Heat Pump Terminology Explained

Are you thinking about getting a heat pump for your home? They’re growing in popularity: they operate more cleanly than a gas furnace and will often save you money.

For many homeowners who are considering a heat pump, it will be their first time learning about them. This means that there is a lot of terminology that will probably be unfamiliar - especially the acronyms! If you’re in that boat, you might wonder what things like SEER and HSPF mean.

Don’t worry - this article will guide you through the key terms and help you understand which specifications will matter to you most.

Heat pump specifications - what consumers need to know

A heat pump installation will set you back a few thousand dollars, even after incentives. Because of that, it’s a good idea to get at least two quotes from local companies. Not only will you get different prices, but you will likely get different equipment and configuration recommendations too.

Getting quoted on different equipment means that you’ll want to understand what those differences are so that you can make a more informed decision about choosing the best product for your home.

If you look up the product brochure or specification sheet for a heat pump, you’ll find that they usually focus on a handful of specifications. Here are the ones that you need to know about.

Tonnage and BTUs: Understanding system capacity

Whether you’re buying a furnace or heat pump, one of the most important specifications is the capacity or size of the system.

When it comes to heating or cooling, the size or capacity of a system refers to how much heat energy it can move in an hour - either putting heat into your house in heating mode, or removing heat when operating in air conditioning mode.

British thermal units (BTU) is a common measurement you’ll see. When it comes to both heat pumps and furnaces, capacity that is expressed as BTUs really means BTUs per hour (BTU/hr), which tells you how much heat energy the unit can move per hour. However, product brochures will often simply list BTU.

In North America, we still use the rather archaic measure of tonnage for both heating and cooling. One ton of cooling is equivalent to 12,000 BTUs/hr. A bit of trivia: this refers to the cooling energy delivered by one ton of ice per day.

For an average sized home with decent insulation in the continental United States, you’ll probably have a heat pump with a capacity between 2-5 tons or 24,000-60,000 BTUs/hr. Very large or small homes, homes that are poorly insulated, or located in climates that experience extreme temperatures will need capacities outside of this range.

The capacity of a heat pump is determined by the outdoor unit. Central ducted heat pumps generally come in a capacity range of 2-5 tons (24,000-60,000 BTU/hr), while mini-split units can be as small as 6,000 BTU/hr and up to about 42,000 BTU/hr (0.5-3.5 tons).

This doesn’t mean that your heating and cooling system is limited to the capacity of a single outdoor unit. If you need a higher capacity system, your HVAC company can design a system with multiple outdoor units. This is especially common with ductless mini-split units.

SEER (and SEER2): Seasonal Energy Efficiency Ratio

If you have an air conditioner, you might already be familiar with Seasonal Energy Efficiency Ratio (SEER) ratings. This is number describes the efficiency of an air conditioner or heat pump when operating in cooling mode. It’s the ratio of the heat removed by the unit, measured in BTUs, to the amount of electricity the unit consumes in Watt-hours. Here’s how it’s calculated:

SEER = total cooling BTUs ÷ Watt-hours consumed

For example, let’s say that your heat pump in cooling mode removes 30,000 BTUs of heat from your home, and consumes 1,875 Watt-hours (1.875 kilowatt-hours) of electricity in the process. That means the calculation would be:

30,000 BTU ÷ 1,875 Watt-hours = 16 SEER

A higher SEER number means better efficiency. If the rating for your heat pump is instead 20 SEER, it would consume only 1,500 Wh (30,000 BTU ÷ 20 SEER) to remove 30,000 BTUs of heat from your home.

In 2023, the minimum rating allowed by federal regulations is 14 SEER in northern states and 15 SEER in southern states - meaning that states with warmer climates, which generally have higher cooling needs, will require more efficient air conditioners and heat pumps.

SEER is measured under standard test conditions, but those test conditions often don’t match real world performance, in the same way that the fuel economy of your car may fall short of its EPA fuel economy rating. To improve the accuracy of the SEER rating, the Department of Energy made changes to its test procedures, with the most significant being an increase in the external static pressure (ESP) on the system.

ESP is the difference in pressure between the supply and return ducts of the system. Higher pressure causes the blower fan of the system to work harder, increasing its power usage. The Department of Energy found that in the real world, home systems may have issues that increase static pressure, such as clogged air filters and ducts that are undersized. The newer test procedure, indicated as SEER2, increases the static pressure on the system to better reflect this.

