How to Configure Complex Systems
Dual Fuel Systems
If there’s an O/B wire and one or more W wires connected to the Nest
thermostat (which indicates that the system uses a heat pump with an
additional heating source), Pro Setup will first ask if the heating system
is dual fuel or single fuel.
For single fuel, any wires inserted into the W1, W2/AUX, or Star (if W3
is chosen) connectors will be treated as auxiliary heat that will be
used simultaneously with the heat pump when needed.
Pro Setup will automatically configure the auxiliary heat wire for an
electric forced air system. The 3rd generation Nest Thermostat can only
support one wire for auxiliary heat; additional auxiliary heat wires are not
supported.
For dual fuel, any wires inserted into the W1, W2/AUX, or Star (if W3 is chosen) connectors will be treated as
alternate heat that will be used instead of the heat pump when needed. Use Pro Setup to indicate the source
(gas, electric, geothermal, etc) as well as the delivery mechanism (forced air, radiant, etc) for each of the W
wires connected. The Nest Thermostat supports two wires for alternate heat; additional alternate heat wires
are not supported.
Humidifiers and Dehumidifiers
The Nest Thermostat supports whole-home humidifiers and
dehumidifiers that connect to it with one or two low voltage wires. There
are multiple Pro Setup settings for the wire in the Star connector to
control a humidifier or dehumidifier. Whenever a humidifier/dehumidifier
is attached you’ll need to select whether or not the fan should activate.
Compatible wires have one of the following labels:
● H, Hum, Hum1, H2, Hum2
● D, DH, Dhum, Dehum, D2, DH2, Dhoom2, Dehum2
Regardless of whether your customer’s humidifier or dehumidifier has one wire or two, you will need to figure
out if it can be activated on its own or if it requires heating or cooling to be on at the same time. You’ll also
need to determine other settings, such as whether the fan is required during operation.
The Nest Thermostat’s Star terminal is a powered terminal. If you’re connecting a humidifier/dehumidifier that
requires unpowered terminals, add a 24 VAC relay suitable for HVAC equipment use. Consult humidifier/
dehumidifier installation manual for wiring requirements.
Important Note: In dual transformer installations (installations with both Rc and Rh wires) it's important to note
that different wires attached to the thermostat will use different power sources. The Rc wire will provide power
to the Y1, Y2, G, and O/B terminals. The Rc wire will also charge the thermostat using the C terminal. The Rh
wire will provide power to W1, W2/AUX, and Star connectors. Never connect wires from dual transformer
systems to connectors from the opposite transformer.
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