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Before crucifying me for the subjective question let me clarify that my question is quite specific if you read on.

I bought an old home with bad insulation and a 55-year-old furnace 3 months ago. I knew I was going to have to replace it eventually but I was hoping to wait until summer. I bought a nest and external sensors for the upstairs floors because I discovered at my last home that it was the cheapest capital investment to make the best of a home that doesn't heat evenly.

The heat exchanger cracked and I had to get a new furnace. I bought a Goodman Greentherm CVM97 modulating furnace. The sales guy told me I would have to go to the Honeywell Lyric because I need millivolt control for the furnace.

After installation the tech told me that the sales guy wasn't exactly correct. The Lyric operated no differently than the Nest and the modulation was entirely controlled by the furnace. I asked him if I should use the Nest instead, and he said he had heard of issues of the logic conflicting between the two, but it could have been user error.

Final question: The Nest has more user end functionality. That is a fact, not a subjective opinion. I would prefer to use it, however I am concerned about the mentioned logic issues. Has anyone gone through a similar experience? Once again this is NOT a brand loyalty question.

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    It does sound like a shopping question. A better ask would be "Can a Nest work in my setup (or am I stuck with Lyric, as they claim)?" – Harper Dec 20 '18 at 0:01
  • You can edit your question with the "share edit delete flag" links just under the keyword list. Yes, I would also revisit the use of the term "millivolt": it is a term-of-art in HVAC, but refers to a wildly different type of heater that requires no electricity at all. – Harper Dec 20 '18 at 0:20
  • Millivolt was the terminology used by the HVAC installers and sales people. In the commercial world, most of the stuff I run across that is current modulated is referred to as "4 to 20", being the most common millivolt range. However, in this case residential HVAC is not something I'm experienced with so I'm simply relying on the experts to pass on valid information. – mreff555 Dec 20 '18 at 0:38
  • @mreff555 -- HVAC "millivolt control" is an on-off control scheme that self-powers the gas valve from a thermopile in the pilot flame. As thus, you'd never see it on a modulating furnace. Can you give us the exact model number of the new furnace by the way? – ThreePhaseEel Dec 20 '18 at 2:00
  • I can’t find a nomenclature on the furnace. The best I can give you is what is stated above. – mreff555 Dec 20 '18 at 2:19

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