Heat Pump Not Defrosting? Here’s What You Should Do.

When your heat pump stops defrosting it will affect the efficiency of your HVAC system or bring it to a halt. 

Eventually it can cause some major problems or damage that may require a very costly repair or even a possible replacement of the heat pump system.  The first question that comes to mind is “Why isn’t my heat pump defrosting?”

This page discusses common causes of a heat pump defrost cycle not working. We recommend solutions too. If you are mechanically inclined and enjoying figuring out issues like this, you might be able fix the defrost issue with your heat pump. Otherwise, a heating and cooling technician should be contacted to troubleshoot and repair your heat pump.

What Does It Mean To Defrost Your Heat Pump?

The outside coil on your heat pump can ice up in very cold weather. Refrigerant in the coil removes heat from the outside air, transferring it through the radiator-like fins of the coil. Perhaps you’d think this would make the coil warm; It actually makes it extremely cold – cold enough to frost up if there is enough moisture in the air.

To prevent this, heat pumps defrost either:

  • On a set schedule, (older models, mostly). Most run a defrost cycle every 30 to 60 minutes.
  • As needed – Newer heat pumps are equipped with a thermostat and a sensor that will start defrost mode when necessary to avoid icing up.

To defrost, whether on a set schedule or as needed, the reversing valve in the heat pump switches. So, let’s say your heat pump system is in heat mode, drawing in heat from outdoors. That makes the outdoor coil very cold. The system will switch to air conditioning mode for a few minutes, pulling heat energy from inside your house and sending the heat to be dispersed through the outdoor coil. The heat defrosts the coil. Done. 

In summer, if your indoor coil freezes up, then the system will switch heat mode. Heat will be brought in for just a few minutes – long enough to defrost the coil without heating up your already-warm house.

The performance of the reversing valve and refrigerant should do the job – Unless, of course, there’s a problem with the heat pump defrost cycle not working. So, let’s explore causes. 

Reasons Why Your Heat Pump Is Not Defrosting

If you are experiencing issues with your heat pump not defrosting properly, check these possible reasons before contacting a heat and cooling technician. Some of the reasons are easy fixes; others should be addressed by a professional HVAC technician.

A heat pump not defrosting can occur in winter or summer, but the location of the ice-up will differ. In winter, the outside heat pump might not defrost properly. In summer, the indoor coil in the air handler can ice up. Most issues are with the outside unit, the condensing unit, so that is primarily what this article covers.

Ice and Snow!

This unit might be headed for trouble!

Ice and Snow

Icing up can occur in colder climates that have extreme temperatures and snow. If your heat pump is covered by snow or ice, then there is a good chance that it will not defrost properly. Those elements need to go!

Pro Tip: Be cautious – take it slow and easy – when removing snow or ice from your heat pump.  Do not use sharp or hard objects to chip away at the snow or ice buildup. This can cause damage to the coils, reducing their efficiency or leading to coil replacement in extreme cases. The best ways are to simply brush the snow off the unit or run water over the heat pump to get rid of the buildup of snow or ice.

Drainage Problems

 A clog of leaves, sticks, snow, ice and other debris could prevent proper drainage. Then when the weather turns freezing, the clog can cause a buildup of ice on the outside unit, and that will definitely cause improper heat pump performance.  To remedy this situation you should make sure everything is clear and that drainage around the unit can occur properly.

Pro Tips: Keep overhead branches trimmed back, so they’re not constantly dropping twigs and leaves onto the unit. Make sure the gutters above are in good condition and emptied, so they’re not leaking or overflowing onto the heat pump.

It is Low on Refrigerant (Leaking)

If the refrigerant or coolant is low then the heat pump will not go to defrost mode or the defrost mode will not be efficient.  The refrigerant is pumped through the coils to heat them up in order to defrost the heat pump.  If the refrigerant is low then there will not be enough coolant to go through the system.  This can be caused by damaged or broken coils or a leak within the heat pump system.  If this occurs a few things can be done.  First of all, you can recharge your system with more refrigerant.  If recharging your heat pump doesn’t work then your coils might need to be replaced due to a break or a leak.

