Heat pump sizing guide

In this guide we’ll go over everything you need to know about the importance of correctly sizing your home’s heat pump system.

We’ll cover:

• The benefits of correctly sizing a heat pump 

• The problems with undersized and oversized heat pumps

• Why rules of thumb and online calculators are almost always wildly inaccurate

• How to properly size your heat pump

Why correctly sizing a heat pump matters

Heat pumps provide heating and cooling to your home all year round. And to maximize their comfort and cost-savings magic, precise sizing is key.

Undersizing a heat pump can create major problems. 

If you undersize your heat pump it will struggle to keep your home warm on the coldest days (though a properly sized and selected heat pump definitely works in cold climates). If you have backup electric resistance heat, also referred to as heating strips, your home will stay warm, but you could end up spending a fortune on electricity as heat pumps have higher efficiency compared to electric resistance heat or heat strips.

Oversizing a heat pump is bad, too. Heat pumps exist in single, double, and variable-speed models, with most popular brands offering variable-speed heat pumps.

Rather than kicking on and off constantly, variable-speed heat pumps can provide a steady comfortable temperature. But if you oversize your unit, even a variable-speed model will kick on and off frequently, as its lowest speed might still overheat your home.

In humid parts of the country this is even more of a problem, because heat pumps also act as dehumidifiers. A properly sized system will constantly pull moisture from the air, leaving you comfortable and your home mold-free. An oversized system will spend most of its time off, allowing moisture to build up. 

But even if you live in a dry climate, an oversized system will result in a less comfortable home. The more time a heat pump runs, the more time it has to distribute warm or cool air throughout your home. If it’s constantly kicking on and off, you’ll end up with hot and cold spots instead of an even, uniform temperature throughout your home. 

Oversized heat pumps also come with financial costs. Besides the higher upfront cost, an oversized unit will have higher operating costs and will need to be replaced sooner than a properly sized unit, since kicking on and off constantly leads to more wear and tear.

Why you shouldn’t trust rules of thumb

If you Google “heat pump calculator,” you’ll probably find a rule of thumb like this: “You need 30 BTUs of heat for every square foot of living space you want to heat or cool.” If you have a 2,000-square-foot home, this rule of thumb suggests you need a 60,000 BTU heat pump.

Call up a contractor and you may hear something similar. For decades, most contractors have sized HVAC systems like heat pumps, by dividing the total square footage of a home by 500 to estimate how many tons of heat a home needs. 

If you have a 2,000-square-foot home, this rule of thumb suggests a 4-ton system. One “ton” in heating-speak is equal to 12,000 BTUs. So this contractor rule of thumb would tell us that we need 48,000 BTUs. 

But this rule of thumb is almost always inaccurate! 

Using data from the Massachusetts Clean Energy Center we compared how the conventional calculation compares to real heating load data. 

If a home is 1,000 square feet, the first rule of thumb would suggest a 30,000 BTU heat pump; the second rule of thumb would suggest a two-ton (24,000 BTU) heat pump. We compared those two numbers to the actual heating load from the MassCEC data (as determined by the industry-standard Manual J test).

Not a single homeowner would have installed the right heat pump if they used the first rule of thumb! In fact, the closest heating load was still off by 12,000 BTUs, or one ton. On average, this rule of thumb oversized heat pumps by 31,000 BTUs. That’s a pretty big price difference between units and a lot of really uncomfortable homes.

The second rule of thumb, dividing by 400, provided more accurate estimates. In some cases it was within a few hundred or a few thousand BTUs.. But 30% of the heating loads were oversized by more than one ton (12,000 BTUs). What’s more (and arguably much worse) is that 32% of homeowners would end up with a seriously undersized unit, meaning some uncomfortable winter nights or expensive backup resistance heat. 

Moral of the story? Poorly sizing heat pumps can defeat the purpose. And using rules of thumb or online calculators won’t cut it.

So, how do you properly size a heat pump?

The best way to ensure your heat pump is accurately sized is to hire a contractor to complete a blower door test, a Manual J load calculation, and a Manual S sizing calculation.

A blower door test uses a door-sized piece of canvas and a large fan to measure how tightly your home is sealed. The test determines your "blower door number," or the air leakage in your home, by measuring ACH (air changes per hour) or CFM (cubic feet per meter). The ACH or CFM helps determine whether air sealing would be a good investment before installing a heat pump and whether ventilation work might be necessary. These numbers help your contractor understand how large a heat pump you will need to heat and cool your home.

A residential load calculation determines the heating and cooling needs of your home based on a ton of factors. Where you live, how many windows you have, the quality of your insulation, the number of appliances you have and much more will help your contractor determine what is needed to keep your home comfortable all year round.

The most popular residential load calculation is called a Manual J, which refers to the technical manual to complete it created by the Air Conditioning Contractors of America (ACCA). 

Other residential load calculations might include whole-building energy modeling or black box modeling, where a contractor uses historical data on a home’s HVAC system performance. Unlike whole-building energy modeling, Manual J is specific to HVAC system design. And unlike black box modeling, Manual J takes into account the particulars of your existing system and accounts for extreme climate conditions and the behaviors of home occupants. 

A Manual S sizing calculation (also created by ACCA) helps the contractor determine what size and type of heat pump will work best for your specific home. This calculation is key in ensuring that the system your contractor installs meets the loss and gain of heat that your home experiences throughout the year. If your system is significantly undersized or oversized, your system won’t provide the efficiency and comfort that makes heat pumps so great. Even worse, system replacement may rival or exceed your installation costs! 

Precision-fit heat pumps deliver the best — so always ask for a Manual S!