What Size Heat Pump Do I Need for My Home?

Allie Ogletree
Written by Allie Ogletree
Updated January 11, 2022
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Photo: John Fedele / Getty Images

Highlights

  • Investing in the right size heat pump can prevent energy waste

  • Calculate your approximate unit size by multiplying your home’s square footage by 20

  • The right-sized heat pump will last longer than one that is too big or too small

  • Your family’s lifestyle, location, shade, and windows can affect what size pump you need

  • Hire a pro to conduct a thorough assessment of factors that could affect the size of your pump

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On a hot day, the last thing you need going for your house is a heat pump that can’t effectively pull the heat out; likewise, no one likes being cold because their heat pump can’t seem to heat up the house in the winter. Choosing the wrong size heat pump can mean your system works overtime to keep up with the demands of your home. With the right system in place, however, you can secure the high comfort level that these heat pumps offer, as well as energy savings.

How a Heat Pump Works

Heat pumps are different from traditional furnaces in that they use a heat transfer system to either pull hot air out of your home and release it outside (in the summer) or pull heat from outside of the home to warm your home (in the winter). There are three types of heat pumps available on the market: air-source, water-source, and ground-source or geothermal heat pumps. 

While each pump operates a little differently, the heat transferal mechanism is similar, and each heat pump uses the same unit of measurement to determine cooling capacity.

Factors to Consider When Sizing Your Heat Pump

Before choosing a heat pump for your home, you’ll want to consider the following factors to help you determine the best size:

  • Your home’s square footage

  • Efficiency rating of the heat pump

  • Features you want in your heating system

  • Installation method

  • Type of heat pump system

  • Soil quality, property size, and potential water sources for ground-source systems

How to Size a Heat Pump

There are two ways to calculate the size heat pump you need. One way is to DIY your calculations, which may mean a less accurate evaluation and sizing. For the most in-depth and professional sizing, the HVAC industry follows the Manual J method, which is a standard sizing practice established by the Air Conditioning Contractors of America. This methodology is used across the country, and it takes into account all relevant factors that influence sizing. 

Both methods require calculating the HVAC load, often noted as BTU, or the amount of energy used per hour to remove heat from your home’s interior. Once you calculate the BTUs, you can choose the best pump for your home. 

DIY Heat Pump Size Calculation

If you want a broad estimation of the BTUs needed to keep your pump at the optimal performance, simply multiply your square footage by 20. Per the U.S. Department of Energy, air conditioners need approximately 20 BTUs per square foot

Here’s how to calculate your square footage and estimate how many BTUs per hour you’ll need:

  1. Measure the rooms in your home. Using feet as your measurement, walk into every room and measure the length and width of each area. 

  2. Calculate the square footage. Next, multiply the length and the width to get the square footage of each room. For more complex room dimensions, you may need to do a little geometry to calculate the square footage. 

  3. Add together the individual rooms to get your home’s total square footage.

  4. Multiply by 20. Multiply your total square footage by 20 to get the final cooling capacity, since HVAC systems calculate cooling capacity by BTUs per hour, which is roughly 20 BTUs per square foot.

  5. Consider factors that affect the cooling capacity. Adjust the total number depending on the unique makeup of your home. Sunny rooms need a capacity increase of around 10%, while shaded rooms should be decreased by 10%. Add 600 BTUs for every person in your household.

Consider speaking to a professional who has experience in accurately calculating the correct BTUs needed in a unit, while also addressing the factors that affect the cooling capacity.

Manual J Method

The Manual J method is used by professionals to ensure that your heat pump is appropriately sized. Your contractor may even recommend making a few changes to your home (such as increasing insulation) to downsize the heat pump system and save you money in the long run.

This method of sizing heat pumps takes eight factors into consideration. These are:

  1. The local climate 

  2. The home's design, such as the overall square footage and the shape of the home

  3. How much insulation the home has and its efficiency rating or R-value

  4. The number of windows and their location 

  5. How much air infiltration occurs

  6. How many people live in the home

  7. How the residents use the home and their preferences for indoor temperatures

  8. Other heat-generating appliances and lighting in the home

When to Hire a Heat Pump Pro

A professional inspecting a heat pump
Photo: Jasmin Merdan / Moment / Getty Images

If calculating the right size for your heat pump has you feeling like you’re about to flunk a complex geometry exam, it might be time to call in the pros. A heat pump professional near you can ensure that you make the most cost-friendly and energy-saving decisions for your new system.

When installing the heat pump, the contractor should calculate the size using the Manual J method and take a few moments to visually inspect your insulation and count the windows in your house. They’ll also ask questions about your family’s lifestyle and preferences to determine the properly sized heat pump system.

Why Sizing a Heat Pump Matters

Heat pumps cost $5,700 on average, so the last thing you want to do is invest in the wrong size unit. Choosing the right size heating and cooling system ultimately helps you save on energy costs; it can also impact the lifespan of your heat pump. The danger of an oversized or undersized heat pump include:

  • Increased on-and-off cycling, which puts more stress on the blower motor

  • Uncomfortable, drastic temperature differences inside the home

  • Higher energy costs

  • Ineffective control of indoor humidity

  • Short-cycling

  • Wasted energy 

With a system sized just right, you'll lock in lower energy costs, a higher system efficiency, fewer equipment breakdowns and repairs, and a longer-lasting system.

Undersized Heat Pump Problems

An undersized pump system won’t have the capacity to successfully cool or heat your home. Instead, your unit will keep running as it tries its hardest to maintain the desired indoor temperature. This not only means higher energy bills, but it also means your unit will have a shorter lifespan as it struggles to meet your home’s energy demands. 

Oversized Heat Pump Problems

Though it might seem like a larger heat pump will do a better job at heating and cooling your home, an oversized pump actually leads to higher electricity bills and increased humidity in the house. When your air conditioning system is too large, it will turn on and off more frequently, as it quickly cools the space. 

The energy required to turn the system on and then shut it off hikes up your energy usage. Likewise, if your unit is constantly running on short cycles, the air won’t blow long enough to dry humidity, leading to increased risks of mold and a general feeling of stuffiness inside your home.

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