Nest Learning Thermostat heat pump & AUX (oil) heat features

[sybernut]

Hello. I just installed an Nest Learning Thermostat (NLT) (3rd get presumably, although there doesn't seem to be a way to verify this on the device itself). I have a heat pump with AUX oil heat. I was unhappy with my previous thermostat (White-Rogers (WR) 1F82-261) because AUX was kicking in early at all temperatures (and the model didn't support the 'Select Fast 2nd Stage' (SF2) feature in the manual), resulting in a waste of oil. I was hoping that NLT would 'solve' this problem and indeed, so far with the setpoint vs. room-temp deltas I have tried, oil is not kicking in... but I would like it to if this delta is above a certain value (as per the missing WR SF2 feature), below a certain ambient (external temperature) threshold. Also, below a certain ambient, the AUX heat should kick in immediately (my heat pump can't really heat well below about -5C (23F), it just keeps running (NLT seems to support this threshold). 

I'm hoping that this would all be possible with a supposedly 'smart' NLT, perhaps it is but I haven't been able to confirm it at this early stage.  It seems that NLT has detected a dual fuel system and there is no Heat Pump Balance (HPB) feature, only a manual 'Dual fuel breakpoint' (presumably, this is what 'lockout' means) below which only oil will heat. Why is HPB not supported? My electricity provider has its own lockout (via a separate relay) at -12C (10F) ambient but as I said, the heat pump won't really work below -5C (23F) or so (depending on humidity, which the NLT likely ignores), so that's what lockout should be set to... however, AUX oil should be able to help the heat pump reach the setpoint at cold temperatures above this.

I would like to know if the adjustable SF2 feature is somehow supported by the NLT: I see no option in Home or Nest to set up an SF2 delta, perhaps it's hard-wired? Would HPB support it, in which case I may have to alter my wiring? Below is my wiring diagram (with comments). Thanks in advance for any and all wisdom.

Y1 - heat/cool,

G - fan

OB - heat pump (O, seasonal)

Rh - power,

* - alt. stage 2 - how can I specify this as EMERG and how can I force emergency oil?

C - 24V common

W2 AUX - alt. heat - this 2nd stage should be kicking in at some point?

[CoolingWizard]

@sybernut , when you set up your system as the oil burner set up as Auxiliary heat, it is a heat pump assistant. That is when the thermostat detects that heat pump is not keeping up with the heating demand, it activates the auxiliary heat. The heat pump also activates the auxiliary heat anytime the heat pump enters the defrost cycle.  

Your other choice to set the system up as a duel fuel system.  This will then require you to setup the temperature point at which the oil burner takes over the heating role and the heat pump is turned off.  

AC Cooling Wizard

[sybernut]

Thank you for your prompt reply. Is it possible that the thermostat decides between Aux Heat and Dual Fuel based on my wiring or an option that I've selected? It's set to Dual Fuel, but I would like it to be set to Aux Heat so that it implements the feature that you described.  My wiring is as per my original post.

One thing: on my old thermostat, oil was also connected to the Emergency setting. On the Nest, if Emergency is chosen for the * (star) connection, it defaults to Electric but I have oil: for this reason, I chose 'Alt heat stage 2' for the * connection, since this offers the Oil option. Is it OK to select Emergency for oil in spite of it being 'Electric'? I want the same profile as on my old thermostat. Perhaps this might allow the Aux Heat option to be used? I would rather understand how all this works before playing around (and retesting). Thanks.

[CoolingWizard]

@sybernut , the Nest is pretty flexible for duel fuel systems, but they are not compatible to be set up as Emergency or Auxiliary Heat since with heat pumps the emergency heat is always set as Electric.  Now with a Duel Fuel set up, you can set the temperature at which the system changes from Heat Pump to Oil Burner and Radiators.  You can choose a value form 50°F down to 10°F.  You can set it to say 25°F.  There are even two options of Never use and Always use. 

With a duel fuel setup, the second fuel source is normally wired to W1 and would be the second stage Heat and would be defined as Oil, Radiators. Now, if go ahead and add a wire between the Oil Burner and W2 it cannot be defined as AUXiliary heat for the heat pump.  This the concept of duel fuel. 