As a result, a SEER2 rating will be lower than a SEER rating. Some manufacturers, such as Lennox, will list both. In cases where they do, I’ve found that a SEER2 rating will be roughly 1-1.5 lower than the SEER rating.

This is the current standard, so any new heat pump or air conditioner you purchase today should list a SEER2 instead of a SEER rating.

HSPF (and HSPF2): Heating seasonal performance factor

The heating counterpart to SEER is Heating Seasonal Performance Factor (HSPF). It’s calculated in a similar way to SEER: it’s the ratio of the heat generated by the system in BTUs to the electricity consumed in Watt-hours, with higher numbers indicating better efficiency.

HSPF = total heating BTUs ÷ Watt-hours consumed

A heat pump with good efficiency might have an HSPF rating around 9. This means that, to generate 30,000 BTUs of heat, the heat pump will consume 3.3 kWh of electricity. Here’s a sample calculation for an HSPF rating of 9:

30,000 BTU ÷ 3,333 Watt-hours = 9 HSPF

The HSPF rating for a heat pump will be lower than its SEER rating because the test standard for HSPF specifies a larger temperature differential between the indoor and outdoor air:

Department of Energy temperature specifications for SEER2 and HSPF2 testing

Because the HSPF2 test involves nearly double the temperature difference of the SEER2 test, the HSPF2 rating will be lower due to the higher amount of electricity needed to reach the target indoor temperature.

As with SEER, the Department of Energy test procedures were updated for HSPF, so new heat pumps should list an HSPF2 rating rather than the older and less stringent HSPF rating.

The federal minimum standard is an HSPF2 rating of 8.8 for all of the United States – there isn’t a different standard for northern and southern states as there is with SEER2.

By the way, the HSPF for an electric resistance heater is 3.4. That’s the lowest efficiency you can expect from an electric heating unit, so when the outdoor temperature is 47°F – the current test standard for HSPF2 – a heat pump that meets the minimum efficiency standard for 2023 will generate at least 250% more heat than an electric resistance heater using the same amount of electricity.

However, this is only true until the outdoor temperature falls below the threshold where backup heat is programmed to kick in. Eventually, the outdoor temperature can be cold to the point where the performance of an air-source heat pump will no longer provide enough heat to maintain the set temperature of your thermostat. When this happens, the backup heat source will turn on. In some setups - like mine - the backup heat source is a gas furnace. Other systems will use electric resistance heat, in which case the HSPF2 rating of the system will drop to 3.4.

For many climates, the number of days when backup heat needed is rare, but if you live in a cold climate, a heat pump that is designed to work well in very cold temperatures will help your system perform more efficiently. You can read more about cold climate heat pumps below.

Minimum standards for SEER2 and HSPF2

The Department of Energy introduced minimum efficiency standards for central air conditioners and heat pumps starting in 1992. Those standards were raised in 2006, 2015, and 2023.

The minimum SEER rating in 1992 was 10 – and that was based on the less stringent SEER standard rather than the newer SEER2 – so you can see that air conditioners and heat pumps today far outperform those from a few decades ago. Here’s a chart of the historical and current standards:

As you can see, these standards have greatly improved the efficiency of air conditioners and heat pumps. If we’re using our benchmark of 30,000 BTUs/hr of cooling, an air conditioner in 1992 might have used 3 kWh of electricity while one in 2023 would need only 1.875 kWh – a 60% improvement.

And that’s just the minimum performance required. You can find many models on the market today with SEER2 ratings that are higher than 20.

Coefficient of Performance (COP)

Another heat pump specification that you will often see listed is Coefficient of Performance (COP). While COP can be used to measure both heating and cooling performance, with heat pumps it will typically be listed for cold outdoor temperatures (ie. heating mode).

Remember earlier that I said that an electric resistance heater will have an HSPF rating of 3.4? This means that for every one kilowatt-hour of electricity used, it will generate 3,412 BTUs of heat. COP is a factor based on this number. A heater with a COP of 1 will generate 3,412 BTUs for every 1 kWh of electricity used. A COP of 2 means that the heater will generate twice as much – 6,824 BTUs of heat for every 1 kWh of electricity.

