  My heat pump is primary.
2. My electric forced air furnace is auxiliary.

Most of the time, my heat pump operates on its own.  Usually, when the auxiliary heat starts, the heat pump stops. There are some corner cases when my auxiliary heat will turn on while my heat pump is still running as if it is assisting instead of replacing.

1. Sometimes, when pre-heating enabled and the scheduled time for the increased temperature  has been reached and the heat has not been reached yet, the furnace will start and "assist" while the heat pump is still running.
2. When the heat pump has been running a long time and the temperature has not yet been reached, the auxiliary heat will start and the heat pump will continue.

When the weather is colder, the heat pump will usually turn off when the auxiliary turns on. When it is really cold, it does eventually run into the same problem but I am able to avoid that just by setting a lockout temperature for the heat pump so that only the furnace is used in those cases. But during the shoulder seasons, there seems to be a greater chance (although still rare) that the heat pump will run at the same time once the auxiliary kicks in.

Changing the heat pump balance seems to changes the likelihood of both heat sources running at the same time but does not ensure that they can't.

If my heat pump coils were placed in the airflow BEFORE the furnace, I would think this would actually be desired behavior. However, with the coils placed AFTER the furnace, it makes it harder to transfer the heat from the coils while the furnace it running.

Some thermostats seem to have obvious settings to ensure that the heat pump can't run once the auxiliary heat is running. I am hoping that these settings exist for the Nest and that they merely happen to be less obvious.

[CoolingWizard]

Paul,

let me see if I can shed a little light on the concerns  you have.  The Heat Pump has a defrost control board(DFC) inside. On that DFC are connections for R, C, Y and W2.  The DFC monitors a temperature sensor mounted to the outdoor coil tube.  The temperature sensor closes at 32°F which signals the defrost board to initiate defrost cycle. In the defrost cycle it is switching the  reversing valve to cooling mode. In cooling mode the hot refrigerant gas goes outside and that melts the ice.  The DFC also outputs 24VAC on the W2 terminal back to the Furnace to force the auxiliary heat on. This is the period you note the auxiliary heat is on and the outdoor unit is still running.  Most DFC have a settable defrost cycle timer that is 30, 45, or 60 minutes.  When the temperature sensor opens again, the DFC will cancel the defrost cycle and return everything to heating mode again.   

Now, let us look at the thermostat.  The thermostats that know how to work with Heat Pumps understand that heat pump start to loose their heating capacity the colder it gets outside.  At -4°F most heat pumps start to really fall behind.  When the thermostat detects  that the indoor temperature is falling when the Heat Pump is running, the thermostat will activate the auxiliary heat to help. The thermostats that work with an outdoor ambient temperature sensors, can and will turn off the heat pump and only use the auxiliary heat.  Before that point, it will also try to use the auxiliary heat to help the heat pump to try and maintain the temperature set point. 

Now the furnace has a control board as well and if it is designed to work with heat pumps, it can help as well.  When the W2 signal is received, it will turn on the auxiliary heat, and slow the blower down from medium speed to low speed. The Furnace, with 3-speed fan motors, use High for AC Cooling and heat pump heating. And Medium for auxiliary heating when Y is not energized.  When Y is energized and W is energized, the blower will switch to low speed to assist with the defrost cycle.

With Gas Furnaces the gas heat exchanger is placed before the coil in the path of air travel. In cooling mode the air has high moisture level and this would cause the gas heat exchanger to rust and fail prematurely.  With Electric strips it does not make as much difference. 

Ken, The AC Cooling Wizard 

[paul_cullum]

Thank you for your response. That is a lot of detail...

There are times where it is obvious that the defrost cycle is running since it sounds different and I can see "steam" rising from the heat pump. And yes, the aux heat seems to run at those times.  I haven't the high-pressure shut-off problem happen at those times... I don't think.

The heat pump high pressure shut-off seems to occur while the auxiliary heat starts blowing hot air over the coils while it seems as though the heat pump is still trying to transfer heat. This can actually happen when it isn't actually that cold outside.

