We just got a new furnace, the Lennox SLP98V. We have it hooked up to a Nest and its working fine. It is set up with two stages. We also have a whole house humidifier set up to it as well.

Lennox offers a thermostat of their own. My question is, does Nest utilize all the capabilities of the new furnace or does it lack or fall short in comparison to the Lennox product?

The reason for this question is that it seems like the Nest turns it into a 2 stage furnace instead of a fully communicating system that can adjust the blower speed and output between multiple stages.

Also, to be clear, I am not looking for a review in terms of which is the best product. I am looking for the technical difference between the two and which utilizes the furnaces capabilities better.

After More Research

After reading the installation manual, I did learn more about how the onboard control board works so the following is based on how I interperted it. A conventional 2-stage thermostat will only have control to set it to 1st or 2nd stage. From there, the control board will auto adjust blower speed and auto modulate itself based on how long its been at a stage. With the propietary thermostat, it has full control over modulation and blower speed. So 2-stage thermostat on a modulating furnace is still a modulating furnance, just without the amount of control as if the propietary thermostat was used.

1 Answer 1



The Nest can only control furnaces that use "on-off", 24VAC control systems, such as conventional and two stage furnaces. Fully modulating communicating systems (such as your Lennox iComfort, or the Carrier Infinity) use proprietary serial protocols that the Nest doesn't know how to speak.

This means that in order to get the full benefits of modulating operation at this time, you need the matching system thermostat -- otherwise, your furnace is simply a rather good single or two stage furnace, not a modulating furnace.

  • 1
    Right. In order for the furnace to modulate its output it would need actual temperature data (or a conversion to output level) provided by the thermostat, not a simple call-for-heat signal.
    – isherwood
    Commented Dec 20, 2016 at 20:38
  • This may be a difficult question, but are there any other universal (non proprietary) thermostats that are 'communicating'? (Lyric, etc)
    – Kalel Wade
    Commented Dec 21, 2016 at 15:36
  • 1
    @KalelWade -- not that I know of -- you'd need a 'universal furnace control board" for that to work, or at least a standard protocol for modulating furnaces... Commented Dec 22, 2016 at 0:16

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