An average heat pump will have a COP of about 3 when the outdoor temperature is around freezing, and improve to a COP above 4 as the temperature increases. As it gets significantly below freezing, conventional heat pumps will start to struggle and may be less cost effective to operate than a gas furnace.

Cold climate heat pumps

Cold climate heat pumps, as the name suggests, are designed to work better in very low temperatures. It’s not just a marketing term either: the Department of Energy has criteria that allows a heat pump to be certified for cold climates.

The most notable criteria is that the heating capacity rating of the heat pump at 5°F (-15°C) must be at least 70% of the heating capacity at 47°F (8.3°C). That’s a really cold temperature, which illustrates how well modern heat pumps perform.

Here’s a list of criteria required by the DoE to meet cold climate certification:

• COP of at least 1.75 @ 5°F
• 70% or better of heating capacity at 5°F relative to 47°F
• SEER2 rating of 15.2 or better
• HSPF2 rating of 7.8 or better

Performance ratings and temperature dependence

Whether you’re looking at SEER, HSPF, COP, or any other efficiency measure related to heat pumps and air conditioners, a caveat common to all of them is that the efficiency – meaning the amount of heat energy moved for each kilowatt-hour of electricity consumed – varies with the temperature difference between inside and outside.

In heating mode, the refrigerant in the outdoor unit is colder than the outdoor temperature. The fan in the outdoor unit blows air over the coils to warm up the refrigerant inside. (This is true even under very cold temperatures when it might not seem like there’s any heat energy available.)

The opposite happens in cooling mode: the refrigerant is relatively hot, and the fan in the outdoor unit blows over the coils to cool it down.

This is why heat pumps are less effective under temperature extremes. If it’s hot outside, it’s harder for the heat pump to cool the refrigerant down. If it’s very cold, there’s less heat available to warm the refrigerant up.

This tends to be more of a problem in cold temperatures, which is why there are standards for cold climate heat pumps. Very high temperatures are a problem too, although even in very hot climates the difference between indoor and outdoor temperatures is less than in cold climate zones. (If you live in a hot region, one measure you can take to improve the efficiency of your central air conditioner or heat pump is to install a screen around the unit to shade it from the sun, but it’s important to ensure that the screen doesn’t restrict airflow.)

The point here is that published measures are accurate only for a specific temperature - 95°F for SEER2 and 47°F for HSPF2, for example. To better understand how your heat pump might perform under colder temperatures, you will want to look its COP data. This can sometimes be difficult to find, but if yours is a cold climate model, the air-source heat pump database from the Northeast Energy Efficiency Partnerships (NEEP) will be useful.

Database of cold climate heat pumps and COP

If a heat pump is certified as a cold climate model, it should be listed in the NEEP database of cold climate heat pumps. Just a warning: it can be difficult to search for a specific model in the NEEP database. For example, you might search “XV18” to find the popular Trane XV18 model, but you will around 58 listings. This is because the XV18, like many heat pumps, have several sub-models that correspond to the different capacities available. There may also be separate listings that correspond to the different indoor and outdoor components in the system. To make sure that you have the right model number, you can look up the manufacturer’s specification sheet or ask your installer.

Once you’ve found a model that you’re interested in, click on it to view its specifications, including COP ratings at various temperatures. Any listed unit should have COP ratings @ 95°F and 47°F because those correspond to the SEER and HSPF test standards. Cold climate heat pumps should also have COP ratings @ 17°F and 5°F. The table will include other data, such as heat or cooling output at that temperature.

As an example, here are the key performance specs for one cold climate Lennox model:

Bottom line: the acronyms might be confusing at first, but are actually easy to understand

For the average consumer, HSPF and SEER are main specifications to know. With both, higher numbers mean higher efficiency, and new models that meet the 2023 federal standard will have an HSPF2 rating of at least 8.8 and a SEER2 rating of at least 14 in northern states and 15 in southern states.

COP can also be a useful number to know, especially if you are shopping for a cold climate model. It’s also a useful number if you’re trying to calculate exactly the amount of heat energy you can expect the unit to produce at lower temperatures, which might be useful if you’re trying to figure out the optimal temperature at which you should switch your heat pump over to a backup gas furnace. Calculating the cost efficiency of a heat pump versus a gas furnace and the optimal temperature at which you should switch to gas backup heat is the topic for a future article.