Have you ever noticed steam coming from your heat pump during winter but weren’t sure why? This phenomenon can be scary, as it leads some people to think the appliance is smoking and could catch fire. While there is a slight chance that it is actually smoke, it’s much more likely that the heat pump is in defrost mode and releasing water vapor into the air.
Why Heat Pumps Need to Defrost in Cold Weather
The way heat pumps work is by using refrigerant to capture heat from outside and then release the heat inside. For this process to work, the refrigerant has to be extremely cold when it flows through the coil in the outdoor unit. The fact that it’s so cold means that water vapor in the air naturally freezes when it comes into contact with the coil, forming a layer of frost and ice. In humid conditions, the coil can start freezing even when the outdoor temperature is in the 40s.
When the coil starts to freeze up, the layer of ice acts like a blanket of insulation that prevents air from coming into direct contact with the coil. This greatly reduces how much heat the refrigerant can capture, resulting in the heat pump heating much more slowly and working less efficiently. To overcome this problem, the heat pump switches to its defrost cycle to quickly melt the ice and thaw out the coil.
How Heat Pump Defrost Cycles Work
Most modern heat pumps have a sensor that monitors the coil and signals the system to switch to defrost mode once the coil drops below a set temperature. Older systems usually just have a timer that switches on the defrost cycle at set intervals. The unique thing about heat pumps is that they can reverse the direction that the refrigerant flows through the system, which is what enables them to provide heating in addition to cooling.
When the sensor or timer signals the system to switch to defrost mode, the reversing valve activates. Instead of cold refrigerant flowing through the outdoor coil and heated refrigerant getting pumped inside, the flow reverses so that hot refrigerant flows through the outdoor coil. This causes the coil to quickly heat up to the point that the ice begins melting. The coil gets so hot that some of the water from the melting ice evaporates, which is what causes steam to come off the unit.
How to Know When the Defrost Cycle Is Running
You’ll know if a heat pump is in defrost mode, as the compressor will still be running while the fan in the outdoor unit is off. Most systems will also shut off the blower inside the house to prevent it from circulating cold air while the heat pump is defrosting. The exception is for cold-climate heat pump systems that have auxiliary electric heat strips. In this case, the heat strips usually come on during the defrost cycle to allow the blower to keep circulating warm air.
If you have a cold-climate system, and you see that your thermostat shows the system is in Aux or Aux Heat mode, it means that the heat strips are currently on. This is typically nothing you need to worry about. As soon as the defrost cycle finishes, the system will switch back to normal heating mode. However, you do need to worry if the system stays in auxiliary mode for an extended period.
As long as it’s not so cold outside that the heat pump can’t work, the heat strips should never stay on very long. If the system is stuck in auxiliary mode, it either means that the heat pump isn’t running when it should or that there’s an issue that’s preventing it from switching back from defrost to normal heating mode. If the heat pump isn’t running, it usually indicates that there’s some type of electrical problem preventing it from turning on. If it’s stuck in defrost mode, it most likely means that the reversing valve is stuck or broken.
Honest, Dependable Heat Pump Services
KOZAK Air Conditioning & Refrigeration is the go-to source when you need any HVAC or heat pump services in Naples, FL or the surrounding communities. Our highly trained technicians have experience servicing and repairing all makes and models of residential and commercial heat pumps. If you have an old heat pump that you need to replace, we offer a range of high-quality new appliances and will expertly take care of the installation from start to finish.
To schedule heat pump service for your home or business, contact us at KOZAK Air Conditioning & Refrigeration today.
