During the winter, the outdoor air is relatively cool with high relative humidity, typically 70% or 80%. Because the air is so cold, it can't hold much water vapor. Consequently, it doesn't take much moisture to almost saturate the air. The colder air is, the less moisture it can hold. Dropping the air temperature even slightly will cause the moisture to fall out of the air as water or frost. When we use a heat pump in the heating mode, the outdoor coil
the temperature of the outdoor air passing across it. This often causes the relative humidity to go up from 80% to 100%. Condensation or frost forms on the coil. If more than roughly one-eighth inch of frost is allowed to collect, this will obstruct the air flow and inhibit heat transfer, effectively making the unit useless. The defrost cycle is needed to melt this ice before it can build up to the point where it inhibits operation. Frost accumulates on the outdoor coil when the coil temperature approaches 32F.
There are two ways that heat pumps get rid of the frost that accumulates in the winter. Some use electric heaters, while others activate the reversing valve to go into the cooling mode. In either case, when the system is in the defrost cycle, the outdoor fan shuts down.
In some systems, the indoor back-up electric heat will kick in so that the occupants of the house don't feel cool air. The indoor house fan keeps blowing air across the indoor coil while the outdoor fan is shut down. With an add-on heat pump (which has a gas or oil furnace), the indoor fan may also shut off during the defrost cycle, to avoid blowing cool air around the house.
Many systems have a
mode. In effect, the heat pump checks the outdoor temperature near the coil every 30 to 90 minutes (depending on how it is set up).
If the temperature is at or below freezing (32F), the system will go into defrost mode for up to ten minutes. This is a somewhat rudimentary defrost cycle.
to be superior. This may be triggered by measuring the air pressure drop across the coil, the coil temperature, or the Freon pressures moving through the coil. The defrost cycle may be as short as one minute. This is a more sophisticated system, but is also more complex and expensive.
There are several concerns with defrosting:
The need to defrost reduces the efficiency of heat pumps, particularly in climates that are cold enough to need frequent defrosting.
It's not always easy to get rid of the melted frost without forming ice dams that obstruct the airflow around the coil.
Reversing the direction of refrigerant flow to defrost the coil can be hard on the compressor.
The water melted from the coil must drain freely away, or it will refreeze. Heat pumps should be well off the ground. This also helps keep drifting or deep snow from obstructing the heat pump.