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Part 3: A Deep Dive into the Efficiency Characteristics of Air Source Heat Pumps

A melting ice cube.

A melting ice cube.

This entry is part 3 of 3 in the series Out of Thin Air a Cozy Home: Air Source Heat Pumps

Dear Reader;

Let’s take a closer look at the efficiency characteristics of ASHPs, including SEER, EER, COP, HSPF, HSPF regions, capacity, and cut-off temperature. We will finish up with the important of matching the various components of an air source heat pump heating system. So, let’s dive in and uncover the secrets of these magical devices.

Efficiency Characteristics of Air Source Heat Pumps

SEER (Seasonal Energy Efficiency Ratio)

SEER measures the cooling efficiency of an ASHP over an entire cooling season. It is calculated by dividing the total cooling output (in BTUs) by the total electricity input (in watt-hours) (AHRI, 2021). The higher the SEER rating, the more energy-efficient the system.

Imagine a car’s fuel efficiency, measured in miles per gallon (MPG). The higher the MPG, the more efficient the car is at using fuel. Similarly, the higher the SEER rating, the more efficient an ASHP is at cooling your home.

SEER ratings typically range from 13 to 26, with higher values indicating greater efficiency. It is becoming competitively common to find air source heat pumps of 18 SEER or more.

EER (Energy Efficiency Ratio)

EER is a snapshot of an ASHP’s cooling efficiency at a specific outdoor temperature, usually 95°F (35°C). Like SEER, it is calculated by dividing the cooling output by the electricity input (AHRI, 2021). EER is useful for comparing the efficiency of different ASHPs under identical conditions.

Think of EER as a sprinter’s performance in a 100-meter race. The athlete’s time represents their efficiency at any specific distance. EER, in the same way, represents an ASHP’s efficiency at a specific outdoor temperature.

EER ratings typically range from 8 to 14, with higher values indicating greater efficiency.

If you find an EER rating but all your comparable products are listed SEER, it is possible to convert the EER to SEER. But it can be pretty math intensive. Head to Google’s search engine and type “convert EER to SEER calculator.”

COP (Coefficient of Performance)

COP measures the heating efficiency of an ASHP by comparing the amount of heat energy output to the amount of electrical energy input. A higher COP indicates a more energy-efficient system.

Imagine a baker who can create 10 loaves of bread using the same amount of energy that another baker uses to make only 5 loaves. The first baker has a higher “baking COP” because they are more efficient at converting energy into baked goods. Similarly, an ASHP with a higher COP is more efficient at converting electrical energy into heat.

COP values typically range from 1 to 5, with higher values indicating greater efficiency. HSPF discussed next, is a more practical way of assessing heat pump efficiency. If you run into this COP and would like it’s HSPF representation you can “math up” this following equation on a good day:

COP = -0.026 × HSPF2 + 0.624 × HSPF

HSPF (Heating Seasonal Performance Factor)

HSPF measures the heating efficiency of an ASHP over an entire heating season. It is calculated by dividing the total heating output (in BTUs) by the total electricity input (in watt-hours). A higher HSPF rating indicates a more energy-efficient system.

HSPF is to heating what SEER is to cooling – both provide a measure of an ASHP’s efficiency over an entire season.

HSPF values will be a key point for you to compare different heat pumps prior to purchasing and typically range from 6 to 10, with higher values indicating greater efficiency. A value of 10 or more is becoming very common.

HSPF Regions

There are five HSPF regions of concern, each with different minimum efficiency requirements for ASHPs based on climate conditions. These regions help ensure that HVAC technicians install ASHPs with appropriate efficiency levels for their local climate. Higher HSPF regions are associated with colder climates.

Think of HSPF regions like clothing sizes, which are designed to fit people of different heights and body shapes. In the same way, HSPF regions help match ASHPs to the climate conditions of different geographical areas, ensuring optimal performance.

HSPF region ratings take into account the local climate and temperature fluctuations in different parts of the country. Heat pumps are tested and assigned a HSPF rating based on their performance in specific regions.

The efficiency rating associated with one HSPF region or another shows it’s capability, under testing conditions, to perform as well as it can. So an HSPF region 5 efficiency rating is a more stringent value to assess than HSPF4 if you live in a colder climate.


Capacity refers to the amount of heating or cooling an ASHP can provide, usually expressed in BTUs per hour (BTU/h). Selecting an ASHP with the correct capacity is crucial for maintaining comfort and energy efficiency in your home.

Imagine a pair of shoes: if they’re too small, your feet will feel cramped and uncomfortable, but if they’re too large, they won’t provide proper support. Similarly, an ASHP with the wrong capacity will struggle to maintain comfort and may waste energy.

