The Heat Pump Defrost Cycle Explained
21 Sep, 2020

The defrost cycle of a heat pump is its way of trying to operate efficiently during cooler months. A defrost control tells the reversing valve when to send hot refrigerant outdoors to thaw the outdoor coil during the winter.

During the defrost cycle, your auxiliary heat takes over, which reduces the heat pump’s overall efficiency by up to 10 percent. The two most common types of defrost controls are time-temperature and demand-defrost. Time-temperature defrost controls activate defrost at regular time intervals for set time periods, whether there is ice on the outdoor coil or not.

What Is Demand Defrost Control?

The heat pump defrost cycle


A demand-defrost control senses coil temperature or airflow through the coil, and only activates defrost if it detects the presence of ice. Obviously, choosing a heat pump with demand-defrost will pay a significant efficiency dividend.

If you stand near your unit outside while it is frosted, you should eventually hear it snap into defrost mode with this function. The ice should disappear as it cycles warm coolant into the unit. Your vents will blow cold air. After this, the fan should start up again. If the icy build-up is not eliminated at the end of your heat pump’s defrost cycle, you should contact smart72 for a service appointment. Your defrost board could be an issue, or it may need a recharge.

Always keep in mind that if you live in an area with even a mild winter, your heat pump unit will ice up. The defrost cycle is natural. If your unit is not defrosting, or the ice build-up on your unit is a thick coat, your unit’s defrost controls are not working properly, and you’ll need service.

Outdoor Thermostats and Heat Pumps

If you leave and return at regular times everyday, you’ll save money by using automatic thermostats, which minimize energy use during the times the home is unoccupied. However, choosing an automatic thermostat’s reactivation time requires considering the duration of heat-pump operation necessary to restore a comfortable temperature. During the heating season, some homeowners also set their thermostats back 10°F, manually or automatically, when they leave home or go to bed.

A two-stage thermostat controls the heating. The first stage activates the refrigeration system. If it’s too cold outside for the refrigeration system to counteract the home’s heat loss, then the thermostat’s second stage activates the electric resistance coils. An outdoor thermostat will prevent the less efficient electric resistance heat from coming on until the outdoor temperature falls below 40°F. An outdoor thermostat also will prevent auxiliary heat from activating when an automatic thermostat is warming the house after a set-back period. Use setback thermostats that are only for heat pumps.

For greater efficiency, don’t locate a thermostat near a heat source or cold draft because they can cause a heat pump to operate erratically. This includes shading thermostats from direct sunlight. Also, do not turn the thermostat beyond the desired temperature. It will not make the heat pump heat or cool your home any faster. It will only waste energy. Residents who duel one another over the thermostat settings, moving it up and down to suit their different comfort levels, cause heat pumps to operate erratically and inefficiently.

Heat Pump Issues Not Related To The Defrost Cycle

If your heat pump continues to act up, it may have been a contractor installation issue. If the install was rushed, or the units did not match in capacity, you may need to adjust the placement or re-install a new heat pump. The unit may have also been improperly charged with refrigerant.

Restricted airflow can also cause icing. Check your air ducts to make sure they are open, and get your ducts inspected to make sure they are not leaking.