If you are in the market for a new heat pump for your HVAC, chances are you've seen the terms SEER, SEER2, EER or EER2 to describe the appliance's cooling efficiency and HSPF or HSPF2 to describe its heating efficiency. In Heat Pump Review , we'll cover all of these terms, but in this post we'll dive deep into wonderful world of HSPF/HSPF2 to help explain how it's measured and how it compares to other performance metrics, hopefully making it easier for our readers to interpret efficiency standards along the way.

Definition

HSPF stands for Heating Seasonal Performance Factor , and is calculated by dividing the heating output of an appliance measured in BTUs by the electricity input to generate that heat measured in watt hours, per season of use.

Diving into this equation, you're likely already familiar with how watt hours are measured (as your electric bill is likely reported in increments of 1,000 of them, or kWh), but if you're like me and needed a refresher on BTU s remember it's the measure of energy required to raise 1 lb of water 1 degree F (in a way, it's like a silly imperial calorie). If you have natural gas on your energy bill you're used to seeing it expressed in 100,000 BTU increments, or "therms."

HSPF2 follows the same equation as HSPF, but it is a better defined standard, requiring more specific and harsher testing conditions than the original metric, lowering the average outdoor temperature used for testing (aligning with Minnesota's harsh winters), as well as increasing the static pressure in the system to better reflect the real load on your system's blower. If you want to dive really deep into these requirements, see this rule published by the US Energy Department.

What is a good HSPF2 rating?

Perhaps an easier question to answer to start is "what's not bad?"

Here, we turn to the famous Energy Star program, run by our Department of Energy. Energy Star helpfully sets a minimum of 8.5 HSPF2 for ductless mini-split air-source heat pump systems to achieve certification, while ducted split systems and "single package" ducted system need to achieve at least 8.1 HSPF2 ( see standards ).

In order to qualify for the Inflation Reduction Act's (IRA) 30%/$2,000 rebate , however, things get a little more complicated: for ducted systems one simply needs to buy an Energy Star certified model (assuming they also meet some cooling criteria not outlined in this article). But for ductless systems, the bar gets raised higher, with residents of the "Southern" Climate Region ( defined by the Consortium for Energy Efficiency , or CEE, an NGO relied on by the Energy Department), needing to meet an HSPF2 of 9.0 while Northern residents need to purchase a system of at least 9.5 HSPF2. This gives us a low end of "good."

On the high end, a Heat Pump Review analysis of over 100K models tracked by Energy Star – for both ducted and ductless heat pump systems – found that while most models hover around this minimum requirement, there are hundreds of heat pump models available between 11.5 and 13.5 HSPF2 for mini-split systems and hundreds around ~10 for ducted systems, giving consumers plenty of options at the top end of the efficiency range. To explore more about some of these models, have a look at Energy Star's list of "2024 Most Efficient" ducted air source heat pump , mini-split , and geothermal systems (geothermal is not covered in the rest of this article, as they still certify using EER, a different efficiency metric geared towards cooling efficiency).

Comparing HSPF2 to HSPF Results

HSPF2 is still relatively new, becoming the standard as of January 1, 2023. That means many manufacturers are still selling models that only have HSPF ratings and we may need to / want to compare the two standards. Because HSPF2 has strict testing standards, and HSPF did not, there is not a universal conversion factor; however, many folks just use 0.85 (in that HSPF2 is 15% lower) as a rough rule of thumb.

Diving a little deeper, we found this reference produced by Washington State's Building Code department, which offers different conversion factors per appliance type, with ducted systems showing the same ~0.85 conversion factor but ductless much closer to HSPF2 at 0.96 (ostensibly because the static pressure of a ducted system adds more load to the fans/blowers, while ductless does not). See table below (avg conversion factor line added by me):

1	HSPF	Ducted Split HSPF2	Ducted Package HSPF2	Ductless HSPF2
2	8	6.8	6.7	7.7
3	8.2	7	6.9	7.9
4	8.8	7.5	7.4	8.4
5	9	7.7	7.6	8.6
6	9.5	8.1	8	9.1
7	10	8.5	8.4	9.5
8	11	9.4	9.2	10.4
	Conversion Factor:	0.85	0.84	0.96

Comparing HSPF2 to COP

Another heating efficiency metric you are likely to see is COP, or Coefficient of Performance . COP is used more extensively in Europe and only measures a heat pump's compressor performance, not the full system's performance, and is done at a set operating environment, usually 5 degrees F. From there, COP is a similar equation, with the amount of heat being added or removed put in the numerator and the "work" required to add or remove that heat in the denominator. While COP is theoretically more a more pure measure of performance, HSPF2 is considered a more realistic measure of performance because it takes into account the energy used to operate the full machine – namely the blower for ducted systems and any other fans or moving parts used to operate the machine – and it takes into account seasonality (while unlikely, a machine to could be optimized to work in the COP environment while doing poorly in other environments).

