Dual fuel Nest thermostat wiring for heat pump & gas baseboard

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Old 10-08-19, 08:28 AM
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Dual fuel Nest thermostat wiring for heat pump & gas baseboard

I have a heat pump setup. Right now, I have a carrier thermostat that I want to change out for a Nest. Old thermostat is a TP-PHP01 which has a seperate control module from the thermostat.

I have the following wires connected to the thermostat control module: Rh (Red), W/W1 (White), G (Green), Y/Y2 (Yellow), C (Brown), O/W2/B (Blue). Also has remote outdoor sensor too.

The tricky thing is that I have a gas baseboard zone connected as well in what I believe to be a dual fuel setup. The white from the baseboard heat and white from air handler (which I assume is electric/emergency heat) are wire nutted together and then a short lead to the thermostat on W/W1. That way, when it calls for emergency heat, it kicks on the baseboard in connection with the electric heat. The red from the baseboard heat is also connected to C (therefore, I have 2 wires connected to my C, a brown and red) which I am not so sure is great.

I think on the Nest, it should be as follows for wiring: Y1 (Yellow), G (Green), O/B (Blue), Rc (Red), C (Brown), * or W2? (White) for wires from air handler. For two baseboard zone wires, I think it goes Rh (Red), W1 (white).

I just need confirmation on how to get the two system connected together on the Nest. I am the most unsure about the white wires.
 
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Last edited by wheeler459; 10-08-19 at 09:08 AM. Reason: adding pics
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Old 10-08-19, 12:16 PM
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The white from the baseboard heat and white from air handler (which I assume is electric/emergency heat) are wire nutted together
Your lucky that worked. Many times combining transformers causes a short.

This isn't really a dual fuel setup if you want both to run together.
When the nest in dual fuel mode.... the W wires are considered sole heat....... no heat pump.
When the nest is in single heat pump mode.... the W wires are in addition to the heat pump.

So in single mode..... your W electric heating line goes to W2/AUX.
The hydronic will go to the * terminal. You can program that as E heat or a third stage AUX heat.
So your heat pump is primary heat. The electric is stage 2. The baseboard is stage 3.
You could reverse stage 2 and 3 if the baseboards are cheaper to run.
 
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Old 10-08-19, 12:46 PM
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Pete, there is some kind of relay that the hydronic system connects to that I forgot to take a photo of (I was in a tight space with no light).

Basically, the two wires from the hydronic zone go to this relay looking device and then the output wires are a white that was in that wirenut bunch and the red went into the 'C' terminal. What is this relay looking block possibly? Is it some kind of step down?
 
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Old 10-08-19, 12:54 PM
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Perfect. That relay is to isolate the two transformers from each other. It was wired correctly.

You need to decide which is least costly to run.
Here where I am in NJ..... I'd use the heat pump, then the baseboards with the electric reheat last. Our natural gas is much less costly than electric
 
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Old 10-08-19, 01:28 PM
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I think I am going to do it in that order (gas then electric). I live in Rhode Island and our electric is ridiculous (like rest of the northeast) I think the way they had it wired was that when electric heat kicked on, so did the gas baseboard. The baseboard (at least from what I recall) never ran on its own. I do remember seeing "Aux Heat" light up at times but the air handler would usually be running. This is the reason why I want to switch to a Nest-to have better scheduling for max efficiency.

Why not use W1 then W2 for the 2nd and 3rd stages on the Nest wiring? Is this because the heat pump is technically 1st stage?
 
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Old 10-08-19, 01:43 PM
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Yes..... the heat pump is considered first stage when in heat pump programmed mode.
 
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