Troubleshooting
Handling unusual thermostat wiring
When a B or X wire is a common wire
Most thermostat manufacturers use
C
to designate the connector for the common wire. However there are
some Trane, American Standard, and York thermostats use
B
for the common wire.
The NEMA standard designated the
B
wire as one of the change-over wires for some heat pump
manufacturers. This is a simple issue to address:
1. If you’re installing Nest to control a heat pump system and the old thermostat has both an
B
and a
C
,
connect to
B
to Nest’s
O/B
connector and connect the
C
to Nest’s
C
connector.
2. If you’re installing Nest to control a heat pump system and the old thermostat has both an
O
and a
B
,
connect to
O
to Nest’s
O/B
connector and connect the
B
to Nest’s
C
connector.
3. If you’re installing Nest to control a heat pump system and the old thermostat only has a
B
wire,
connect to
B
to Nest’s
O/B
connector.
4. If you’re installing Nest to control a conventional system (aka not a heat pump) and you see a
B
wire
connected to the old thermostat, that
B
is a common wire and should be connected to the
C
connector
on Nest.
Important Note:
If you have a
B
wire that is actually a common wire, not following the rules above can lead
to blowing a fuse on your HVAC controller board and, most likely, damaging Nest.
When X, W1 or W2 is an AUX wire
Some heat pump installations may have the auxiliary heat connection labelled as
X
,
W1
or
W2
on the
thermostat.
You should verify that the
X
wire on the old thermostat is not a common wire. Once you’re sure , you can
assume that the
X
is the auxiliary heat and should be inserted into the
AUX/W2
connector on Nest.
Compressor and Auxiliary Heat Lockout Temperatures for Heat Pumps Requires Wi-Fi
Both auxiliary and compressor lockout temperatures can be changed in the Heat Pump section of the
Equipment menu.
The lockout temperatures are only enforced when Nest is connected to Wi-Fi so it can track outdoor
temperatures. If Wi-Fi is not enabled, Auxiliary heat will come on automatically when it takes longer than
expected to reach the current target temperature.
Search nest.com/support for
auxiliary heat
for more details.
Nest Learning Thermostat installation & configuration guide
Page 28
