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HVAC Iced Up?

January 21, 2018

One of the common problems we encounter is a heat pump that has iced up.  A heat pump in good condition should be able to operate in below freezing temperatures without icing over, so why does this happen?

Things to Eliminate

Here are some circumstances that cause icing which you may be able check, fix or eliminate:

  1. Check to make sure the outdoor unit is not blocked by leaves or covered in any way.  Some people mistakenly think that covering the outdoor unit helps protect it from bad weather in the winter, but this is dead wrong. It always needs proper airflow around it.
  2. Check to make sure you do not have water dripping into the unit by a leaking or blocked gutter.  As temperatures drop, that water will harden into ice and could cause the fan blades to cease up.
  3. Check to make sure there is room for water to drain away from the unit… that it hasn’t sunken into the ground.  If water pools around the unit, it will freeze as temperatures fall, and the ice will impede the operation of the unit.
  4. Check to make sure that nothing is restricting airflow inside the house, such as dirty filters or blocked vents.

If you’ve eliminated these as possible sources of the problem, then it could be a mechanical or electrical problem inside the unit itself.  First, let’s look at how a heat pump should work.

How Heat Pumps Work

The refrigerant inside the heat pump transfers heat back and forth as needed. Even in the winter, there is heat that can be extracted from the outside air to help heat the inside of your home.  But to do so, the refrigerant needs to get very cold… much colder than the outside temperature.  When coils get this cold, water vapor in the air will start to crystalize into ice around them. To prevent ice build-up, the heat pump will periodically go into a defrost mode.  A valve switches so that the outdoor evaporator becomes the condenser.  This allows the coils to get warm enough to melt any ice that may have formed.  While in defrost cycle, the fan turns off so you won’t get cold air blowing on you, or a second-stage heater comes on to offset this cold air.  After the outdoor unit reaches a certain temperature, or after a certain amount of time goes by, the valve switches back and the system returns to normal heating mode, reversing the evaporator and condenser. This cycling on and off happens transparently to the user while the heat pump is in use.

Reasons Heat Pumps Ice Up

  1. Perhaps the most common reason heat pumps ice up, is that the reversing valve gets stuck.  This prevents the heat pump from going into its defrost cycle and the ice on the outdoor coils continues to accumulate until it impedes the turning of the fan blades.
  2. If the reversing valve is fine, it could be that something is wrong with the defrost timer, sensor, or control module, so that the defrost cycle is not completing or not happening often enough or not being triggered at all due to a faulty thermostat or sensor.
  3. If the system is low on refrigerant or the outdoor fan motor dies, this could also cause the system to ice up.

Each of these reasons will likely necessitate a service call.

In the Meantime…

If you can visually see your outdoor unit has iced over, turn the unit off.  If you are able to, turn on the emergency heat mode until help can arrive.  Do not continue trying to run the unit “normally” as this will only cause more damage, and don’t wait too long before seeking help, as this may increase the extent of repairs that are necessary.  Do not try to pick off the ice with a sharp object, as the coil and fins will damage very easily, and you may cause a refrigerant leak.


As with most HVAC systems, problems with a heat pump can often be prevented with proper maintenance.  Getting a fall maintenance check before the worst of the cold weather sets in, and getting a spring maintenance check before the heat of the summer, will help lessen the probability that you’ll find yourself all iced up!

If your HVAC system needs attention and you live in the Nashville area, call on Interstate AC Service at 615-802-2665.  We’re here for you.