For you the most efficient approach is Duel Fuel with Heat Pump and Oil Burner.  Set the change over to 25°F. If you add an additional wire between the *(E) and the oil burner, the system will define the Emergency Heat as Electric.  That will not matter since you will just manually activate emergency heat as if the heat pump has failed and you need heat.  However, you have an additional choice, set your change over value to “Always use Alternate Heat”. Then the NLT will never use the heat pump for heat.  So if you make this change when the heat pump fails, it is sort like the emergency heat function. 

So, you need to go to SETTINGS > EQUIPMENT > CONTINUE > CONTINUE > PRO SETUP > CONTINUE > DUEL FUEL > you will now see a picture of the nest backplate showing wires that are attached. The wedges are green if nest is ok with the assignment. They are yellow when the nest needs your help to define what the wire controls.  Your W1 should be Alternative Heat, oil, radiators.  You might need to set those values.  If you decided to wire the oil burner to *(E), it will be defined as Electric, Forced Air and you cannot change anything but the function.  Just it to E. emergency heat.

AC Cooling Wizard

[sybernut]

Hello and thank you for your reply.

My primary reason for switching to the NLT was that on my old thermostat, the oil heat would kick in whenever the difference between set and room temperatures was greater than 2C (3F), regardless of outdoor temperature; this resulted in a waste of oil.

My heating arrangement is such that the set temperature is set very low overnight so that the heat pump doesn't activate while I'm sleeping (due to the noise). My heat pump can maintain a set temperature even at relatively low outdoor temperatures, but it needs an initial boost from oil, typically on Winter mornings (I live in Eastern Canada) after the house has cooled. Let's say that the heat pump can maintain heat above -10C (14F) but may need a boost let's say below 0C (32F) in order to heat the house reasonably quickly e.g. in the mornings. So if I set the 'the temperature at which the system changes from Heat Pump to Oil Burner' to -10C (14 F), it will take ages to heat the house in the morning. If I set it to 0C (32F), I will be burning only oil for most of the Winter.

For this reason, I want to set up the system with the oil burner set up as Auxiliary heat, as a heat pump assistant and not as a dual fuel system. This should be possible as I continue to have both heating sources 🙂 and that is how the old thermostat used to work (although at a too-small delta between set and house temperatures). Does the NLT allow the user to specify the delta to be used, or does it just switch to Auxiliary Fuel if the set temperature is not reached within a certain time (also perhaps user-settable)? I expect a (smart) thermostat to allow the user to set such parameters (I did find one thermostat that allows this, but it had no hysteresis, which strains the system).

I wired the NLT identically to the old thermostat. As a result, I have no W1 connection, for example, only W2 AUX, is this the reason why the NLT defaults to dual fuel? What wiring change (if any) must I implement to get the NLT to function with oil assist?

Thanks for the * suggestion, I can specify Type = Emergency, Source = Electric (in spite of Emergency being Oil). Note that if I specify Source = Oil for *, the Type defaults to 'Alt. heat stage 2 (dual fuel)', perhaps this is the only reason that the system defaults to Dual Fuel, and only this needs to be changed?

Here again is my current wiring (labels are as per the Dual Fuel mode?):

Y1 - heat/cool,

G - fan

OB - heat pump (O, seasonal)

Rh - power,

* - alt. stage 2 

C - 24V common

W2 AUX - alt. heat

Thanks again

[CoolingWizard]

@sybernut , I guess I failed to fully explain the valid system configurations for heat pump systems. By default an heat pump is always considered the primary heat source.  Heat pumps are by nature a single heat type and that is electric.  The basic concept is that auxiliary heat will also be electric. These HVAC system configurations are:

- 1 Stage Heat Pump

- 1 Stage Heat Pump with Auxiliary Heat(W2)

- 1 Stage Heat Pump with Auxiliary Heat (W2) and Emergency Heat (*)

- 2 Stage Heat Pump

- 2 Stage Heat Pump with Auxiliary Heat(W2)

- 2 Stage Heat Pump with Auxiliary Heat(W2) and Emergency Heat (*)

