Multi-stage Heat Pumps: Are They Worth It?

If you’re shopping for a heat pump or air conditioner, one of the key features that you’ll see mentioned is whether the unit is single-stage, two-stage, or multi-speed.

A heat pump or air conditioner with more than one stage is able to vary the output of its compressor to suit different heating and cooling needs. While a single-stage heat pump can only operate with its compressor at 100% power, two-stage and multi-speed heat pumps can set a lower output when needed.

This results in a steadier indoor air temperature, better comfort, and higher energy efficiency. The tradeoff, as you would expect, is a higher price tag. Is it worth it? That depends on what’s most important to you in your heating and cooling system. This article will discuss the pros and cons of single- and multi-stage heat pumps and hopefully help you make a smart decision for your home.

Note: this article is focused on heat pumps, but the details apply to central air conditioners as well.

What is a single-stage heat pump?

Single-stage stage heat pumps are designed to operate at full power only. When your thermostat decides that your house needs heating or cooling, it sends a signal to the heat pump to turn on. After the target temperature is reached, the system will turn off.

This simple on-off operation means that single-stage heat pumps are the lowest cost choice offered by manufacturers. They can still be efficient: you can find single-stage heat pumps with a SEER2 rating up to about 17.

What is a two-stage or dual-stage heat pump?

As the name suggests, a two-stage heat pump can operate at two power levels. It can operate at 100% power when needed, and reduce power when your home’s heating or cooling needs are lower. The low power mode of two-stage heat pumps is typically in the range of 60% to 70%.

When operating in low power mode, the heat pump will use less electricity, but it will also run more efficiently because there is less energy wasted in the refrigeration cycle¹.

Most manufacturers offer two-stage heat pumps, and they will have a higher maximum efficiency than single-stage models. For example, two-stage heat pumps offered by Trane start have a minimum SEER2 rating of 16, and those from Lennox have a minimum SEER2 of 17. In contrast, single-stage heat pumps by both manufacturers have SEER2 ratings as low as 14.

What are the advantages of a two-stage heat pump?

Having two stages allows a heat pump to better match the heating or cooling needs of a home. For example, let’s say you set your thermostat lower when you’re sleeping at night, and program it to warm up again in the morning. In this scenario, a two-stage heat pump would operate at 100% power to quickly warm up the house. After the target temperature is reached, it would switch to low power mode. While the heat pump may eventually overshoot the target temperature even in low power mode, it will take longer to do so, resulting in a steadier temperature in your home.

A two-stage heat pump is usually paired with a variable speed blower. When the heat pump is running in low power mode, the fan will run at a lower speed as well. This means you’ll hear less noise from your ducts, and you’ll be less likely to feel breezes coming from the ducts as well.

Finally, a two-stage heat pump will dehumidify the air better, which is often an underappreciated function of air conditioning. Because the low power stage operates for longer, the heat pump is able to extract more moisture out of the air. While a higher power stage will bring down the air temperature more quickly, the shorter run time means that the heat pump will have less time to dehumidify the air. This means that the air can be cold but feel clammy due to high humidity. This is not only uncomfortable, but can cause mold issues.

In fact, lower humidity can be a more important factor for comfort than lower temperatures². This won’t be an issue in all climates, but if you live somewhere with high temperatures and very high humidity in the summer, a two-stage heat pump or air conditioner can keep your home comfortable with less energy usage.

What is a variable speed heat pump?

Finally, variable-speed heat pumps are able to operate at a wide range of power levels, often from about 30% to 100% output. This allows the heat pump to run at a very low output to maintain a steady temperature in a home.

This has the same advantages of a two-stage heat pump: better humidity control, high energy efficiency, quieter operation, and steady temperature control. But because a variable-speed heat pump can vary its output with more flexibility, it’s even better at these things than a two-stage heat pump.

Variable-speed heat pumps are at the top end of any manufacturer’s product lineup, so you can expect them to be the most expensive option. They will give you operating cost savings because they’re the most energy efficient: variable-speed heat pumps will have SEER2 ratings starting around 17, and go as high as 25. That’s a big increase over a single-stage unit.

You can expect some cost savings from this higher efficiency, but don’t count on these savings to make up for the higher cost of the unit. Instead, the main reason to choose a variable-speed heat pump is better comfort. With the ability to throttle down to a very low output, a variable-speed heat pump could run continuously at the power level needed to maintain a steady temperature and comfortable humidity level.

While you might be concerned about the electricity usage of the blower motor operating continuously (which can be a few hundred watts at full power) when it’s running at a lower speed it will consume significantly less. This is not only because the fan is running at low power, but because there’s lower air pressure in the ducts, which results in less back pressure on the fan. (High air pressure causes a blower motor to work harder to push air through the ducts, increasing electricity usage.)

Finally, continuous operation can mean cleaner air, specially if you have an advanced filter system add-on.

