My old thermostat has two labels for each connector
Some thermostats have two labels for each connector: one for conventional systems and one for heat pump systems. To determine which labels are correct for your wires, you'll need to know which type of system you have.
Many thermostats work with both conventional systems (like furnaces, boilers, and AC) and heat pump systems (condenser units that both heat and cool). These two systems use a slightly different set of wires, so to accommodate both systems some thermostats have two rows of wire label or have two labels listed side by side.
On this thermostat, the labels for conventional systems are on the inside of the connectors and the labels for heat pump systems are on the outside of the connectors.
If you know what kind of system you have, use the labels for that system.
If you're unsure, the color of the wires can be a clue. For example, in the photo above, the connector labeled O/B for heat pumps is also labeled W for conventional systems.
The color of the wire that has been inserted into the O/B and/or W connector in the photo above is orange. Therefore, you might guess that this system is a heat pump. However, the colors of the wires don't always correspond to their labels, so it’s best to consult the user guide for your thermostat to be sure.
If the same connector label lists a second letter in addition to O or B then that connector can be used for heat pump or conventional systems. In the example below the connector labeled W/B is actually a W wire, not a B wire in this case, because the heating system is a conventional heating system not a heat pump. The white color of the wire is another clue.
If you aren't certain what wire labels you should use, consult an HVAC professional. Check our Pro Finder to locate a Nest Certified Professional near you.