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 a Nest thermostat to control a heat pump system and the current thermostat has
both an B and a C, connect to B to the Nest O/B connector and connect the C to the Nest C
connector.
2. If you’re installing a Nest thermostat to control a heat pump system and the current thermostat has
both an O and a B, connect to O to the Nest O/B connector and connect the B to Nest’s C connector.
3. If you’re installing a Nest thermostat to control a heat pump system and the current thermostat only has
a B wire, connect to B to the Nest O/B connector.
4. If you’re installing a Nest thermostat to control a conventional system (non heat pump) and you see a B
wire connected to the current thermostat, that B is a common wire and should be connected to the C
connector on the Nest base.
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 the Nest Thermostat.
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 the Nest
Thermostat.
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 the Nest Thermostat 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.
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