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?
Many homes in the Houston, Texas and the surrounding Gulf Coast area may use on average a heat pump system that distributes 3 tons of air conditioning. That size is the most commonly purchased size of heat pump and central air conditioner in the United States. Tons refers to the amount of air conditioning needed to cool your home.
What is a BTU?
BTU stands for British Thermal Units and it technically refers to both heating and cooling. One ton of air conditioning is equivalent to 12,000 BTUs. As a shorthand, you’ll hear many HVAC technicians refer to “tons” of air conditioning.
Should I use a heat pump calculator?
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 northern regions of the US.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) collects comprehensive weather information to create a design temperature that sets the working parameters for calculating a heat load. You may not have known that cooling system design 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 available for the Houston area are sized for the average summer or winter temperature, not the most extremely hot or cold 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 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 what year 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?
The type of exterior siding and even the color of your roof can play a role in the heat load calculation for your home. The darker the roof color, the more radiant heat your roof absorbs and the more air conditioning you’ll need.