Is my furnace working properly. I hear this a lot when people have heat pumps. They see that the A/C unit on the outside of the home is frozen over.
As the air moves over the evaporator coils, the gas starts to warm back up. The warm gas moves out of the furnace and back to the exterior air conditioner unit where the fan on the exterior unit cools the gas back down again. The gas then enters the compressor and sends the gas back into the home starting the cycle back over again.
This picture shows the inside of a typical furnace. On the
top of this furnace is the evaporator coil. When your Air Conditioner is
running is sends the gas into the furnace, through the expansion device causing
the gas to cool off.
The evaporator coil starts to freeze over, the fan on the
furnace turns on and blows the warm air over the evaporator coils, this causes
the warm air to cool and this is how you get the cold air into the home.
Now when you have a heat pump for a furnace. The air conditioner is the furnace. Instead of the cold gas going into the home and the warm gas going outside. The compressor runs backwards. This is what causes the outside unit to freeze over.
So instead of the unit looking like this. It looks like this.
This is normal.
The warm gas heats the interior and the cold gas freezes the exterior unit up. The colder it is outside the more often it will freeze the exterior unit up. When this happens the exterior unit shuts itself down and goes into what is called a defrost cycle. If the home is in need of more heat. The furnace usually has several heating elements inside the furnace to provide the heat when the heat pump is not running due to the defrost cycle.
These heating elements as serve as extra heat if the heat
pump cannot produce enough heat on those very cold winter nights.
When the unit has defrosted it will turn itself back on producing the heat that the house needs. There are a couple of things to look for when the heat pump is running. If the unit does not freeze evenly, it can mean that the unit might be clogged or damaged as seen in the picture below.
Just remember that this can just be the unit in defrost mode. So be careful to monitor the unit while it is running as well. This can take several hours. No sense calling a service repair man it you only see this one time.
This unit happens to be damage. A damaged unit will not produce enough heat for the home. It can also damage the unit further causing it to fail completly.