Nest thermostat for two-wire systems

February 4 2017

If you're following the instructions included with your new Nest learning thermostat and the inside of your thermostat looks like this (two wires), don't stress. It's very simple to set up a Nest with these systems and unfortunately very poorly documented by Nest. Read on for instructions.

Labelling diagram

This is an older-style, mercury based thermostat which was invented long before we had complex heating, humidification, and cooling systems with neatly labeled wires. Nest's site and the internet is sparse on information on how to connect these systems to Nest. You do not need a common wire as some suggest.

Disclaimer

These instructions are for owners of a mercury-based thermostat that has two wires with a 24v load attached to it. My system is a forced air gas furnace. Your mileage may vary and I take no responsibility to damage or injury that may result from my instructions. Don't forget to turn your furnace off at the breaker before attempting any of this. That said, these steps worked for me and are from Nest's customer support technicians. I have a generation 3 Nest thermostat.

Instructions:

  1. Remove and label the two wires from the screw posts as shown in the image above (circled in purple). These two wires are essentially just the ends of a switch. You do not need a "C" common wire as others say. Nest works great with these systems as it will just close and open the switch whenever it senses the heat dropping below your desired temperature.
  2. Remove the rest of the existing thermostat from the wall (see the screws circled in the picture above in blue). If it's stuck because it's been painted around like mine was, I recommend taking a razor blade to the sides of it before attempting removal. You could damage your wall by ripping the paint/drywall if you don't cut the sides loose.
  3. Attach the wall plate (if needed) and Nest backing plate (the ring-shaped thing with the grey wire inserts). Wire it like this:

You can't really screw this up. The polarity of these wires shouldn't even matter, but this is the arrangement that Nest's support technician suggested.

Turn your furnace back on at the breaker and you should see the little home icon on the screen. The Nest is now performing a series of automated tests on itself to see what kind of system you've given it. Mine took around two minutes to complete. Follow the instructions on the screen and soon yours will look like this:

Set up Nest

That's it. If you have issues feel free to reach out on Twitter.