Table of contents
Heat Pump Sizing Guide
In this guide we’ll go over everything you need to know about sizing a heat pump for your home.
- 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
Sizing a heat pump is one of the most important parts of the installation process. But many homeowners, and even experienced installers, don’t get it right.
Incorrectly sizing a heat pump comes with some major problems.
The problem with undersizing
Undersize your heat pump and it will struggle to keep your home warm on the coldest days. If you have backup electric resistance heat, also referred to as heating strips, your home will stay warm, but you could spend a fortune on electricity.
The problem with oversizing
Most heat pumps have variable speed motors, meaning they adjust how much heating or cooling they provide based on the temperature outside. 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 a steady temperature throughout your home.
Oversized heat pumps also come with financial costs. Besides the higher upfront cost, an oversized unit 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’ll probably 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 these rules of thumb are almost always wildly inaccurate.
For the last few years, the Massachusetts Clean Energy Center has been publishing data on the heating loads of homes in the state. It’s a great dataset because the state requires Manual J and blower door tests (which we’ll explain below). Using this data, I decided to see how the rules of thumb I mentioned above compared to this actual heating load data.
For every home in the dataset, I ran the calculation based on each of the rules of thumb I mentioned above. For example, if a home was 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. Then I compared those two numbers to the actual heating load of the home, as determined by the industry standard, Manual J test, which I’ll explain in the next section.
So what did I find?
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 pretty uncomfortable homes.
And Massachusetts is a cold state. Imagine how much this rule of thumb would oversize heat pumps in relatively moderate Virginia or Washington.
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, which isn’t bad. But 30% of the heating loads were oversized by more than one ton (12,000 BTUs). What’s more (and arguably much worse), 32% of homeowners would end up with a seriously undersized unit, meaning some uncomfortable winter nights or expensive backup resistance heat.
All of that is to say: rules of thumb and online calculators are a really bad way to size a heat pump.
How to size a heat pump?
There are two ways to accurately size a heat pump. One involves an experienced contractor or building performance contractor. The other requires a smart thermostat.
Get a Manual J Load Calculation and Blower Door Test
The safest and easiest way for a homeowner to figure out what size heat pump they need is to work with an experienced contractor or get an energy audit.
In both scenarios, someone will come to your home and do what’s called a “blower door test,” in which they put one of these things in your doorway:
This blower door test, combined with some other data, like the climate of the city you live in, will help them produce a Manual J Load Calculation, which is the industry standard way to size an HVAC system.
Here’s an example of the energy audit I got for my house before we made any upgrades. If you look at the bottom section, you’ll see the heating and cooling load calculations. Our 2,200 square foot home was incredibly leaky and had a massive heating load when we moved in.
Use smart thermostat data
If you have a smart thermostat and a furnace, you can estimate your home’s heating load without a blower door test. Here’s how you can do it.
First, you’ll need to know how many BTUs of heat your furnace provides. I found the manual sitting next to my furnace in the crawlspace, then looked up the model number on Google. My natural gas furnace was 100,000 BTUs.
Next, you’ll need to figure out the “Winter Design Temperature” of your local climate. To quote Allison Bailes at Energy Vanguard: “This is the outdoor temperature that your location stays above for 99% of all the hours in the year, based on a 30-year average. Turning it around, the outdoor air where you live is going to be colder than this temperature for only 1% of the hours in a year. That happens to be about 88 hours per year. In Atlanta, the 99% winter design temperature is 23° F.”
You can look up the design temperature of your county here. Mine in Boulder, Colorado is -2° F.
Finally, you’ll need to figure out how many hours your furnace runs when it’s that design temperature.
To find my design-temp day, I used Wunderground’s historical weather data and found a cold stretch in January, when the temperature was consistently close to the design temperature. I wrote down five hours, where the temperature was -2 degrees.
Then I went into my Nest smart thermostat app, looked at historical usage, and I wrote down how many minutes the furnace ran during those hours:
- 3-4pm: 42 min
- 4-5pm: 36 min
- 5-6pm: 35 min
- 6-7pm: 40 min
- 7-8pm: 37 min
I added those numbers up to figure out the total minutes the furnace ran during those 5 hours (190 minutes). And I divided that by the total amount of minutes in 5 hours (300 minutes). That gave me 63%.
Finally, I multiplied that 63% by the capacity of my furnace (100,000 BTU/hr) to find that, on average, at the design temperature, my furnace provides 63,000 BTUs of heat.
Et voilà, 63,000 BTUs is my home’s heating load.
Of course, the irony is that after writing hundreds of words disparaging the 30 BTU per square foot rule of thumb, that would have gotten pretty close to the actual heating load of my home. But imagine if I lived in North Carolina where heating loads are half as big. Or imagine if I lived 100 miles west in the much colder mountains of Colorado. In both cases I’d have a poorly sized heat pump.
What impacts heat pump sizing the most?
The most important factor for your heat pump size is your local climate. The hotter and more humid your climate, the more cooling you need; the colder and wetter your climate, the more heating you need.
You can use the following map developed by the Pacific Northwest National Laboratory as reference for what your climate zone is.
Insulation and sealing
Another important consideration is home insulation and air sealing.
As anyone who has lived in a drafty home will tell you, some homes are terrible at keeping warm in the winter and cool in the summer. On the other hand, the most advanced “passive homes” are so airtight that turning on the oven will increase their temperature noticeably.
Generally, older homes suffer from poor insulation and air leaks. As a result, they have higher heating loads.
As ductwork ages, it generally becomes less efficient. In other words, less of the warm air provided by your heating system and cool air produced by your cooling system gets distributed throughout your home.
According to Energy Star, “In a typical house, 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.”
Windows and doors
Just as there are a lot of leaky ducts in America, there are a lot of leaky windows and doors. If your home has single-pane windows or doors with gaping cracks, you’ll probably have a high heating and cooling load.
Shade and sunlight
The more sun that comes into your home, the more free thermal energy you get. In the winter this means less heating. In the summer it means a higher cooling load.
Use of space
How you use your home, the number of people expected to be in it, and the appliances you have all impact your heating and cooling needs.
The more people and the more heat-generating appliances you use (such as dryers, refrigerators, and ovens), the less heating you’ll need in the winter and the more cooling you’ll need in the summer.
When in doubt, ask an expert
As you can see there are a lot of factors that can change your heating load. This is why even homes built in the same year on the same block can have vastly different heating loads.
That’s always why we strongly recommend working with an installer who will do a proper blower door test and Manual J load calculation.