One concern some people have is that their HVAC system may freeze during the winter. Generally speaking, this really isn’t a concern unless you have a central or ductless heat pump system. In that case, you can expect the coil in the heat pump unit outside to routinely freeze. This sometimes happens as often as every hour or two when it’s fairly cold outside. It’s also something you normally don’t need to worry about since the system’s defrost cycle will switch on occasionally to thaw the heat pump out.
What Causes a Heat Pump to Freeze in Winter?
During a heat pump’s heating cycle, the refrigerant in the system continually changes from an extremely cold, low-pressure liquid to a high-pressure, superheated gas and then back to a cold liquid and so on. The refrigerant is in a cold liquid state when it flows through the system’s outdoor coil. After it captures heat from the air, it gets compressed and instantly boils and changes to a gas. Once it releases the heat inside, it again changes back to a liquid. The temperature of the refrigerant, when it’s in a liquid state, is below its boiling point, typically somewhere around -15 to -22 degrees Fahrenheit. The fact that the refrigerant is so extremely cold leads to frost and ice forming on the coil and the heat pump starting to freeze up even on much milder winter days.
How a Heat Pump’s Defrost Cycle Works
Whenever a heat pump systems need to defrost, it activates the reversing valve and the refrigerant immediately changes the direction it’s flowing in. That means cold refrigerant starts flowing through the indoor coil and hot refrigerant flows through the outdoor coil, resulting in all of the ice on the outdoor coil quickly melting. This is the same direction the refrigerant flows when the system is cooling. The difference is that the condenser fan outside and the blower inside both shut off when the heat pump is defrosting. The blower has to shut off so that the refrigerant doesn’t start removing heat from the house, and the condenser fan in the heat pump shuts off just so that cold air isn’t blowing over the outdoor coil and it defrosts more quickly.
What Can Prevent a Heat Pump From Defrosting?
It typically doesn’t take more than a few minutes for a heat pump to defrost and resume heating as normal. It can take longer when it’s much colder outside, but even then, the defrost cycle will usually last for a maximum of 15 minutes or so. Any longer than this and the system will usually switch to auxiliary heating mode and turn on the electric heat strips inside the air handler. You can tell when this happens because your thermostat will then show the system is running in auxiliary mode.
If you notice your heat pump system has been running in auxiliary mode for quite a while, it usually means one of two things. The first is that there’s some type of issue preventing the heat pump from turning on. The other is that the system failed to switch to defrost mode, which usually happens due to the reversing valve being stuck or possibly because the valve has failed. If the system doesn’t switch to defrost mode, the heat pump will, sometimes, continue running. However, it won’t be able to produce any heat due to all of the ice on the coil. That means the thermostat will eventually signal the system to switch to auxiliary mode once the temperature in the house starts to decrease.
If you notice a large buildup of ice in your heat pump and it doesn’t melt within a couple of hours, you’ll need to have it professionally inspected. You’ll want to have this done as soon as possible, as the system uses far more energy when running in auxiliary heating mode, which can quickly spike your heating costs.
With an unmatched level of service and expertise, Smith & Willis Heating & Air Conditioning is the company to contact for all of your cooling, heating, and indoor comfort needs. We’ve been serving Commerce City and the Denver area for over 20 years and have a team of certified technicians dedicated to providing top-tier repair, maintenance and installation services, as well as indoor air quality services. If you have issues with your HVAC system freezing or not heating effectively this winter, give us a call, and we’ll quickly get to the bottom of the problem.