Given that nuance, there is no definitive conversion factor, so we are forced to make one. In a Heat Pump Review analysis over 100K Energy Star models where both HSPF2 and COP are reported, we find an average HSPF2 to COP ratio of 0.227 for ducted systems and 0.192 for ductless mini-split systems .

Comparing HSPF2 to AFUE

AFUE, or Annual Fuel Utilization Efficiency, is the efficiency at which a fossil fuel-based furnace converts fuel into heat, following a similar algorithm as HSPF but with input BTU in the denominator).

Why, then, do we care about converting HSPF2 to AFUE? Well, heat pumps still very much compete against conventional natural gas models , and the largest cohort we need to convert to heat pumps in the coming decade are those who will be rolling off of fossil fuel furnaces. Being able to compare the two allow us to think more clearly about both the financial and CO ₂ e costs of our investments.

While again there is no published standard way to compare HSPF2 to AFUE, we can contemplate it two ways:

CO ₂ e Conversion

Let's say that in a given season we need to produce 400 therms (40,000,000 BTU) to heat a home. With a typical AFUE 80 natural gas furnace, this would require burning 500 therms of fuel, which – according to the EPA's handy calculator – would produce a whopping 5,832 lbs of CO ₂ e during the season.

Using the same CO ₂ e calculator, along with the HSPF2 algebra above of BTU/watt hours, we can quickly find that an equivalent HSPF2 would be 6.5 , giving us CO ₂ e conversion factor of 12.3:1 HSPF2 to AFUE. Said differently, using today's North American electrical grid mix, a heat pump with an HSPF2 of 6.5 would produce the same amount of CO ₂ e as our old, conventional, fossil fuel burning furnace. Also of note, this calculation does not account for the electricity required for the gas furnace's blower, which can draw upwards of 1kW while in use).

The good news is that you are not going to buy any heat pump with an HSPF2 of 6.5. As we outlined above, most heat pumps sold today are above 8, with the minimum requirement for a ducted system to achieve an Energy Star rating (required to receive an Inflation Reduction Act rebate) being 8.1 and ductless 8.5.

While the top of the line gas furnaces today can get very close to an AFUE of 100, using the conversion factor above you can see that you would need an AFUE of 99.6 to compete with the bare minimum standards for new ducted systems and 105 to meet the minimum standards for ductless systems (the latter, of course, defying the first law of thermodynamics); and again that is a generous comparison for natural gas, as AFUE does not take into account the energy input of the system's blower.

Financial Conversion

While the conversion of therms to CO ₂ e is normalized in all markets and kWh to CO ₂ e somewhat normalized across markets (with renewable-heavy grids being much more favorable than those with coal), a financial comparison can be all over the place depending on energy rates in your region and market. In Maryland, where retail cost / therm is ~$1.91 and retail cost / kWh is ~$0.158, converting from gas to electric without also buying one of the top-of-the-line models may not be cheaper. Again, using our 400 therm heating requirements above, with an AFUE 80 system we can expect to pay ~$764 per season (not counting electricity for the blower). If we were to buy a non-Energy Start equivalent (we wouldn't) and match on CO ₂ e, that would require 6,154 kWh of electricity, or ~$972. However, if we were to get a top of the line ducted system at ~10 HSPF2, we would only need to use 4,000 kWh at $632 - an annual savings of $132. Meanwhile, for those of us with rooftop solar producing surplus kWh, of course this amount can improve even further.

Conclusion

HSPF2 is the new standard for air source heat pump efficiency, and should be maximized as you purchase your next unit in order to maximize both the environmental and financial benefits of electrifying your home heating. As grids continue to get cleaner, these gains are likely to improve over their conventional fuel counterparts (which can never get cleaner), improving the returns over time.