Heat Pump with AUX, Fuel Oil Heat, 2 nests.

[cwbywy]

I have one Nest hooked up to a TRANE HeatPump with AUX heating unit in the attic.  I have another Nest hooked up to a fuel oil heating system in the crawl space under the house.  The thermostats are located in the same location a few inches apart from one another.  Is it possible for the HeatPump Nest to tell the Fuel Oil Nest to turn on instead of the AUX, OR in addition to AUX?  Jumper wire from one W connection to the other W connection?

Best case would be for Fuel oil system come on, and the Recirculation Fan for the Heat Pump system to come on at same time basically sucking heat from one system and distributing heat as well.

[dhx227]

You would have a number of problems with that.   First, you can't intermix the signals from two different transformers.   Each heating system most likely has it's own 24vac transformer that powers the Nest and all it's control functions.   So, you can't take a wire from one transformer and expect the other transformer to balance it to where it shows voltage (and hence turn something on/off).

But, let's say you solved that part...   Rewired everything to be power from one larger 24vac transformer...

The next problem is that the Nest is not designed to recognize a signal to be on/off from somewhere else (pretty much no thermostat is).   So, you can't use a wire from one thermostat to signal to another thermostat that it should come on.

A far simpler (though not simple) approach would be to programmatically monitor the one Nest and then programmatically tell the other Nest to come on.   There is an API to the Nest Home that the Home app uses, but I have not played with it, so I can't even say how feasible that it.

As a final note, and per you last comment: the most feasible approach would be to use an independent relay installed on the recirculation fan that is triggered from the fan control on the oil furnace.   You just need to make sure the relay control circuit is tied to the oil furnace and the relay switched circuit is connected to the heat pump system fan circuitry.  In this scenario, when your Nest connected to the oil furnace turned on, it would then effectively also turn on the fan in the heat pump system.

[Patrick_Caezza]

Yes, you can do what you want, it's called a dual fuel system, and only the Nest 3rd Gen Learning Thermostat can do it. The Nest Learning Thermostat is designed to work with two separate 24V transformers (that's why there is an Rc & Rh terminal).

How are the two thermostats currently wired?

When you want heat, the heat pump will be your primary heat source, the fuel oil furnace with being the 2nd stage heat source and the aux heat would be your 3rd stage heat.

Is that what you want?

[dhx227]

I believe he said he already had two Nests and that he somehow wanted them to talk to each other... 

And yes, using a dual fuel setup with a single thermostat would be the normal way to resolve this kind of configuration.

[JoM0r88]

Sorry but your comment is the first I have found that directly addresses an issue I have been having.

I have been having issues with the HVAC company that installed exactly what you said - Nest 3rd Gen. I have my AC/Heat Pump in Rc and Oil Boiler in Rh. However since they have installed it my AC/HP does not switch between cooling/heating so they have come every 6 months and "fixed it" each time, but when they leave its only either cold air or hot air on my unit. 

So currently when I try to use the heat pump it will blow cold air for 2-3 minutes before switching over to the Oil Boiler. They have my boiler wired into the W2AUX. T

The system details are 'Heat source 1 Electric' with 'Heat Type 1 Forced air' and 'Alt. heat source 1 Oil' and 'Alt. heat type 1 Radiator'

Is there anything I could look into or do/test to see if they are perhaps overlooking something?

[Markjosephp]

Hello JoM0r88,

I'm sorry for the late reply. I'd be happy to take a look into this for you.

Could you please send us a picture of your thermostat's wiring setup so that we can isolate the problem? Also, to clarify, does your AC activate unexpectedly whenever you turn on your heat?

I did find a helpful article with some troubleshooting steps. Please let me know if that helps, or if the issue still persists.

Best regards,

Mark

[Jake]

Hey there,

I wanted to check in and see if you managed to see Mark'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

[Jake]

Hey there,

I wanted to check in, and let you know that I will be locking the thread in 24 hours.

Best regards,

Jake

[cwbywy]

Thank you both for replying.  Yes, I did want to keep both thermostats.  But it doesn't look like that is possible without complication.

[dhx227]

What you could try is to add a relay that additionally controls (calls for heat on) the Fuel Oil furnace.

However, I have no idea how a Nest would behave if something external to it switches on the call for heat.

Basically, the (R) wire provides power to a "switch" (the thermostat) that when it's time to call for heat, merely connects the (R) to the (W) wire.   When the furnace sees voltage on the (W) line, it turns on.

So, if you were to take the AUX (W2) output from the heat pump thermostat and connect that to one terminal of the power side of a relay, connect the other terminal of the power side of a relay to (C)ommon - this would switch on the relay when W2 was called for.   Then connect the switched (Normally Open) side of the relay to the (R) and (W) lines of the fuel oil furnace.  That would then bypass the fuel oil thermostat and turn on the fuel oil furnace whenever the AUX came on from the heat pump thermostat.

The big question mark I have is how the fuel oil Nest would behave when it sees that something else has turned on the furnace; ie, there is a short between (R) and (W) outside of the Nest.   It may not care or it may go into a "wiring error" mode.

[dhx227]

Also note: if you want both AUX and fuel oil, it would be most ideal to use a double pole relay (DPST or DPDT) and then use other "pole" of the switched relay to control the AUX heat pump.  To do that connect another wire from (R) on the heat pump thermostat, and then move the (W2) wire that would go to the heat pump from the Nest and connect that to the other side of that switched "pole".    Using this additional pole would be more reliable since the relay itself consumes a tiny bit of power from the W2 thermostat output and could cause the heat pump to not sense that it's on.   The cost difference between a SPST relay vs DPST/DPDT relay is really minimal.   As a matter of fact, DPDT relays are often easier and cheaper to find and can serve all these purposes plus more.

[dhx227]

Oh, after re-reading your last paragraph in the OP - you could alternatively use a relay to have the fuel oil thermostat control the fan in the heat pump system.   The power side of the relay would be connected to the (G) and (C) wires of the fuel oil thermostat, and the switched side would be connected between the (R) and (G) wires of the heat pump system.    Again, I have no idea how a Nest behaves if/when it senses something else has shorted/connected the wires external to itself...

Note: you could test that merely be taking a jumper wire and connecting (R) and (G) together on your heat pump system.   The fan should come on when you do that and then just monitor what the Nest does/says.

[Ryan_G]

Hey all,

I just wanted to follow up and see if you're still in need of help. Please let us know as we would be happy to answer any questions you may have. 

Thanks for your insights, dhx277.

Best regards,

Ryan

[Ryan_G]

Hey there,

Thanks for being here in the Google Nest Community. 

I'd like to bump in here and make sure that you saw our response.

Please let us know if you have any other questions or concerns as I will be locking this thread in 24 hours.

Have a great day.

Best regards,

Ryan