Air source heat pumps are sometimes listed with capacities in kilo-Watt-hours (kWh) or tons for heating as well as cooling.

12,000 BTU/h is one ton of heating or cooling,

About 12,000 BTU/h (11,942.5) is 3.5 kWh of heating or cooling, and it follows that,

one ton of heating or cooling is roughly equal roughly to 3.5 kWh.

Cut-Off Temperature

The cut-off temperature is the lowest outdoor temperature at which an ASHP can efficiently operate. Below this temperature, the system may require a backup heat source, such as a furnace, to maintain indoor comfort.

Think of the cut-off temperature as the point where a hot cup of coffee becomes too cold to enjoy. Just as you might reach for a fresh cup when your coffee cools, an ASHP may need backup heating support when the outdoor temperature drops below its cut-off point.

Components of an Air Source Heat Pump Heating System

The Coil or Indoor Unit

The coil, also known as the indoor unit or evaporator, is a critical component of an air source heat pump (ASHP) system. Its primary function is to absorb heat from the indoor air during the cooling mode and release heat into the indoor environment during the heating mode. The coil comprises a series of refrigerant-filled tubes, surrounded by fins that promote efficient heat transfer. As air passes over the coil, the refrigerant within the tubes either absorbs or releases heat, depending on the mode of operation. Ensuring that the coil is clean and functioning properly is essential for maintaining the overall performance and efficiency of the ASHP system.

The Condenser or Outdoor Unit

The condenser, or outdoor unit, is another vital component of an ASHP system. It houses the compressor, which is responsible for pressurizing the refrigerant and enabling the heat transfer process between the indoor and outdoor environments. The condenser also contains a coil, similar to the indoor unit, which either releases heat into the outdoor air during cooling mode or absorbs heat from the outdoor air during heating mode. The outdoor unit’s performance is crucial for the ASHP system’s overall efficiency and effectiveness, and it is essential to ensure that it is properly maintained, free of debris, and protected from harsh weather conditions.

The Air Handler or Furnace

The air handler, also known as the furnace or blower, is responsible for distributing conditioned air throughout the home. In an ASHP system, the air handler works in conjunction with the coil or indoor unit, moving air across the coil to facilitate heat transfer between the refrigerant and the indoor air. It is essential for the air handler to be properly sized and maintained to ensure optimal airflow, energy efficiency, and indoor comfort. Proper ductwork design, insulation, and sealing are also crucial in maximizing the performance of the air handler and the overall ASHP system.

The Importance of Matching Components Based on the AHRI Directory

Matching the components of an ASHP system, such as the coil or indoor unit, the condenser or outdoor unit, and the air handler or furnace, is crucial for achieving optimal performance and efficiency. The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Directory serves as a valuable resource for homeowners and professionals to ensure that the components of an ASHP system are compatible and meet the required efficiency standards. Using the AHRI Directory to select a matched system helps guarantee that the ASHP components will work together seamlessly, providing reliable and energy-efficient heating and cooling for your home. – Matching components for efficiency ratings

A quick word on navigating the AHRI directory. Using the search field can be quite frustrating, believe me. Search in this way and you should retrieve the listing. Once you’re viewing the listing, write down the AHRI Reference number. This will allow you to search the same system on rebate databases and clarify estimates from contractors. There is only one AHRI number per designed system! But thousands of model numbers for coils, condensers, and air handlers.

Using the AHRI Directory:

  1. Navigate to the website link above.
  2. Choose the model number radio button (below the search field).
  3. Enter the model number for the indoor coil unit.
  4. Leave a single space.
  5. Enter the model number for the outdoor condenser unit.
  6. Leave a single space.
  7. Enter the model number for the air handler unit.
  8. Click the orange search button next to the search field.
  9. If there is no match, then there is no guarantee of full system operating efficiencies. Some systems made by Tosot, for example, are still in the process of registering. This doesn’t mean their engineers haven’t designed an efficient system by any means.
  10. Make sure the status column shows ‘production stopped’ or ‘active.’

The world of air source heat pumps is a fascinating one, filled with an array of efficiency characteristics that can help homeowners and professionals make informed decisions. Understanding metrics like SEER, EER, COP, HSPF, HSPF regions, capacity, and cut-off temperature can empower you to choose the right ASHP for your specific needs and climate. We hope you’ve enjoyed this three-part series and that air source heat pumps are little less mysterious.

Series Navigation<< Part 2: Choosing, Installing, Maintaining, and Operating Air Source Heat Pumps

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