According to our Heat Pump and Air Conditioner Repair Guide, fixing  refrigerant leak and recharging the system costs anywhere from about $200 to a $1,000 or more depending on the damage and the amount of refrigerant the system requires.

Improper Installation

If your heat pump isn’t defrosting or even functioning properly, then there is a chance that the entire installation of the system is not done correctly.  The original installation of a heat pump should be done by a heating and cooling company that specializes in heat pumps. Some common mistakes that lead to improper installation are that the system isn’t charged with enough refrigerant or there is a kink in the refrigerant line due to careless workmanship.  Another possible installation issue is if the heat pump is positioned in an area that does not have proper air flow.  If you suspect issues with installation, call the company that installed it to have them come out and check and make the required repairs in order to get your heat pump defrosting properly.

Faulty Reversing Valve

When mentioned this valve earlier. It changes the direction refrigerant flows, allowing your heat pump to switch from air conditioning to heat and back.  If this valve becomes faulty then the defrost mode on your heat pump will not work properly.  When this occurs a new valve will need to be installed.  This would be a job for a heating and cooling technician, but it can be done by a handy, “do it yourself” person

Electrical Issues

Older wiring can fail, preventing the heat pump from getting the signal to defrost. Connections can also get bad over time, resulting in a failure.  You can look over the wiring of your heat pump to make sure that all the wires are connected properly and that they are in good condition.  If a wire needs to be replaced and you don’t have much experience in the electrical field, then an electrician or a heating and cooling specialist should be hired to do the job.

Damaged Coils

 Damage to a coil, such as corrosion or getting run into with a lawn tractor, can cause a refrigerant leak that will prevent defrosting. Inspect the coil for signs of damage, or call your HVAC tech to do the inspection, check refrigerant levels and look for other repair issues.

Faulty Sensor or Thermostat

If the heat pump is one that uses an internal thermostat to signal it to defrost, the thermostat could be bad. Manually switch the wall thermostat to AC mode for a few minutes. That should cause the system to absorb heat from inside your home and send it outside to the frosted-over coil and begin the defrost process .Obviously, that’s not a long-term solution. You’ll need to have the system properly diagnosed and repaired.

Outdoor Fan Issue

The fan motor on the outside condensing unit / heat pump can go bad and stop working or stop pushing out enough air to effectively defrost the coils. This could also be an issue with a part called a capacitor that stores a little extra electrical charge to give the fan a boost for starting. See our Pick HVAC Capacitor Repair Cost and Guide for details on diagnosing and repairing this common problem. Remember to follow safety precautions when working with electrical systems.

Related Articles:

Quick Defrost Issues Checklist

If your heat pump is not defrosting the first thing you should do is make sure that the unit doesn’t have any leaves or debris blocking any areas where air needs to flow freely.  This is a simple fix that most homeowners can do. 

Next, check to make sure that there isn’t any ice or snow buildup on or around your heat pump. This build up is usually located on or near the coils.  If there is ice buildup on the heat pump, including the coils, remember that the best solution is to run water over it to melt the ice.

If leaves and debris or ice and snow are not the issue then the problem with your heat pump not defrosting properly is most likely something mechanical or electrical.

After troubleshooting and repairing the problem with your heat pump not defrosting or calling a heating/cooling technician to fix the problem, you should consider installing a supplementary defrost kit. A well-known one is the EDIDS, or Energy Docs Intelligent Defrost System.  There are several types available, however, and a kit is an added element that will allow your heat pump to defrost more effectively and efficiently. 

The EDIDS system is unique in that it has a timer, thermostat and sensor, but it also has a pressure sensor that makes the system better than just a standard temperature or sensor model that is  located on a heat pump. When the coils on the heat pump begin to freeze, then pressure increases and the EDIDS system senses it – and kicks into defrost mode.  The best attribute defrost kits with sensors is that they only has your heat pump go into defrost mode when it is necessary, not only during certain times of the day/night.

In conclusion, there are multiple reasons why your heat pump is not defrosting.  Troubleshoot and use the ideas listed above to try to fix the problem.  If the problem cannot be fixed easily then you should call a professional HVAC technician to repair the heat pump.  The longer you run a heat pump that is not defrosting properly, the more damage it will cause over time.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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