The Duel Fuel systems are anytime the secondary Heat type is other than electric.  You have a duel fuel system.  The valid system configurations are:

Duel Fuel - 1 Stage Heat Pump, 1 Stage Heat(W1)

Duel Fuel - 2 Stage Heat Pump, 1 Stage Heat(W1)

Duel Fuel - 1 Stage Heat Pump, 2 Stage Heat (W1 and W2)

Duel Fuel - 2 Stage Heat Pump, 2 Stage Heat (W1 and W2)

These are the only system configurations that the NLT can support.  When there is 2 Stage Heat, this is a gas valve with 2 stages. Oil burners are not manufactured with dual stage oil output.  They have dual stages, but this has to do with pump impellers.  They do not increase the output like a gas valve does.  

When is the only time or situation you want to use your Oil Burner hot water radiator system? 
With a heat pump, Auxiliary heat is defined as electric heat supplement.  It is used by the thermostat anytime the heat pump operating and the house temperature continues to drop. At that point the NLT will activate Auxiliary Heat to boost the heat pump.  The other time auxiliary heat will be called on is by the Heat Pump defrost control board.  The air handler has to have a Heat Pump knowledgeable control board.   In this case the defrost control board in the outdoor unit will initiate the defrost timer, and activate W2 in the air handler which will turn on auxiliary heat and slow the blower down.  In your case this will not be desirable. In defrost mode, your heat turns into an air conditioner to use the hot gas refrigerant to melt the ice on the coil.  Since your auxiliary is an oil burner, there is no heat in the air handler to offset the cool air.  If you insist on using the auxiliary heat approach, the air handler control board must turn the fan off during defrost. This is typically a DIP setting on the control board.  If the control board is not Heat Pump smart, this will not work for you.  Again, you technically have a Duel Fuel system and as such, should be configured on W1 as type Oil, Radiator.  

AC Cooling Wizard

[sybernut]

I seem to have it working as I want. I specified 'Single Fuel' in PRO SETUP, and * is set to Emergency (Electric, even though it's oil, I don't know why Nest insists on electric). It now functions as a Heat Pump with Aux Heat (listed as electric once again, although it's oil), and has been tested as such. 'Compressor lockout' is set to -10C (14F). 'Auxiliary heat lockout' is set to 4.5C (approx 40F)... if this is set to 'As needed' in Google Home, the oil kicks in with a set-to-home temperature difference as low as 2C (approx 4F) at any temperature, which was the problem with my old thermostat. So now oil only assists the heat pump between 14F and 40F, with oil-only below that range and heat-pump-only above 👍.

[CoolingWizard]

@sybernut , I thought I covered this before.  It is not Nest that specifies the auxiliary and emergency heat to be electric, this is specified by the HVAC industry.  Again, the concept is simple.  When a Heat Pump system is installed, the primary heat is electric since the heat pump is powered by 240VAC.  When the heat pump air handler is installed, it is an electric blower fan with option to install electric heat strips.  
The industry recognized the possibility of a second fuel type for heating.  The thought is is the second type fuel, say natural gas, is less expensive than electric heat pump, then there needs to be a way to pick a change over point when the thermostat can change from primary heat pump to secondary natural gas.  The calculations are bit advanced, but basically most heat pumps begin to loose efficiently at -10°F. The more modern systems can work to -35°F.  These systems are variable refrigerant flow (VRF) compressors that are DC motors.  

At any rate, I am glad you have your system working the way you desire.  

AC Cooling Wizard

[sybernut]

Thank you for your input

[Markjosephp]

Hello everyone,

I wanted to ensure that everything was covered here.

Thanks for the detailed information, @CoolingWizard. It's a big help!

@sybernut, I hope you've found the answer you're looking for. If you're still in need of assistance, feel free to let us know.

Regards,

Mark

[Markjosephp]

Hi sybernut,

Checking back in — how's it going? Still need our help?

Thanks,

Mark

[Markjosephp]

Hello sybernut,

It has been a few days since your last reply, and we're going to lock the thread within 24 hours. If you have any questions, please feel free to create a new post.

Best regards,

Mark