Nest multi-stage unit limitations

[dhs910]

Google specifies that the base Nest Thermostat can control either 2 single stage units (furnaces or ac compressors), or a single stage and a 2 stage unit (either furnace or AC), but not 2 2 stage units. The upgraded Learning thermostat can do three and two.

My question is why? I thought that in modern HVAC systems, the real intelligence and control is in a separate controller, like the Honeywell HZ322 or HZ432, to which the thermostat connects with 3 wires (by convention, red, white and blue), and the controller then connects to the furnace, AC and usually other things.

Is Google assuming that I don't use a separate controller, and the thermostat is directly connected to the furnace and AC units? In a zoned system, for example, you need the controller to operate the zone dampers, among other things.

[Markjosephp]

Hello dhs910,
 
I understand that the multi-stage function of the Nest Thermostats can be confusing. But I'd love to help.

HVAC systems with heat and cooling capacities up to 2-stage heating and single-stage cooling or single-stage heating and 2-stage cooling are Nest-compatible. But this differs depending on what type of Nest Thermostats are used.

Note: Most users don’t need a C-wire for their system, but some should have a stable power source when they set up the Nest Thermostat (2020).

Do you have a Nest Thermostat installed? How's it going? Let me know if you need help with the configuration.

Regards,
Mark

[dhs910]

That's fine Mark, but that doesn't really answer my question. I think I figured out the answer, however, by talking with a friend who has Nest thermostats. He has two separate systems (upstairs and downstairs), and a Nest thermostat for each. The thermostats are wired directly to the furnace and AC units, without an intervening control unit.

And I think that's the sort of configuration where the "stage-ness" of the Nest thermostats matter - when you're wiring directly to the units themselves. If both units are two stage, you need the Nest thermostat; otherwise the base Nest is fine. With an intervening control unit, you're just going to use 5 wires on the Nest - heating, cooling, fan, power and the common (C) wires - the control unit does the rest.

So in my case, since I need a controller because I have a 2 zoned HVAC system and the controller needs to manage the zone dampers (which the Nest and other smart thermostats can't/don't do), either Nest is fine because I don't need the nest to control the stages for the unit.

Doug