HSPF5 Vs HSPF4: Why 2 Heat Pump Efficiencies On Specs Sheets?

If you open heat pump specs sheets, you will see the HSPF5 rating and HSPF4 rating. Obviously, we want to know what is the heat pump HSPF rating – denoting the heating energy efficiency – but this HSPF5 vs HSPF4 rating might give us a pause. Let’s explain what both HSPF5 and HSPF4 ratings are and how to differentiate between the two.

Here is what HSPF is defined as:

HSPF stands for Heating Seasonal Efficiency Factor (‘Seasonal’ here plays the key role when we will differentiate between HSPF4 and HSPF5 further on). HSPF is defined as a ratio of heating output (heating BTU/hr) over energy input (electrical energy or wattage).

Example: A heat pump with a 12.0 HSPF produce 12,000 BTU/hr of heating output for 1,000W electric input. At a lower HSPF rating – let’s say 10.0 HSPF – this heat pump could produce only 10,000 BTU/hr of heating output for 1,000W electric input.

Now that we understand what HSPF is, it’s a bit unusual to be looking at HSPF4 and HSPF 5, right? Let’s look at some examples where these HSPF5 and HSPF4 ratings appear. Here is what you will find in some mini split heat pump specification sheets:

• 12,000 BTU Mitsubishi MSH12TN outdoor unit has an HSPF5 rating of 5.9 and an HSPF4 rating of 6.8. HSPF4 is 15% higher than HSPF5.
•  24,000 BTU MrCool Advantage series A-24-HP-WMAH-230B outdoor unit has an HSFP4 rating of 8.0 and an HSPF5 rating of 9.5. HSPF4 is 19% higher than HSPF5. Here is a screenshot of this specification sheet:

  

• 12,000 BTU Cooper & Hunter Hyper Heat has an HSFP4 rating of 12.0 and an HSPF5 rating of 10.5. HSPF4 is 14% higher than HSPF5. Here is how these HSPF5 and 4 ratings can be read off the EnergyGuide label:

  

You will notice that the HSPF4 rating is always higher than the HSPF5 rating. But how can the same heat pump have two different ratings?

We compare how energy-efficient heat pumps are by comparing their HSPF rating. How can we say that a 9.8 HSPF rated heat pump is more energy efficient than a 9.6 HSPF rated heat pump, if the first rating can be that high HSPF4 rating and the second one can be lower HSPF5 rating? If this were the case, the 9.6 HSPF heat pump would actually be more efficient than the 9.8 HSPF heat pump.

Let’s look at why HSPF4 and HSPF5 ratings are not one and the same:

HSPF4 Or HSPF5 Rating (Climate Zone Component)

Measuring heating seasonal performance factor has a lot to do with COP or Coefficient Of Performance. Heat pumps have a COP of around 3.7 at 47°F (this is the average temperature that HSPF ratings are measured as well). At below 10°F, the COP value can drop below 2.0. That’s why we always say that heat pumps are very efficient but can be inefficient at low temperatures. You can check the heat pump efficiency vs outdoor temperature graph here.

Where do HSPF4 and HSPF5 come into play here?

Essentially, the HSPF rating is a seasonal rating; it depends on the average outdoor temperatures. If you are living in cold climates, a heat pump will have a lower COP value and thus a lower HSPF rating. If you are living in warmer climates, the same heat pump will have a higher COP value and a higher HSPF rating.

Essentially, heat pumps extract heat from the outdoor air. It’s easier to extract heat from 47°F air than from 10°F.

That ‘4’ and ‘5’ in HSPF rating refer to the US climate zone where the HSPF was measured. The seasonal rating obviously depends on the average winter temperature in a specific climate zone. Here are all 8 US climate zones (note that zones 4 and 5 are about in the middle of the country):

As you can see, it’s quite simple to understand now why HSPF4 is always higher than HSPF5:

• HSPF4 is measured in Climate Zone 4. Winters in this area are, on average, milder in Climate Zone 5. Measurement of HSPF in Climate Zone 4 (HSPF4) will reflect that; at a higher temperature, air source heat pumps will have higher energy efficiency.
• HSPF5 is measured in Climate Zone 5. Winters in this area are, on average, colder in Climate Zone 4. Measurement of HSPF in Climate Zone 5 (HSPF5) will reflect that; at a lower temperature, air source heat pumps will have lower energy efficiency.

Here is the important note when choosing the most energy-efficient heat pump:

You have to compare HSPF ratings, yes. But you also have to make sure that you are comparing the HSPF5 rating to the HSPF5 rating, and the HSPF4 rating to the HSPF4 rating. If you mix these heating energy efficiency ratings up, you will likely be choosing a less energy-efficient heat pump in the end.

Now, if you either have only HSPF4 or HSPF5 rating, and the other unit has only one rating (HSPF4 or HSPF5), you will need to convert HSPF4 to HSPF5 or HSPF5 to HSPF4 in order to get an idea on which heat pump is more energy efficient.

Here is how you can convert these HSPF ratings:

HSPF4 To HSPF5 And HSPF5 To HSPF4

Because all of these HSPF ratings are measured, there is not an exact conversion formula for converting HSPF4 to HSPF5, and HSPF5 to HSPF4.

For a rough estimate, however, we can say (check the 14%, 15%, 19% HSPF differences in the example above) that the HSPF4 rating is about 15% higher than the HSPF5 rating. This is a rule of thumb we can use to convert between these two HSPF ratings.

Here is a chart that converts HSPF5 ratings from 6 to 12 to HSPF4 ratings:

With this table, you can put the HSPF specifications of heat pumps you are comparing on the same common denominator (either HSPF4 or HSPF5).

We hope that you have a better understanding of the nuances between HSPF4 and HSPF5 ratings and that you will know what to do when you are comparing HSPF5 vs HSPF4 in heat pump’s specs sheets.