Google Nest Learning Thermostat 4th Gen Installation And Configuration Manual page 26

Installation Overview - Pro Setup
Key wiring and compatibility notes
Boilers
Must use an external transformer for a source of constant 20-30 VAC when using the power connector.
If the boiler is DC powered, you must install a SSR between the transformer and terminal block.
Zone Panels
Nest Thermostats will not work with zoned systems in which the thermostat does the activation and
deactivation of the zoned dampers; usually indicated by "L" terminals. Many zone panels will require an
upgraded transformer when using a smart thermostat such as the Nest Learning Thermostat. Check the
requirements of the specific zone panel or board being used.
Emergency Heat
The Nest Learning Thermostat supports a dedicated wire for emergency heat in the star terminal, but it is
not required. When emergency heat is activated, it will either energize the equipment connected to the
Star terminal or it will energize Aux heat or Alt heat.
For heat pump systems equipped with auxiliary (Aux) heat, engaging the emergency heat function will activate
the Aux heat strips as the primary heat source. The heat pump itself will be temporarily disabled while emergency
heat remains active. In systems utilizing an alternate heat source (e.g., gas furnace), activating emergency heat
will engage the highest stage of the alternate heat source. This alternate heat source will remain operational until
emergency heat is manually deactivated.
Variable Speed Systems
The Nest Learning Thermostat does not modulate voltage; therefore, it cannot control variable speeds on the
thermostat side. If the equipment can modulate itself with a non-proprietary 24VAC input, then the Nest Learning
Thermostat will work, but will require adjustments to settings to ensure efficient operation of the equipment.
Unsupported Wires
Some common terminals that we do not work with are non-standard HVAC terminals; if you see terminals
labeled 1, 2, 3, or A, B, or D, there is a pretty good chance that the Nest Learning Thermostat will not work with
the equipment.
There are some terminals that might fall under the standard HVAC terminals that the thermostat can treat a little
differently. HVAC equipment that has sensors wired to it usually have an "S" terminal designation. Since the Nest
Learning Thermostat does all its sensor data via Wi-Fi and local weather data, it does not need the "S" wire. In
this case, if all other terminals are standard HVAC wiring, you can proceed as normal, and you will just cap off the
"S" wire and not use it.
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