You're not alone if you’re wondering about all the hype around heat pumps. How do they work in a cold New England winter, and do they really cut down on heating bills?
Take it from hundreds of our customers: heat pumps are a solid choice, even when temperatures plummet. In the long run, heat pumps are more efficient and affordable than traditional heating and cooling methods (heat pumps also double as air conditioners), and the technology has come a long way. These days, heat pumps can work at full efficiency down to 10°F, and they’ll even keep running when the temperature drops as low as -20°F.
To see how heat pumps handle the cold, it helps to understand how they pull warmth from the air. Believe it or not, there’s still heat energy in the air, even when temperatures drop below freezing. Temperature measures how fast molecules move; the faster the molecules move, the more heat energy we feel. Even at temperatures below freezing, the molecules in the air still move, which means they retain some heat energy. This energy exists in the air until temperatures reach absolute zero (approximately -460°F), when all molecules stop moving. Thankfully, we’ll never experience that here on Earth, and if we do...we’ll have much bigger problems to worry about than our heat pumps.
People often refer to heat pumps’ capabilities as “magic.” While heat pumps are remarkably efficient and cost effective, the magic comes from a process humans have been utilizing for thousands of years: phase change. In phase change matter shifts from one state to another, like when water boils and turns into steam at 212°F. We’ll keep using this example of boiling water to illustrate how heat pumps work.
If you seal boiling water in something airtight, like a pressure cooker, the water can get much hotter without turning into steam. As the water heats up inside the sealed cooker, the molecules move faster and press harder against the walls, building up pressure. With more pressure, the water needs more energy (a higher temperature) to boil because the pressure keeps the molecules more tightly packed together. Boiling is just what happens when a liquid’s molecules are moving fast enough to break away from each other and turn into vapor. So, even though it’s above 212°F in the pressure cooker, the water stays liquid because of the added pressure packing the molecules together.
Heat pumps operate using this same concept, but instead of water, they use refrigerant. However, this refrigerant boils at about -50°F, meaning the moment it gets exposed to temperatures above -50°F, it instantly turns into a vapor. Even in freezing outdoor temperatures, this refrigerant will still boil.
The heat pump process begins with sending compressed refrigerant to an outdoor unit through a coil. Once the refrigerant reaches the outdoor unit and is no longer under compression, it absorbs heat energy from the outside air and turns into vapor because of its extremely low boiling point. After picking up this heat, the vaporized refrigerant carries it back to the compressor through the coils. The compressor then squeezes the vapor, making the molecules move closer together and faster, which increases the vapor’s temperature – just like the water in the pressure cooker. This repeated process creates what's called "superheat."
A good way to understand superheat is by using the example of a bike pump. When you push down on the handle of a bike pump, you're compressing the air inside. As the air gets compressed, it heats up. If you touch the pump after using it for a while, it feels warm. This heat is the same idea as superheat with a heat pump – the vaporized refrigerant gets compressed, its molecules move faster and closer together, and heat is generated. The superheat builds up after many cycles of refrigerant and is then used to heat your home.
The beauty of heat pumps is that they can deliver three to four kilowatts of heat energy for every one kilowatt of electrical energy used to power the compressor. This is because heat pumps don’t generate heat from electricity like a space heater; instead, they use electrical energy to transfer and amplify heat from the outside air into your home. This concept is where the “coefficient of performance” (COP) comes from, a term used to describe how efficiently a system outputs heat energy compared to the energy it uses. Fossil fuel heating systems just aren’t as efficient as heat pumps - they’ll never reach 300-400% efficiency. In fact, they can never reach 100% efficiency because some energy is always lost in systems like propane or heating oil while turning the fuel’s chemical energy into heat. Heat pumps, on the other hand, can move more heat energy than the electrical energy they use, which is why they are an incredibly efficient and effective alternative for those cold winter months.
For a heat pump to run properly in the winter, however, one thing must be true: the system needs to be properly sized. Check out our recent blog post on why missing the mark with system sizing can be a costly mistake for homeowners installing heat pumps.
