When a Bigger Heat Pump Is Not What You Need to Stay Cool

When the summer heat arrives in Texas, do not be tempted to buy a heat pump system that is too big. Bigger is not always better when you’re purchasing a replacement HVAC system to keep your home or business cool.

The most efficient forced-air heat pumps are sized just right for your home or office.

How do you know what size heat pump to install?

The size of a heat pump system is measured in tons for cooling and BTUs for heating. One ton is equivalent to 12,000 BTUs. Many homes in the Houston, Texas and the surrounding Gulf Coast may use on average a heat pump system that distributes 3 tons of air conditioning.

If you search on the internet for heat pump calculators, advice abounds with ideas that can guesstimate how large of a system you might need. But as Michael Thomas from Carbon Switch demonstrates, those rule-of-thumb calculations can be very poor indicators indeed.

Many factors affect how much cooling and heating your home needs, so it is best to hire a professional who knows what they are doing. Purchasing a heat pump system to keep your home cool is a significant investment in your peace of mind and comfort. Suffice it to say that you definitely do not want to wing it and hope for the best.

What affects the size heat pump system that you need?

In our warm, humid climate, we are most concerned with cooling because we spend more of the year with the thermostat set to cool. Before a home is constructed, engineers determine a home’s heat load.

This information gives HVAC engineers the information they need to properly size a forced- air heat pump system. They will likely use a Manual J calculation from the American Contractors of America, which takes many factors into consideration to properly choose the right size heat pump system for your home.

If you renovate your home or make significant changes to the building footprint, such as adding insulation, other energy-efficient features, or even landscape changes, it can change the size heat pump system that you need.

What factors affect the proper sizing of a heat pump system?

Similar to a central air conditioning unit, a force-air heat pump system must be properly sized to preserve your comfort and to keep your home in good repair.

Climate & Weather

The climate where you live affects the heat pump system you choose. Along the Texas Gulf Coast, heat pumps efficiently remove humidity from the air all year round. They can provide heating, too.  Differently calibrated heat pumps are available here than are available in other regions of the US.

If you lived in New York, for example, you might need a cold-climate heat pump system that has a greater capability to heat a home when the weather gets especially chilly.

American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) collects comprehensive weather information to use for these calculations. You may not have known that heat load calculations are based on the 30-year average the outdoor temperature will be warmer (or cooler) than the outside design temperature. This means that heat pumps calibrated for the Houston area are sized for the average summer or winter temperature not the most extreme days.

Cooling Season Design Temperature

If you’re curious, you can download ENERGY STAR Certified Homes County-Level Design Temperatures.

  • In Fort Bend County, the 1% Cooling Season Design Temperature is 93 degrees Fahrenheit.
  • In Harris County, the 1% Cooling Season Design Temperature is 95 degrees Fahrenheit.

Why is Houston and Harris County so much warmer than its far western suburbs? My guess is that it is due to the heat island effect. Even in Alief, the neighborhood where my sister and I grew up, they are taking action to rebuild the urban tree canopy to counteract the heat island effect, which makes an area seem hotter than it should be. We laugh sometimes that we could cook an egg on hot pavement, but that effect is real and it affects your outdoor and indoor comfort at the end of the day.

Your Home’s Efficiency

Do you know how your home was built and how energy-efficient it is? These are factors that are part of the equation that determines how large a system you will need. Is your home very well insulated or is it drafty? How are your windows and doors performing in terms of efficiency? Even the color of your roof and type of exterior siding can play a role in the heat load calculation for your home.

Rob Terry surrounded by duct work in an attic as he is maintaining a heat pump system

Terry’s A/C & Heating Owner Rob Terry working on a home’s ductwork as part of maintaining a heat pump system in Fort Bend County, Texas.

Existing Ventilation

Ducting or ductwork which distributes air to all the rooms of your home, except maybe the garage, is also a factor in your home’s heat load calculation. In a perfect world, ducts would be located indoors in a conditioned space, but we don’t have cool basements along the Texas Gulf Coast. Many times residential ducts are located in unairconditioned attics because that is the space that is available. The placement of air distribution systems, like ducts, affects the efficiency of your existing cooling and heating system.

Because ductwork can become less efficient over time, sometimes homeowners select ductless mini-split heat pump systems to cool an extra room or an area that just never stays cool enough on the hottest days. Mini split heat pumps are exceptionally efficient because the cool air they emit is never lost through a network of ducts.


How your home is oriented on your property can affect its overall heat load. A home with trees that shade part of the home is going to need less cooling. If those trees are damaged in a storm and provide less shade, it can affect your indoor comfort.

Home Size and Use

Many times people think they can guess how a heat pump system they might need by simply having the square footage of their home. That metric alone just doesn’t produce an accurate Manual J heat load calculation, unfortunately. A heat load will also look at how many people live in the home, whether it has more than one level, how high the ceilings are, what types of heat-generating appliances you have in the home, and more.

By now, you are starting to understand that this can be a complicated math equation that goes far beyond a calculation of area or perimeter. And you thought you’d never have a use for that Algebra class. Ha.

What happens if a heat pump system is too big?

If your forced-air heat pump system is oversized for your home (aka it is too big), this is what might happen:

  • When your heat pump is too big, it won’t be able to remove all the humidity from the air that it should. This is why it feels “cool” but it doesn’t seem comfortable.
  • If humidity builds up in a home over time, it can cause mold problems in a home. No one wants those.

This Old House Episode 40 illustrates the factors that affect the sizing of a heat pump system. Keep in mind that the home featured in this episode is located in a different climate, so the point of view focuses on heating more so than cooling. Using a small-scale home model, the host is able to show the effects of the climate, humidity, solar gain and more. You can see why it is so important to correctly size a replacement heat pump system.

Expert Help to Choose the Right Heat Pump

If you are looking for expert advice to pick a heat pump system that will stand the test of time, call Rob and Summer Terry at Terry’s A/C and Heating at 281-495-7830 in the Southwest Houston, Texas area and points nearby of Katy, Richmond, Sugar Land, Fulshear.

We know that changing HVAC equipment is a long-term investment in your home’s resale value and your comfort. This is why we provide each customer options and educate them about their choices. Whether you pick a variable speed heat pump or something in the mid-range, we listen to your needs, so we can provide affordable, reliable solutions to keep your home comfortable year-round.

Find out what our customers say about working with our family-owned business and what makes us different from other HVAC companies in Richmond, Texas.