Heat pumps provide homeowners with a range of benefits – cleaner, more affordable heating and cooling year-round. But to get these benefits, the system must be correctly sized for the house, explains Dave Ragsdale, one of ReVision Energy’s Thermal Technical Specialists. Before coming to ReVision, Dave designed and installed fossil fuel-based heating systems for over twelve years.
"The biggest mistake I’ve seen installers make with heat pumps is making them too big or too small for the space,” says Dave reflecting on his years in the heating industry. To understand why getting the size wrong can be harmful, we need to understand how installers calculate heating system sizes in the first place.
The heating industry uses something called “heating load” to figure out what a room or house needs to be adequately heated, measured in BTUs (British Thermal Units) per hour. One BTU is the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit. More BTUs means more heat. This load calculation, based on the design and layout of the house, helps determine the size of the heating system.
Traditional fossil fuel-based heating systems have a fixed amount of heat they can generate at a given moment, which we call the system’s “capacity”, also measured in BTUs. If a burner is rated at 100,000 BTUs, it always produces 100,000 BTUs when running – no more, no less.
Heat pumps, however, can vary their capacity. For example, a single heat pump unit rated at 9,000 BTUs can adjust its output to as low as 1,400 BTUs or ramp up to as high as 15,000 BTUs. The rating of 9,000 is simply the midpoint, and the system can work above or below it as needed. This ability to modulate is key to what makes heat pumps less expensive than traditional heating methods; a correctly sized heat pump will only use the electricity needed to provide exactly the right amount of heat to keep a room at a desired temperature. And electricity (especially when made by solar) is far cheaper than propane or heating oil.
With traditional heating, it didn’t matter much if an installer oversized the system, because no matter if a boiler is rated for 90,000 BTUs or 100,000 BTUs, it’s still going to output heat until it satisfies the thermostat. It was less important for the house’s heating load calculation to be spot on. Dave says this mindset can be problematic for installers transitioning from traditional systems to heat pump installations because with heat pumps, size does actually matter.
“Those who don’t understand heat pumps and their ability to modulate [their capacity] will just look at a house and say “Hey, you have a radiator in every room with your boiler, let's just put a heat pump in every room.”
But this idea can cause problems. If you put a heat pump in a room with a heating load that only requires 300 BTUs per hour to maintain a temperature of 65°F, and the selected heat pump for that room can’t produce less than 1,400 BTUs, it will short cycle, turning on, overheating the room, shutting off, and repeating this cycle. This frequent cycling wastes energy and causes uncomfortable temperature swings and unexpected electric bills.
On the other side of the spectrum, if the heat pump is too small, it simply won’t be able to keep up with the heating demands of your home during the coldest days. The system will run constantly, trying to generate enough heat, which wears out the compressor faster and leads to higher bills. Undersizing isn’t an issue if you use heat pumps alongside an existing boiler or furnace as a supplemental system. It becomes an issue when the heat pumps are the sole heating system for the home.
It’s critical to accurately determine the heating load for each room, so you don’t oversize or undersize the system. The industry standard is the “Manual J”, which considers the size, insulation, and layout of each room to determine the correct heating load. If you’re replacing your entire heating system with heat pumps, and your installer doesn’t perform a thorough heating load calculation, you should consider using a different company. As we know, quality matters when it comes to heat pump installations. But if you're working with a company who does a comprehensive heating load calculation and seamlessly installs your system, heat pumps will have no trouble keeping up with cold New England temperatures this winter.
Want to know more? We have a team of experts who can help walk you through the options and figure out what’s best for your house. Reach out today to see how you can stay warm and save money this winter.
