Important Information to Know as the Temperatures Falls Below 42 Degrees

During winter months when temperature falls below 42 degrees many heat pumps will need to go through a “defrost” cycle during operation. These temperatures are not normal all winter in Phoenix, but when it does happen, it is common for a heat pump to have some ice buildup and need to be defrosted. Many of our customers are not familiar with this process. Often this has not been explained well before installation, which leads to a lot of confusion about why the heat pump is not working.

What is a “defrost cycle”?

In heating mode a heat pump extracts heat from the outside air and transfers it inside your premises to warm it. When the ambient temperature outside gets very cold (close to 0°C or below) the moisture in the air freezes on the outdoor unit’s heat exchanger as the fan blows the air across it. A defrost cycle is simply the system recognizing that ice has formed or begun to form and automatically fixing this.

Why does my unit have to do a defrost cycle?

Any ice building up on the outside heat exchanger reduces the airflow across it, which will effect the efficiency, sometimes reducing it dramatically. In extreme cases this can also cause damage to the outdoor unit.

How do I tell if my unit is in a defrost cycle?

Inside you will notice the unit will stop heating, the indoor fan will stop and depending on the model there will usually be some form of visual indication like a light on the unit (usually the “run” light) will blink continuously. Outside, the outdoor fan will also have stopped and the compressor will be running.

The following is what you can expect to experience during a defrost cycle:


  1. You may or may not hear a loud swishing noise as the system goes into and out of the defrost cycle.
  2. Compressor can sound louder than normal.
  3. The outdoor motor will shut off during the defrost cycle.
  4. As the outdoor unit defrosts, you may see steam from the unit.
  5. The machine will shed water onto the roof and or ground by the unit. This water is distilled and will not harm the roof.


  1. It’s normal to feel cold air coming through the supply vents during a defrost cycle (unit in a/c mode).
  2. During the defrost cycle; Do Not Turn off the thermostat. This can delay the defrost cycle, as it can last up to 20 minutes.
  3. Your thermostat may drop a few degrees depending on the length of the defrost cycle (this is normal).
  4. After system has gone thru a defrost cycle, it will return to heating.
Watch this video on how Heat pumps are great at maintaining a temperature, but will not raise temperature in the house as rapidly as a gas or electric furnace.
During the cold winter nights and mornings it is best to keep your thermostat at a consistent comfortable temperature. We recommend between 69-72 degrees. The key thing to remember about your heat pump is to SET IT AND FORGET IT!!

Wolfgang's Cooling & Heating has a team of professionals that can come to your home and provide a complete inspection of your Heating System. The investment that you have made into your home is one that you want to protect. Call or Text Message Wolfgang's Cooling & Heating at (480) 719-7779 Today.