Beware of Heavy Icing on Your Heat Pump When It’s in Heating Mode

If you use an air-source electric heat pump as your primary heating source during the winter months, more than likely you’ve experienced it icing up. This probably is not a serious issue if it’s just a light layer of frost. That’s normal, and the heat pump eventually should switch into a defrost cycle to melt any ice on the heat-exchange coil. A less welcome sight is heavy frosting on the outside unit of the split-system heat pump, with ice covering both the top of the outside unit and the inside coil. This situation likely will waste energy and result in performance issues, and eventually lead to damage to the heat pump equipment.

What Causes an Iced-Up Heat Pump?

The problem is probably a malfunctioning defrost cycle; however, there are other potential causes. When a properly operating heat pump receives an electronic signal to defrost during its heating mode, a reversing valve should switch the heat pump from heating to cooling/AC mode, and the outside fan should turn off. As a consequence, refrigerant flowing through the outside coil will heat up and melt the ice off the coil. The heat pump system then will return to normal heating mode, awaiting the next signal to defrost the coil.

As is the case with any process that has multiple steps, any of the steps in the defrost progression in an air-source heat pump has the potential to malfunction – sensors, reversing valve, thermostat, timer, compressor, fan motor, coil, refrigerant line, etc. The more areas where there’s a potential for something to go wrong, the greater the chance it might happen, in this case resulting in an iced-up heat pump.

For example, the problem might originate with your system’s air filter. If it’s clogged with dust and debris, that could prevent your heat pump from working correctly. You can check to see whether the air handler blower is the problem by putting your hand next to a supply vent when the heat pump is running. You should feel moving air; if you don’t, the blower likely is malfunctioning. Test this by turning the thermostat to its “Fan” setting; that should kick on the blower. Similarly, the fan in the outside unit should be spinning during heat pump operation, blowing air through the coil fins. Debris such as leaves and dirt also could be impeding air movement through the fins.

How to Address an Icy Heat Pump

Most problems that can lead to a badly frosted heat pump should be diagnosed and addressed by your trusted Cincinnati HVAC contractor. However, the homeowner can do a few things when experiencing an iced-up heat pump. If the top of the outside unit has a coating of ice, turn off the system and carefully remove the ice (avoid using sharp objects). Don’t allow a leaky rain gutter to drip on the outside unit and keep snow and leaves away from it. Avoid allowing the outside unit to settle into the ground or become uneven.

Foremost, schedule seasonal maintenance on your air-source heat pump before each high-use season. A competent technician should be able to pinpoint any areas that might lead to an iced-up heat pump system during winter operation.