Understanding HSPF2 Rating For Heat Pumps (New 2023 Metric)

In HVAC, we deal with a lot of energy efficiency ratings. In order to determine energy efficiency as accurately as possible, new metrics are being introduced over time. HSPF2 rating or Heat Seasonal Performance Factor 2 rating is just such an example, and it goes into effect on the 1st of January 2023.

This change will likely impact the 2023 minimum HSPF rating (from 8.8 HSPF to 7.5 HSPF2) as well as eligibility for tax credits.

Most of us are already familiar with the HSPF rating. This is (well, was) the main heating energy efficiency rating for heat pumps, furnaces, and water heaters. We have explained what the HSPF rating means here.

In short, the HSPF rating that ceased to be used on the 31st of December 2022 was calculated by dividing heating output during the heating season (in BTUs) by the amount of electricity used to create that output (in kWh).

HSPF2 rating follows the same formula as the HSPF rating but under harsher test conditions. We will see the effect of these harsher conditions further on but here is a good summary by the US Department Of Energy on HSPF2 vs HSPF rating (from 10 CFR Parts 429 and 430 document):

“DOE also reviewed test data and confirmed that the 15% reduction from HSPF to HSPF2 that DOE observed for split-system and single-package heat pumps.” (DOE on new HSPF2 rating)

heat pump energy efficiency for heating
Same heat pump will have a 15% lower new 2023 HSPF2 rating compared to the now older HSPF rating.

Example: Let’s say that a heat pump generates 10,000 BTU of heating output under 2022 and prior test conditions. For this, it needs 10 kWh of electric input. According to the new harsher conditions for 2023 and the ongoing HSPF2 rating, the same heat pump would produce only 8,500 BTU of heating output.

Here is how we can calculate both HSPF and HSPF2 rating for the same heat pump:

HSPF Rating = 10,000 BTU / 1,000W = 10 HSPF

HSPF2 Rating = 8,500 BTU / 1,000W = 8.5 HSPF2

What is the reason for this sudden difference between HSPF and HSPF2?

These energy efficiency rating are very useful when you want to determine which heat pump to buy. Heat pumps with higher HSPF rating are more efficient and will provide you with energy savings.

Given that we already have the HSPF rating, here is the US Department Of Energy introducing this new HSPF2 rating:

HSPF2 Rating Explained

The whole point of energy efficiency ratings is to accurately determine the realistic energy efficiency. With the introduction of HSPF2 (as well as SEER2 and EER2), DOE has devised a new metric that more adequately determines the real-life energy efficiency of heat pumps.

The change between HSPF and HSPF2 is a rather small one but it has big consequences (we will talk about how the minimum required HSPF rating (now HSPF2 rating) changed from 2022 to 2023 and forward).

Namely, DOE has a set of test conditions when determining HSPF rating. One of these test conditions is the external static pressure. This is the pressure the vent is at. Here is the key difference in test procedures for HSPF vs HSPF2 ratings:

  • HSPF rating uses 0.1 inches of water column (in. WC) external pressure test condition.
  • HSPF2 rating uses 0.5 inches of water column (in. WC) external pressure test condition. Basically, when measuring HSPF2 rating, the external pressure is 5 times higher external pressure used when measuring HSPF rating.

Clarification: Inch of the water column is a unit of air pressure. 1 in. WC is equal to 0.0361 psi or about 2.5 mbar.

When you increase the external static pressure, the blower motor has to work harder to expel air (burn more watts for the same amount of work), and that’s why the HSPF2 rating cannot be as high as the HSPF2 rating. We talk about harsher test conditions.

In short, DOE deemed the external pressure test conditions used for HSPF to be too low for adequately capturing the real-time conditions heat pumps operate in. That’s why the increase from 0.1 in. WC to 0.5 in. WC external pressure.

The result of changing these test conditions is that we will no longer talk about the HSPF rating; we will talk about this new HSPF2 rating.

A relevant result for homeowners and manufacturers alike is that this changes the minimum HSPF rating (or minimum HSPF2 rating now).

New Minimum HSPF Rating

DOE usually dictates the minimum HSPF rating for air-source heat pumps. Here are the historical minimum HSPF ratings (Source: US Energy Information Administration):

  • Years 1992 – 2005: Minimum HSPF rating is 6.9 HSPF.
  • Years 2006 – 2014: Minimum HSPF rating is 7.7 HSPF.
  • Years 2015 – 2022: Minimum HSPF rating is 8.2 HSPF.
  • Years 2023 and onward: Minimum HSPF rating is 8.8 HSPF.

Now, this is not surprising at all. The energy efficiency of newer heat pumps is higher and the federal government stipulates what is the minimum HSPF rating these heat pumps should have. Obviously, they have higher HSPF ratings.

From the US EIA listing above, it seems quite clear that manufacturers in 2023 can only sell heat pumps with an HSPF rating equal to or higher than 8.8.

However, here comes a conundrum:

In 2023, we don’t have an HSPF rating anymore. We have the new HSPF2 rating.

Inevitably, the new minimum heating energy efficiency rating in 2023 won’t be 8.8 HSPF. Instead, we have to convert this 8.8 HSPF rating into the HSPF2 rating.

As the DOE has discovered, the new 5 times higher external pressure test conditions reduce the heating seasonal performance factor by 15%.

Based on this we can calculate the minimum HSPF2 rating in 2023 like this:

Minimum HSPF2 Rating = 8.8 HSPF × 0.85 = 7.5 HSPF2

In short, DOE is making these two changes as far as the minimum HSPF rating requirement for heat pumps from 2022 to 2023:

  1. Increase of HSPF rating from 8.2 HSPF (established in 2015) to 8.8 HSPF.
  2. Changing HSPF test conditions and thereby introducing a new HSPF2 rating (that is 15% lower than the standard HSPF rating).

In 2023, we will start seeing more and more HSPF2 ratings on heat pump specification sheets and EnergyGuide labels. We hope that this explanation is astute enough to adequately answer what the HSPF2 means for heat pumps.

3 thoughts on “Understanding HSPF2 Rating For Heat Pumps (New 2023 Metric)”

  1. Good article, but a couple of clarifications:
    *In the air source heating cycle a HSPF of 10 would only take 1 KW to produce 10,000 btu/hr of heat. 10,000 BTU/HR / 1,000 watts = 10 BTU/watt-hr.
    *ESP is external static pressure, not the pressure at the vent. It is the total system pressure drop outside of the blower (or furnace) fan box. It typically includes the pressure drops through the return air grille, supply air grille or diffuser, ductwork, intake and discharge plenums, and usually includes filter and coil pressure drop. For residential applications anywhere from 0.3″ to 0.8″ is usually in the ballpark, so moving the standard from 0.1″ to 0.5″ was appropriate.

  2. We may now see an even greater difference between ducted and ductless systems. Under HSPF, it was typical to see a lower HSPF for a ducted system, because more energy was used to distribute the heat through air ducts. Under HSPF2, we will likely see an even greater difference. This is good, because it gets closer to a real-world results, which is the reason for the change. It will help with decision-making, and could result in more heat pump projects choosing ductless based on lower operating costs.


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