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Defrost cycle, Don’t call the Fire Department!

How to know if your unit is in defrost…Every winter as the temperatures drop in the Scottsdale/ Phoenix area, into the 20 degree or teens, then people all over the valley decide to call the Fire Department, over for breakfast.  The Fire Fighters are all just waiting to be served a delicious breakfast by someone who calls them to put out the fire in their air conditioning systems which is actually called DEFROST CYCLE.  

In our neck of the woods, the great South West, many of us heat our homes with a system called a Heat Pump.  These systems provide cooling is summer and heating is winter, please disregard this article if you have a gas furnace or an electric furnace as it does not apply to you.  It only applies to those of us with heat pumps.

Cold mornings, where there is some moisture, tend to make the heat pumps turn into blocks of ice, as they try to provide enough heat.  There comes a point where the system has to get rid of the ice that it has made on the outside unit before it can continue its heating mission.  This process of getting rid of the ice outside is called the “Defrost Cycle” and when it happens, the unit gets noisier and at some point water vapor (that kind of looks like smoke from a fire) comes flying out of the top of the outside system.

Fire departments all over town love the cold early morning 911 calls that come in because of heat pumps catching fire.  Fire departments have learned that all Heat Pumps are designed to catch on fire on cold mornings, so there is no need to panic.  “Well that’s just stupid!” you might say. And you’d be right of course. They don’t really catch on fire, but here is what really does happen.


Heat pumps use the available heat with the outside air to operate in heat mode.  In the early hours of a cold morning, there is not enough heat to provide adequate operation and the outside system starts to ice up.  

If you are UN-fortunate enough to have to leave your warm home in the morning, look at the a/c units that you pass on the way in to work.  You may see some systems with a light frost coating OR others that are frozen to a solid block of ice. (See photo to the right)

The heat pump will leap into DEFROST MODE at some point to remove the ice buildup. While doing this, hot refrigerant is pumped through the outside coil to melt any ice, turning it to moisture, and ultimately into a vapor.  It also stops the outside fan so that the defrost works faster. When the defrost is over, the outside fan starts again and all of that vapor from the melted ice is blown out of the top of the outside system giving the appearance of smoke from a fire.  This vapor is blown away in less than a minute and normal heating operations continue.



…have defrost controls that are basically timers. Normally, older systems, take too much time in defrost making them less comfortable in heating mode.  Newer systems use sensors attached to circuit boards to do a much more accurate defrost in less time keeping you more comfortable.  Many systems have the ability to provide short term electric back up heat to help keep you warm during the defrost cycle.  Ask your service provider, if your system has that capability.

Its at that point that the uninitiated Heat Pump Owner shuts the system off in mid defrost and calls out the Fire Department.  When they get there, they find no fire but they are very hungry and you should feed them.  Also, you have just turned the system off and interrupted the defrost process.  So when you turn it on again, many things can happen to delay the return to normal heating function.

So then….heat pumps will at times make funny noises and produce water vapor that kinda looks like steam, and also kinda looks like white smoke from a fire.  It is best to just let the system alone to do its job or….start making some bacon and eggs…..make lots of bacon…..Firemen love bacon.

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