What is multistage heating and cooling and how do I tell if I have it? (2024)

Part of figuring out if your system is compatible with a smart thermostat is determining if your 24v HVAC system is single stage or multistage. (Wait a minute, how do I tell if I have a 24v HVAC system in the first place?)

Single stage systems are best described as “on-off“.

They’re either blowing hot (or cold) air at full force, or they are off entirely.Single stage systems tend to be older HVAC models, or newer models found in smaller homes and/or in milder climates.

Multistage systems have steps, usually two, such as “low” and “high“.

A 3-stage system might have “low”, “medium”, and “high”.) Like a hair dryer or a ceiling fan, they have some granularity to their output.Multistage systems are more common in larger and newer homes, and are often found in regions with very cold or very hot weather.

Multistage systems are generally more energy-efficient, too. Running at full blast might get the house to temp faster, but you can usually reap greater energy savings by getting to your desired temperature more gradually. Smart thermostats are sophisticated enough to make this determination for you – no need to micromanage which stage is currently in use.

As you research smart thermostats, you might see designations such as “2H/2C“. This shorthand means the thermostat supports up to 2 stages of heating and 2 stages of cooling. It’s okay if you have just 1 stage of heating or cooling.

How to tell if your HVAC system is multistage heating or cooling

We think the best way to determine whether you have multistage heating or cooling is to pop your current thermostat off its base and take a look at its wires. You’ll need to look at your wires anyway to determine if the smart thermostat you’d like to get is compatible with your HVAC equipment, so you might as well get familiar with what’s back here.

(Note that this only applies to 24V systems. If you don’t know what kind of HVAC system you have, check out our articleHow can I tell what kind of heating/cooling system I have in my house?)

Once you’ve popped your current thermostat off its base, look at the wires leading into the terminals, particularly the W, W1, Y, and Y1 terminals.

Identifying a single stage system

• Single stage conventional systems systems will only have one wire for heating (in the W or W1 connector) and one wire for cooling (in the Y or Y1 connector)
• Single stage heat pumpswill have one wire leading into the Y1 terminal for both heating and cooling

Identifying a multistage system

Multistage systems have multiple heating wires and/or cooling wires. The heating wires are typically inserted into terminals labeled with a “W”, and the cooling wires are typically inserted into terminals labeled with “Y”.

• 2-stage conventional heatingwill have wires leading into the W1 and W2 terminals
• 3-stage conventional heating will have wires leading into the W1, W2, and W3 terminals
• 2-stage conventional cooling will have wires leading into the Y1 and Y2 terminals

Some HVAC systems rely primarily on a heat pump until the outside temperature gets too cold for the heat pump to keep up with on its own. These systems have AUX heat, and a wire leading into the W2 or the AUX terminal.

What if there are two sets of labels?

Sometimes a thermostat will have two sets of labels for its terminals – one set is for what’s called a “conventional” system and the other set is used if you have a heat pump.

What if there’s a jumper connecting W2 and E?

In this example, there’s a heat pump backed up by “emergency” heat. (This is in case the heat pump fails for some reason.) If you have a heat pump, you might have a setup like this one.

Since this is 2-stage heating, the wire that’s fed into “E” terminal can go into the W2 terminal or the “*” (star) terminal on a Nest thermostat and the thermostat will probably ask if it’s emergency heating during setup.

Which smart thermostats work with multistage heating and cooling systems?

The short answer is they all do. Nearly all of the most popular smart thermostats on the market today support up to 2 stages of heating and 2 stages of cooling.

However, there are some nuances to be aware of. Nest Learning Thermostat 3rd gen. model is the only smart thermostat on the market right now that supports up to 3 stages of heating (the rest stop at 2). The Nest Thermostat E (Nest’s budget model) supports 1 stage of heating and 1 stage of cooling, plus one more stage of either heating or cooling (so you can have 2H/1C, or 1H/2C, but not 2H/2C with a Nest E).

Also, take note that how many stages are supported varies with the type of HVAC system. For instance, some smart thermostats support more stages if your system includes a heat pump.

Conventional forced-air systems

Heat pump systems

*The Nest Thermostat E supports a single stage heat pump with/without AUX heat and a single stage heat pump with separate single stage furnace heating (dual fuel). It does not support virtually any other kind of heat pump, such as a 1-stage heat pump with separate 2-stage furnace heating (dual fuel) or a 2-stage heat pump with/without AUX heat. For more help with Nest Thermostat E compatibility, check Nest’s compatibility page.

These are just some of the most popular smart thermostats available today. We hope this article has helped you understand the differences between single stage and multistage heating and cooling, and narrow down your set of choices as you shop for a smart thermostat!