Make Sure Your Smart Thermostat Is Wired Correctly

By Jody Howe
CEC Sr. Energy Specialist

With all the desire to have a smart thermostat, Nest and Ecobee being two of the more popular brands, I’d like to give users a heads up. The wiring on thermostat diagrams can be a little confusing.  For all the DIYer’s and installers here are a couple tips so heat pump systems can achieve MAX savings with resource efficiency.

If your current thermostat is 120V or 240V, or has L1 and L2 terminal blocks, your system is high voltage and is not compatible with the Nest thermostat. Ecobee is also designed for low voltage heating and cooling systems.

This tip is also for heat pump systems that use a fuel back-up such as natural gas or propane. View diagrams below.

A Heat Pump with fuel back-up use the Aux W1

A Heat Pump with electric back-up use the Aux W2.

For this model Nest in particular, if we attach wires to the O and W1 terminals it will think it is dual fuel by default. If configured this way it will only switch between heat pump use and the W1 wire at the selected switchover temp. If it were a heat pump with electric backup, it would then run the heat strips (auxiliary heat) by themselves any time temps dropped below that set outdoor temperature –NOT GOOD!

The advantage in savings; heat pumps run at low outdoor temperatures. A switchover temp could be set for 30-35 degrees because this is a default for fuel use. Depending on that duel fuel system, this switchover temp could be set lower. A variety of electric systems preform at 20 degrees and lower. Fully electric heat pumps offer a greater value than auxiliary heat strips when properly set up and commissioned. Again, If your heat pump has electric backup, install the aux heat wire to W2 for NEXT and W1 for EcoBee  Check the CEC website for more heat pump information.

Then in setup wizard, you can set backup heat to electric.