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Heat Pump Freezing Up: Causes & Solutions



Being a homeowner, you depend on the heat pump to maintain a comfortable indoor climate during the winter season. But, if you’ve ever observed your outdoor heat pump frosting up in cold weather, you surely understand how inconvenient and alarming it can be. A frozen heat pump can contribute to lower efficiency, higher energy bills and, in some cases, system failure if the issue is not addressed promptly. Figuring out the causes of a heat pump that is frozen, especially during cold weather when the defrost cycle may be crucial, is very important for a good running HVAC system. Through the determination of the real cause of the problem, you can implement steps to prevent further occurrences and the heat pump will work at the best of its ability. Therefore, it is time to dive into the realm of heat pump clogging and provide you with the skills to always keep your home warm and comfortable through the winter.

heat pump freeze up

Understanding Heat Pump Basics

Before we delve into the causes and solutions of a frozen heat pump, we need to know how a heat pump works so that we can understand its basic principles. A heat pump is a multi-purpose HVAC system, which can both heat and cool your home, by simply transferring heat from one place to another. In winter, a heat pump pulls heat out from the outside air and transfers it inside, but in summer, it reverses the process, taking heat out from your home and releasing it outside.

The key elements of a heat pump consist of an outdoor unit that houses the compressor, condenser coil and expansion valve and an indoor unit that includes the evaporator coil and the air handler. This refrigerant circulates between these units, taking up and releasing heat as its state changes, from liquid to gas and back again. Knowing these basic principles will aid you to better get a grip on the causes of a heat pump failing to work properly.

When Heat Pump Freeze Ups Occur

The most common reasons for heat pumps to freeze up are the low temperatures that usually occur in the winter season when the outdoor temperature drops below the freezing level. The heat pump will try to draw heat from cold outdoor air. Therefore, moisture can build up on the outdoor coil. The moisture would start to condense on the coil if the coil’s temperature falls below 32°F (0°C), creating a temperature difference that can lead to ice formation. Due to the presence of the ice layer, the coil’s surface will become frozen.

A little frost or ice on the coil placed outside is quite common and does not usually indicate a problem. On the one hand, the heat pump is able to operate normally even if there is some ice buildup on it. But, on the other hand, if the ice buildup becomes excessive, it can hinder the heat pump’s capacity to absorb heat from the outdoor air efficiently. This could lead to a drop in the heating capacity of the system, more power consumption, and might even damage the compressor if the problem is not addressed. Detecting the symptoms of a frozen heat pump, like decreased airflow from the vents or ice forming on the outdoor unit, requires keen observation to allow for timely action.

How Heat Pump Defrost Systems Work

To deal with the problem of a frozen heat pump, manufacturers have developed the defrosting systems, which are used periodically for removal of ice buildup from the outdoor coil. These systems achieve the cooling of the heat pump by running it in the reverse mode, where the refrigerant in the outdoor coil is heated up and melts the ice.

Heat pump operates in defrost mode during which it switches from heating mode to cooling mode for a specific time interval, typically 5 to 15 minutes. The switch valve changes the path of the hot refrigerant to the outdoor coil, and the outdoor fan is turned off to not let cold air in the house. Once the ice has melted, the heat pump should be able to continue its normal heating operation. Modern heat pumps are fitted with sensors and control boards that can start and stop the defrosting cycle automatically once the outdoor temperature and the coil temperature are considered.

Air source heat pump outdoor unit covered with snow and frost

Common Causes and Solutions of Heat Pump Freezing Up

Low Airflow

Low airflow is a common culprit behind a heat pump freezing up, as it can cause the outdoor coil to become too cold and accumulate ice. The most frequent reasons for insufficient airflow include dirty air filters, blocked air vents, or debris around the outdoor unit. In fact, a clogged air filter can reduce airflow by up to 50%, putting significant strain on your heat pump system. To resolve this issue, start by checking your air filters for dirt and debris, replacing them if necessary. Additionally, ensure that your outdoor unit is free from obstructions such as leaves, grass clippings, or snow, as these can impede proper airflow and lead to a frozen heat pump.

Refrigerant Leaks

If a leak develops in your heat pump unit, the refrigerant levels will drop down, preventing the heat pump from absorbing heat as efficiently as it should. When the refrigerant charge is low, the evaporator coil becomes too cold. This causes ice buildup and freezing of the whole system. It is of great significance to have a professional HVAC technician inspect your refrigerant levels, as low refrigerant levels are often a sign of a leak. Working with refrigerant is dangerous and requires special training and equipment. A good technician can detect and fix the leaks, thus improving the heat pump’s efficiency.

