By Albemarle Heating & Air
How a Heat Pump Works
In order to understand what “defrost mode” does, you need to know how a heat pump works. An easy way to explain it: think of it like an air conditioner in reverse. Air conditioners collect heat from the air in your home and expel it outside, while a heat pump collects heat from outside and releases it indoors where you need it.
But wait, you might think, how does a heat pump collect heat from outside when the temperatures are already so cold? The answer is that yes, it might be what we perceive as cold, but that doesn’t mean there isn’t thermal energy out there. There is simply a lot less than there would be during a warmer period, and that means a heat pump needs to work harder to collect it, concentrate it, and send it inside where it can keep you warm.
The heat pump cycle starts at an outdoor unit, where refrigerant is run through a compressor. Compressing refrigerant causes it to absorb a ton of heat extremely quickly, making it boiling hot. However, this heat has to come from somewhere, and that somewhere is the air around it. Even if it is extremely cold outside, this process is remarkably effective at collecting this energy. From there, the refrigerant is sent inside and passed through a metal coil. Air passed over this coil then
absorbs the heat out of the refrigerant, making it extremely warm before it is sent through your home.
Once the refrigerant passes through this coil, it becomes significantly cooler. From there, it is sent through an expansion chamber, which allows it to decompress. This process causes it to expel a ton of heat, making the refrigerant extraordinarily cold (often well into negatives in Fahrenheit). This temperature difference is what allows the refrigerant to effectively collect heat from outdoor air—the refrigerant is so cold that even the cold air around it is significantly warmer, which therefore heats it up.
What Is a Defrost Cycle?
So where does this “defrost mode” come into play? Well, the answer to that has to do with that last step of the cycle we discussed: the expansion. When this refrigerant is brought down to sub-zero temperatures, it is then run through your outdoor coil again so it can absorb more heat. However, this brings the temperature of the coil itself down to an extremely low level. At temperatures this low, any water molecules in the air in vapor form that touch the metal of the coil can instantly condense and freeze. After a while, so much vapor has accumulated and frozen that the coil can no longer collect the thermal energy needed to warm your home. There may not be a lot of humidity in the air, but there is enough to make this a problem, and for that reason, your heat pump needs to periodically “defrost” itself.
So, yes, in a sense your heater actually is freezing, as ridiculous as it might sound. However, it isn’t a sign that anything is wrong; it’s just a byproduct of the heat collection process, and it is something that engineers have accounted for by creating “defrost mode” in your heat pump’s operating code.
During “defrost mode,” a few things happen. First, the outdoor fan motor shuts off so the cold air doesn’t pass over the coil. Second, the flow of refrigerant reverses. This sends hot refrigerant through your outdoor coil, melting any accumulated ice and water vapor off of the outdoor coil. Once the outdoor coil is free from frost and ice, it can be used to start collecting thermal energy once again.
Depending on your system, a defrost cycle will take roughly about five to ten minutes to complete. But does that mean you’ll be without heat for that amount of time? During a particularly freezing day, going for even ten minutes without heat might seem like a huge burden. The answer is yes and no. Yes, your heat pump will not be providing heat because it’s actually running in air conditioning. And no, because with any heat pump system there’s some type of supplemental heat. In a 100% electric heat pump system an electric heat strip located in the air handler provides the needed heat. If you have a dual fuel heat pump system, your gas or oil furnace provides the supplemental heat. An electric heat pump can usually only work efficiently down to temperatures as low as the low teens in Fahrenheit before there simply isn’t enough thermal energy in the atmosphere to allow them to work efficiently and that’s when the supplemental heat is activated.
- Heat Pump