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Correctly Sizing Variable-Capacity Heat Pump Equipment

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Bottom Line: The new ACCA Manual S® (2022) will allow heat pumps to be sized for heating when you follow the rules.


The growing use of renewable energy is pushing many communities and energy efficiency programs towards “electrification,” the shift to using all electrical products. This translates to an increased use of electric heat pumps as opposed to fuel-fired appliances. Heat pumps have always done a great job providing cooling, and some heating. Modern variable-capacity equipment provides for very robust heating capacity too.

When heat pumps are oversized in areas where there are humid summer conditions, comfort is often compromised, especially during milder (and wetter) shoulder seasons.  Another big problem with any type of equipment oversizing is excessive wear. Oversized equipment starts and stops far more often than right-sized equipment. The excessive starts jolt the compressor and motor windings into action, and a few minutes later, it shuts down. Original equipment manufacturers (OEM) do a great job building durable products. However, even great products will fail faster when they must start and stop thousands of times more often than right-sized equipment.

To increase comfort and reduce excessive wear, ACCA’s Manual S® sets boundaries for all types of residential equipment. This article focuses on variable-capacity heat pumps.

The Basics

This article quickly dives into draft language in the new Manual S® variable-capacity equipment sizing requirements. If you want to learn more about electrification, please look here. If you need an introduction to heat pump technology, you can learn much more in ACCA’s Manual H®. Finally, if you need an overview of variable capacity heat pump equipment, I recommend you speak with your favorite distributor.

The Terms

To understand how Manual S® developed its sizing guidance, it’s important to know a few key terms: equipment capacity, minimum compressor capacity, heating loss, and size factor.

Equipment Capacity: Variable-capacity equipment produces a wide range of heating/cooling over a wide range of operating conditions.  Some OEMs do this better than others. The only way to tell how the equipment you sell will perform is to look at the OEM’s performance data.  This will tell you how it works in your area because Seattle is different than Phoenix, which is different than Denver, which is different than Miami.

Variable-capacity equipment can operate at several different speeds, but each product has a maximum and minimum compressor speed. So, to use the new heat pump sizing guidance, the OEM will need to provide the capacity at these speeds for a range of operating conditions, and the maximum and minimum compressor speeds.

Minimum Compressor Capacity: This is the lowest capacity documented in the performance data at the operating conditions. Also known as Minimum Capacity (MinC). The new Manual S® requires that the equipment is sized to meet the load and that the lowest compressor speed must be less than the design heating load.  Manual S sets this new limit at 0.80 (80 percent) of the heat loss. A 45,000 BTU/h heating load would need a heat pump with a minimum compressor heating capacity of 36,000 BTU/h. Another way to think about this is that the least amount of heat that the variable-capacity equipment can produce must be 20 percent smaller than the heating load. This ensures that the heating system cannot satisfy the heating load at low speed. This new definition and sizing metric opens the door to use heat pumps to be the sole source of heat in a home.

Heating Loss: Homes lose heat at different rates for different reasons. Location: homes in Chicago lose heat fast than homes in Houston.  Construction: tight and well-insulated homes lose less heat than leaky uninsulated homes.  You get the idea. Perform a rigorous Manual J® load calculation to determine the heat loss and gain.

Size Factor: This is the equipment capacity divided by the load. Since there are different types of capacities and loads (heating, total cooling, sensible cooling, latent cooling, etc.), there are also different size factors. The intent of these terms was to simplify the guidance, create concise requirements, and make them easier to understand and use. This article discusses how to use the minimum compressor (heating) size factor.

New Manual S® Guidance – Variable-Capacity Equipment

Manual S® requires the heat pump to meet the heating and cooling requirements:


  • The cooling capacity must be between 1.30 and 0.90, and
  • The latent cooling capacity must be greater than or equal to the latent load.


  • Meet the heating load at maximum heating capacity (the heating size factor must be 1.00 or greater), and
  • The minimum heating compressor heating size factor must be less than or equal to 0.80. As mentioned, the variable-capacity equipment can produce must be 20 percent smaller than the heating load.


Example: Daytona Beach, FL


Design Temperature: 39F

Heat Loss: 17,095 BTU/h

Heating Capacity – High: 18,682 BTU/h (109 percent)

Heating Capacity – Low: 12,982BTU/h (76 percent)

The heating capacity neatly falls between the heat pump’s maximum and minimum capacities and meets the Manual S® requirement to meet the heating load. The minimum compressor heating meets the “under-sized” requirement (80 percent) so it will not suffer from excessive short cycling.


Design Temperature: 91F

Heat Gain (Total): 22,357 BTU/h

Heat Gain (Sensible): 20,144 BTU/h

Heat Gain (Latent): 2,213 BTU/h

Cooling Capacity – High: 22,700 BTU/h (102 percent)

Latent Cooling Capacity: 5,406 BTU/h (244 percent)

Winter heating is necessary, but in Daytona, cooling is king.  This unit provides the cooling capacity to meet the load and has an abundance of latent capacity.  In the previous version of Manual S® (2014) there was a limit to the latent capacity, the new manual removed this limitation.

ACCA’s Manual S® must complete the public review process before it is published.  Then it can be adopted by code, or promoted by an energy efficiency program, or accepted by your local jurisdiction. This will provide the option to select a heat pump to deliver all of the home’s heating needs.

Give your customers the comfort they want and a system that will serve them for years to come.
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