Improper Defrost Settings or Defrost System Malfunction

The defrost system in your heat pump is constructed in such a way that it melts off any ice on the outdoor coil, so that the heat pump operates in the most efficient way possible. But, defrost settings may be wrong or the system can fail and the heat pump will become frozen. Many reasons of defrost system failure are related with the thermostat, timer or control board malfunction, including a stuck or broken reversing valve. Continuous maintenance and professional check-ups are a must to make sure the defrost system is working well and the coil is free of ice buildup.

Dirty Coils

A very dirty evaporator and condenser coils will impact the heat pump’s performance of absorbing and releasing heat effectively. When the coils are covered with dirt, dust or debris, it forms a kind of insulating layer that impede heat transfer. This decrease in efficiency may make the outdoor coil to be too cold, and this will cause the ice buildup and freezing of the heat pump. While a 0.042 inches of soil on the coil can decrease the heat transfer efficiency up to 21%. To eliminate this issue, the coils of your heat pump should be cleaned periodically. A professional HVAC technician can provide a full cleaning service using unique equipment and methods, which will guarantee the system works well and the risk of future freeze-ups is reduced.

Air conditioner Outdoor Unit in Winter

Outdoor Conditions

The freezing low temperatures and extreme humidity can help the coil of your heat pump to become an ice-maker. Wind could make the situation even worse by speeding up the heat getting out of the coil which would lead to the coil being more prone to freezing. If this is possible, you may probably consider lowering the temperature setting on your thermostat to lessen the amount of time the heat pump operates in defrost mode. In locations with the most severe winter conditions, a heat pump cover or blanket may help improve insulation and shield against the environment.

Improper Installation or Design

Incorrect installation of heat pump system may result in a couple of problems like improper air flow, drainage problems, or inappropriate protection from weather elements. This can be the case of a frozen heat pump which can further lower the heat pump’s effectiveness and may even cause damage to the system. The right design and proper installation of a heat pump by a professional HVAC technician are critical to ensure that it is able to serve your home uniquely. A professional will help you take the right actions to avoid costly problems and will ensure that your heat pump will work in the most efficient way for a long time.

Low AirflowReduced airflow from vents Ice buildup on outdoor unitCheck and replace dirty air filters Clear obstructions around outdoor unit
Refrigerant LeaksReduced heating performance Ice buildup on coilsContact professional HVAC technician to identify and repair leaks
Improper Defrost SettingsExcessive ice buildup on outdoor coilEnsure proper defrost settings Schedule professional maintenance
Dirty CoilsReduced heat transfer efficiency Ice buildup on coilsClean evaporator and condenser coils regularly Schedule professional maintenance
Outdoor ConditionsIce buildup during extreme cold and high humidityAdjust thermostat settings Consider installing heat pump cover or blanket
Improper InstallationUneven airflow Drainage issues Insufficient protectionConsult with qualified HVAC technician to ensure proper installation and design

Preventive Measures to Avoid Heat Pump Freezing Up

Regular Maintenance

Maintaining preventive measures is the key to avoiding heat pumps freeze-ups and ensuring the system functions at optimum efficiency. Scheduled maintenance, performed by an HVAC expert, is one of the major ways to stop ice formation on your heat pump. Schedule every year a tune-up with a qualified HVAC technician who is able to perform a complete inspection of the system, clean the coils, check the refrigerant level, and solve potential issues before they get to the point of freezing the heat pump. Also, be active in the removal of any barriers to the free flow of air, say, by clearing debris around the outdoor unit or furniture blocking the indoor vents. The monitoring of outdoor conditions and making the necessary adjustments to the thermostat settings can as well help in lowering the risk of ice accumulation.

Residential Heat Pump and AC HVAC unit in the Snow

Proper Installation and Design

A right installation and design are the key factor that help avoid heat pump freezing. An air conditioner that is either too big or too small will most likely be cycling a lot and in the process, the outside coil will become too cold and freeze. A HVAC contractor can evaluate the load calculation of your house and determine the correct size heat pump for your home. The factors that are taken into consideration include square footage, insulation, and climate. Besides, they can help with correct installation of your heat pump in a good place that is not exposed to direct sunlight and protected from extreme weather conditions. Through the proper installation and design, you will be able to operate a reliable and efficient heat pump system which will give you consistent comfort throughout the whole year without having to worry about expensive freeze-ups.


Heat pump freezing up can be a really frustrating and worrying problem for homeowners, but knowing the most common reasons for this and the solutions for them will help you fix the problem properly. Keep in mind that a regularly maintained, well-functioning heat pump is not only a reliable source of heating and cooling but also can save you money on energy bills and prolong the life of the system. With knowledge and being active for the health of your heat pump, you can provide yourself with a warm home all the time in the winter.

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