Nest thermostat doublespeak

The Steam Whisperer

We had some problems recently with a nest....check out what they say about the common wire....

"The Nest Thermostat E works with 85% of 24V heating and cooling systems, including gas, electric, forced air, heat pump, radiant, oil, hot water, solar and geothermal.
Heating and Cooling: 1 stage of heating (W1) and cooling (Y1) and a second stage of either heating (W2) or cooling (Y2).
Heat pump: with auxiliary and emergency heat (O/B, AUX)
Fan (G)

Power (C, Rh or Rc)
The Nest Thermostat E is designed to work even if your home doesn’t have a common (C) wire, but installing a new C wire may be required.
If you have a dual fuel system, we strongly recommend professional installation." ( bold added)

pecmsg

I believe I’ve been saying that for years!

Jamie Hall

I think -- but an not sure -- that the problem is very closely related to the cycle timing. It seems that if the heating system runs rather short cycles -- typical of forced air -- with short run periods and short off periods, they do work without a C wire. Why/ Because when they are NOT calling for heat, the 24 volts difference between Rh or Rc and W can be used to charge them up (question for an Electonics geek; do they actually use a battery, or just a big capacitor?) but if the on cycle is too long -- typical for most hydronic and steam -- the battery or whatever it is goes flat and they lose their pointy little minds.

EdTheHeaterMan

pecmsg said:

I believe I’ve been saying that for years!

I know that you secretly like the technology of the smart thermostats. Once you got your flip phone to work