For example, if:

• Outdoor temperate is 45°F
• Indoor temperature is 60°F
• Nest starts pre-heating in anticipation of scheduled  70°F ( for example, 7AM)

What happens is the heat pump turns on to start pre-heating.  Shortly before 7AM, the auxiliary heating kicks in while the heat pump continues to run.  Chances are good that I will hear a loud noise as the high-pressure shut-off is triggered. Once this has happened, I have to go reset the heat pump by shutting power off at the breaker panel.

This seems less likely to happen when it is colder. When it is below 28°F the heat pump will usually stop when the auxiliary turns on. I've tried to lock out the heat pump when it is below 12°F so it will only ever use auxiliary below that.

FYI I have

• a Broan 21B20M B Series electric furnace which now acts as auxiliary
• a Lennox EL15XP1 heat pump

I don't think my furnace was originally designed to work with heat pumps. I believe that I either need to get a relay added to stop the heat pump from running or get a get/configure a thermostat that can do this. The thermostat that Lennox provides would handle this. The Ecobee has an explicit setting. I am hoping that the Nest thermostat has a similar setting. 

At the moment, I have to proactively change the Nest configuration at different points of the year to reduce the chance of this happening. Changing heat pump balance in the shoulder seasons or setting explicit lockout temperatures that avoid the situation altogether.

[CoolingWizard]

You’re a Bron 21B20M it was known as an electric 20KW forced air furnace. It does not have advanced control system for working with heat pumps. If your nest was set up that your electric heat is indeed auxiliary heat, there are limited things you can do but if it’s set up as a dual fuel system, you can change settings as to when the auxiliary heat comes on including never.  What the Nest is doing is trying to speed up the heating in anticipation of your set time on the schedule. My recommendation would be to not use auxiliary heat in conjunction with the heat pump and instead set it up as emergency heat for those times when unfortunately your heat pump might be not working during the winter.  In that case you would have a back up heating source being the furnace electric furnace

Ken, The AC Cooling Wizard  

[Jeran]

Thank you for helping out on this thread, CoolingWizard!

Hey there paul_cullum,

I'm just checking up on this thread, do you have any more questions or need any additional help? 

Best regards,
Jeran
 

[Jeran]

Hey there paul_cullum,

Do you have any more questions or need any additional help? 

Thank you,
Jeran
 

[paul_cullum]

Sorry @Jeran  and thank you @CoolingWizard .

I appreciate your replies. From the sound of it, I'm not sure what I want to do is possible with the Nest. Aside from setting compressor and auxiliary heat lockouts to prevent it, there does not appear to be a good way of preventing them from running at the same time. I was hoping that there might be some obscure setting that would allow me to still make use of the heat pump balance and make sure that the heat pump wouldn't continue to run when the auxiliary starts to run. Regardless of outdoor temperature.

I might give the Ecobee a try. While it offers settings for what I am trying to do I'll have to see what trade-offs there might be to get those.

Thanks again!

[CoolingWizard]

In the Nest Thermostat you can go into Pro Settings and set the conditions for the use of Heat Pump and or auxiliary heat.  You can set the temperature set point for when the Heat Pump is disabled and the alternate fuel heating system takes over.  

Ken, The AC Cooling Wizard

[WillR]

Cooling Wizard. We need your help. Having a similar issue to this:

- nest 3 rd generation

- heat pump with forced air

- secondary source for below freezing temps is electric furnace

system is set to switch to aux heat at 28 degrees. When it switches over (or when system is tested) only cool air is blown from the electric furnace.

wiring is correct according to Nest install with electric furnace connected through W2 wire. I would think heating element of furnace is broken and except when old thermostat is connected six heat works just fine.

any suggestions?

Thanks.

[CoolingWizard]

WillR, please open a new thread and post your question.

Ken, The AC Cooling Wizard

[Jeran]

Thank you for helping out on this thread again, CoolingWizard!

Hey there paul_cullum,

I'm following up again on this thread, since CoolingWizard provided another solution. Did you still have any more questions or need any help?

Best regards,
Jeran

[Jeran]

Hey paul_cullum,

Do you have any more questions that we can address? If not, I'll go ahead and lock up this thread in 24 hours.

Just checking up,
Jeran