How to find the right size heat pump
Installing a heat pump is one of the best ways to save money as a homeowner. But choosing the right heat pump is a bit confusing.
One of the most confusing parts of the buying process is finding the right size heat pump.
So in this guide we’re going to explain everything you need to know to buy the right size heat pump.
Heat pumps measure heating and cooling capacity
First things first: the size of your heat pump isn’t about the actual size of the machine. It’s about the maximum amount of heating and cooling capability it has. A 1-ton heat pump will be able to heat less in absolute terms than a 3-ton heat pump.
If you’re just trying to figure out what size to buy, skip to the next section. But if you’re curious about what the sizes mean, keep reading. It’s a nice mix of a science and history lesson.
BTUs vs. Tons — What’s the difference?
In terms of heating and cooling, “ton” refers to the amount of cooling caused by melting one ton of ice. Or the amount of heat needed to melt that ton of ice.
Why do we measure that way? Before the advent of refrigeration, home cooling was provided by literally letting ice melt. Ice was sold by weight, and tons was the standard unit of measurement. One pound of ice at 32 degrees Fahrenheir takes 144 BTU of thermal energy in order to heat up by 1 degree (and become water). Multiplying this by 2,000 (1 ton = 2,000 lbs) means one ton of ice required 288,000 BTU of energy in order to melt completely.
The first air conditioners and coolers were sold to directly replace the use of ice in cooling the home. Salespeople advertised their units against how many tons of ice melting over 24 hours you could replace by installing one. Dividing the energy needed to melt one ton of ice over 24 hours (288,000 BTU / 24 h), you get exactly 12,000 BTU per hour. Thus, the industry standard was created.
How to convert BTUs to tons?
For those of you that don’t already know, BTU stands for British Thermal Unit, and it’s the most commonly used unit of measurement for heat in the United States. 1 BTU is equal to the amount of heat it takes to raise one pound of water one degree Fahrenheit.
In order to convert from tons to BTUs, just multiply the tons by 12,000 BTU/ton. Below we’ve provided a tons to BTU comparison chart to make things a bit more simple.
What size heat pump do I need?
Picking the right size for your heat pump is crucial.
- Too small a unit, and you won’t be able to fully cool or heat your home—and the pump will have a much shorter lifespan due to constant operation.
- Too large a unit, and you’ll waste money installing and operating excess heat and cooling capacity. You also may not be able to dehumidify your home effectively.
To find the perfect size get an energy audit using the Manual J system
General recommendations are to size your heat pump to the larger of your heating and cooling demand, up to 125% of your cooling needs, to ensure efficient operation year-round. (We’ll get into how to calculate your capacity below).
If you’re serious about installing a heat pump, we’d recommend getting an official energy audit from someone using the Manual J standard from the Air Conditioning Contracts of America (ACCA, the official trade group for space conditioning in the United States). From there, you’ll want to find a contractor who uses the ACCA Manual S standard to find the right heat pump and other efficiency methods for you.
This process (often just called Manual J and Manual S procedure) is the most effective and robust way of finding what type and size of heat pump you need, and it’s the industry standard. Any number you get online is based on (a) a decades-old estimate that you should use 1 ton (12,000 BTU) for every 500 square feet of floor space and (b) this EnergyStar guide for new manufactured homes (mobile homes) in different climates.
The difference between homes is often as large as the difference between people, and you’re getting a machine that will last decades if you pick the right size. Without using the Manual J and Manual S procedure, you risk over- or under-sizing your heat pump.
How much will the Manual J and Manual S procedure cost?
It depends where you live and what company you go with. Sometimes the audits will be provided for free by your local utility or government, or even as a complimentary service from the HVAC company you’re going to buy the heat pump from. Other times, you’ll have to pay for it, which will cost anywhere from $100-300.
Here’s the program for my local utility as an example. It provides the audit (which includes recommendations for sizing) for free, and it provides rebates if you work with one of their vetted contractors.
What impacts heat pump sizing the most?
Even though you’ll likely have a professional take care of it, you should know the factors the Manual J takes into account when calculating the heating and cooling needs for your home. Here’s a summary of each of the most important factors, what they mean, and how to measure them.
Climate — 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.
Ductwork and shape of home — Different shaped homes are going to allow temperature to move differently and in different ways. A two-story home is going to have very different temperatures on each story, and that needs to be taken into account. Additionally, if your home has ducts, their location might increase or decrease space conditioning efficiency.
Number and type of windows — The temperature seepage from windows is an important factor overall for heating/cooling needs.
Shade and sunlight — The amount of natural light and thermal energy from the sun that comes will change your cooling needs.
Insulation — The amount of heat that can be absorbed or released through the walls dramatically impacts the size of heat pump needed, and a well-insulated home can easily be run using a much smaller heat pump. Generally, the older your home is the less insulated it is, and the larger a heat pump you’ll need.
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 your heating need is and the more your cooling need would be.
The internet is good for estimates, but get an energy audit to be safe
You can see why it’s important to have a professional do an audit—there are just so many variables. Here’s a cool chart from the Department of Energy that shows the impact that all the different factors can have for two different homes in Raleigh, North Carolina. For example, the insulation, type and location of windows, and location of ducts cut the size of the heat pump needed by more than half. That’s why we’re being pretty forward with our PSA to get a professional audit.