Summary of pros and cons of multi-stage heat pumps

Here’s a summary of the main pros and cons of multi-stage heat pumps:

A big advantage of two-stage and variable-speed heat pumps is that they can maintain a steadier temperature in your home by using a lower power output and fan speed. This is illustrated by the graph below. (Note that this graph is just an illustration of the concept. It’s not based on real world data.)

Integrating a heat pump into an existing forced air system

In addition to higher cost, another disadvantage of two-stage and variable-speed heat pumps is that they are less likely to be compatible with an existing forced air system.

Most two-stage and all variable-speed heat pumps that are integrated into a forced air system require a blower motor (ie. fan) that is variable speed. (While you might be able to install a two-stage heat pump with a single-speed blower, you would lose many of the advantages.)

However, most forced air furnaces are installed with single speed blowers. This means that if you want to add a multi-stage heat pump to an existing forced air system, there’s a good chance that you will also need to upgrade the blower motor and furnace along with it. (Unless you decide to use a different option for backup heat.)

Another potential issue is that the indoor heat exchanger – also known as the coil – that is required for two-stage and multi-speed heat pumps may not fit within an existing system.

This issue isn’t because of the multi-stage operation of the heat pump, but the higher SEER2 ratings that advanced heat pumps have. To help achieve their higher efficiency rating, high SEER2 heat pumps often need larger heat exchangers than single-stage heat pumps or air conditioners do. A larger coil may not fit within an existing duct that is sized only for a gas furnace or single-stage heat pump or A/C. This may cause you to have to choose between a single-stage unit or spending additional money on new duct work.

Are variable-speed heat pumps more expensive to repair?

One common question that people have about two-stage and variable-speed heat pumps is whether they are less reliable or more expensive to repair.

The parts can certainly be more expensive, especially the inverter motors used in variable-speed equipment. In addition, these advanced heat pumps tend to have more electronics, which are another potential failure point.

However, variable-speed heat pumps tend to come with longer warranties. For example, the Signature series of variable-speed Lennox heat pumps come with a 10-year warranty, while their Merit line of single-stage heat pumps have only a 5-year warranty. It tends to be the case that longer warranties mean that the manufacturer has higher confidence in the durability of the product, so while variable-speed heat pumps are more complex, it doesn’t necessarily mean that they are more prone to failure.

Communicating thermostats for multi-stage heat pumps

Earlier in this article, I described how a typical thermostat works when it calls for heating or cooling from a single-stage heat pump or air conditioner. When the air temperature falls outside the set temperature, it sends a signal to the HVAC equipment to turn on. Once the set temperature is reached, the thermostat asks the system to shut off.

However, two-stage and variable-speed heat pumps don’t work in a simple on-off fashion. Instead, the thermostat evaluates how much heating or cooling is needed based on the indoor temperature, and then asks the equipment to set the power output at the appropriate level. For example, if the room temperature is only a little below the set point, the thermostat would ask for a lower power output.

A conventional thermostat doesn’t have the capability to request differing power levels like this. Instead, multi-stage heat pumps require a communicating thermostat. “Communicating” doesn’t refer to Wi-Fi or “smart” thermostats like the Nest or Ecobee (although they can work with two-stage heat pumps in a limited fashion.)

Communicating thermostats use a communication protocol called BACnet. BACnet allows a thermostat to intelligently “talk” to connected HVAC equipment to find out what features they support. It can then turn on features to match the thermostat’s settings: for example, turning on a heat pump at a low level to raise the temperature gradually while also asking a humidifier to raise the humidity level.

At the time of writing, popular smart thermostats like the Nest and Ecobee are not communicating thermostats. While some models do support two-stage heat pumps, they only support single-speed blowers.

Instead, you should choose a fully communicating thermostat. There are many on the market, but one example is the Trane ComfortLink series³, which supports 5 heating stages and 2 cooling stages.

Summary: two-stage and variable-speed heat pumps have many advantages, but the cost can be prohibitive

To summarize, there are two main reasons why you might choose a two-stage or variable-speed heat pump:

• You want maximum comfort in your home
• You want a cold climate model that will work efficiently in very cold temperatures

A two-stage or variable-speed heat pump can meet both these goals. Unlike single-stage systems that cycle on and off at full power, heat pumps with two or more stages will maintain a steadier temperature in your home. They also have higher efficiency, with SEER2 ratings as high as 25.

Other advantages include better overall air quality, including humidity control, and quieter operation. With a variable-speed heat pump, you might not even notice when the system is running.

The downside is that these systems cost more. Not only is the equipment itself more expensive, but if you are trying to integrate a two-stage or variable-speed heat pump into an existing forced air system, you might find that the blower or ducts are incompatible and would require an upgrade.

While the higher efficiency will reduce your energy costs, it may not offset the higher installation costs.

Bottom line: talk to your installer to weigh your options, costs, and the potential rebates you can receive. I think in many cases, a two-stage heat pump represents a good balance between performance and price.

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