Heat pump with backup electric heat - W1 and W2 wiring question

[Hammersmith]

I've got an HVAC system consisting of a single stage heat pump and an air handler with 4 electric heating elements grouped into 2 stages(HE1+HE2 and HE3+HE4). (5KW each for a total of 20KW)

The original setup had W1 and W2 tied together in the air handler so the heating elements were acting as a single stage(HE1+HE2+HE3+HE4). When replacing the original tstat with the Nest, I separated them and configured the Nest software to treat it as 2 stage electric heat(I had a spare tstat wire so I could run a separate W1 and W2).

My question is whether the Nest keeps W1 energized when switching to Alt. Heat Stage 2?

If the Nest only energizes one at a time, then all I'm doing is switching from HE1+HE2(10KW) to HE3+HE4(10KW) and I'm never running all four elements(20KW) at the same time. 



A second question is whether this wiring config is any better or worse than the traditional setup of Aux/Emergency that is normally used with a heat pump?

My heat pump does not have an external ambient temp cutoff sensor, so I use the Nest for that. I'm currently testing an ambient temp cutoff of between 10-15F as my heat pump is a regular one and not a cold-climate heat pump with inverter.

I don't think the Nest can do both aux heat(heat pump + electric) and 2-stage electric heat at the same time. So I don't know if I'm better off wiring it as aux heat or as 2-stage electric.

[Derickf]

Hi @Hammersmith,

Thank you for reaching out to the community. I understand you have questions about the proper wiring configuration for your system; I'll be more than glad to solve any question you may have.

I just have a couple of questions. 

• What model of Nest Thermostat do you have?
• Could you please provide me with a picture of the current wiring?

In the meantime you can take a look at the following articles: Learn about multistage heating and cooling system, Heat Pump Balance.

I’ll be waiting for your reply.

Regards,

Derick.

[Hammersmith]

Thermostat is a 3rd Gen Learning

Wiring

I'm comfortable that the wiring setup is fine as long as W1 and W2 energize together for 2nd stage heating. Again, my question is just whether the Nest energizes W1 and W2 at the same time for 2-stage heating, or does it do W1 for the 1st stage and only W2 for the 2nd stage?

I discovered today that a big part of my problem is that 2 of my 4 heating elements are actually burnt out. I'm currently waiting on parts. I also picked up a clamp meter so I'll be able to answer my own question once I fix the backup heat unit, but it would still be nice to know before that so I can setup the Nest and wire things differently if W1 and W2 are never energized together.

[Derickf]

Hi there!

Thanks for your reply. In response to your inquiry, the thermostat will activate or deactivate the emergency heating or the second heating stage as necessary to assist the central heating system in maintaining a pleasant temperature in your house. Not all stages will work always at the same time.

Please don't hesitate to continue posting if you have any other questions.

Best regards,

Derick.

[Hammersmith]

Yeah, that doesn't help at all.

It's a simple question. Is the Nest 3rd gen Learning programmed to do this for electric Alt heat:

Alt 1 - W1
Alt 2 - W2

or this:

Alt 1 - W1
Alt 2 - W1+W2

That's all I need to know.

Current Equipment Setup:
Heat source
Stage 1 Electric (heat pump - using Y1 and O/B)
Alt. 1 Electric (electric strip heat using W1)
Alt. 2 Electric (electric strip heat using W2)

Heat type
Stage 1 Forced air
Alt. 1 Forced air
Alt. 2 Forced air

[Hammersmith]

Never mind. I figured it out on my own with the clamp multimeter I recently got.

In case anyone stumbles across this thread in the future looking for a similar answer, the W1 is left energized when the Nest calls for Alt heat 2.

So it's:
Alt 1 - W1
Alt 2 - W1+W2

Which is what I always expected it was, but it's nice to now have confirmation. It turns out the reason I thought it might've been the other is completely because of the burnt out elements.

I was expecting to get 10KW of heat on Alt 1 and 20KW on Alt 2, but I was actually getting only 10KW on Alt 2. I thought it might be because of the thermostat only energizing one W wire at a time, but it was actually because of the burnt out coils.

[Derickf]

Hi there!

I'm happy to hear that you were able to find the answer you needed on your own.

If you have any more questions, don't be afraid to keep posting.

Best regards,

Derick.