There are three types of thermostat systems used in homes today:

• Low-voltage systems (24 volt; the most common)
• Millivolt systems
• High voltage (also called line voltage) systems

Nest only works with low-voltage systems. Both millivolt and high voltage systems are relatively uncommon and are not compatible with Nest.

Millivolt systems

Millivolt systems are typically wall heaters or floor heaters that don’t use electricity (for example, gas powered wall heaters). These systems use much less electricity than low voltage systems. If you install Nest on a millivolt system, Nest won’t be able to electrically detect the wires. During setup, some wires on the wiring screen will have red dots to indicate wiring errors.

High voltage systems

High voltage systems are usually electric baseboard heating systems or other radiant systems powered by electricity. High voltage thermostats are built into the wall and have 2-4 thick wires that come out of the back of the thermostat. These wires connect with wire nuts to corresponding wires coming out of the wall. Only high voltage thermostats have thick, stranded wires coming out of the thermostat itself. Other thermostats will only be attached to the smaller copper wires that come out of the wall.

If your current thermostat has the following labels, you can be sure that your system uses high voltage and won’t work with Nest:

• 110 VAC
• 115 VAC
• 120 VAC
• 240 VAC

High voltage thermostats should only be installed or uninstalled by a professional due to the high voltage that they carry.

Low voltage systems

Low voltage systems are the most common type of system. Low voltage thermostats can control nearly any kind of system: furnaces, boilers, radiant heat, air conditioning, heat pumps, hydronic baseboard heating (to name a few). Nest is only compatible with low voltage systems.

If your system is low voltage, you can remove the cover of your current thermostat to expose the thin, copper wires. With the cover removed, a low voltage thermostat something like this:

Low voltage systems that work with Nest can have between two and nine small gauge wires connected to them. Your thermostat may have more or fewer wires depending on what type of system you have. Some low voltage systems have more than nine wires.

Note how which wires from inside the wall are connected and how the corresponding connectors are labeled. Enter this information into Nest’s Compatibility Check to confirm that your low voltage system is compatible with Nest. If you’re still unsure what kind of system you have, contact a local Nest Certified professional for help.