Maximizing Performance: The Heat Pump Defrost Cycle

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By Debarghya Roy

Heat pumps are a highly efficient way to heat and cool both residential and commercial buildings. But they come with an important feature to help them work efficiently in cold weather: a defrost cycle.

In this blog, we’ll discuss why the heat pump defrosts mode is so important, how it works, common issues related to the process signs that your heat pump might be running too often or not at all, and how you can maintain its effectiveness.

Key Takeaways

  • Heat pump defrosts cycles are essential for maintaining efficiency and preventing ice buildup.
  • Defrost cycles consist of reversing the refrigerant flow, activating auxiliary heat, and using defrost sensors.
  • The frequency of a heat pump’s defrost cycle depends on ambient temperatures outside as well as weather conditions.
  • Proper maintenance can extend a unit’s lifespan by ensuring components remain in working order while avoiding unnecessary energy losses due to inefficient operation.

What is a heat pump defrost cycle?

heat pump defrost cycle

A heat pump defrost cycle is a system that occurs when the unit senses that outdoor temperatures are too low for efficient heating, so it switches into reverse operations and begins to draw in warmer air from outside. 

During this process, excess frost builds up on the evaporator coil which needs to be quickly removed to prevent damage and keep system efficiency high.

How Does A Heat Pump Defrost Cycle Works?

The heat pump defrost cycle works by reversing the flow of refrigerant, and activating auxiliary strip heaters and sensors to monitor the temperature.

Reversing The Refrigerant Flow

When the defrost cycle activates, it triggers the compressor and heat pump reversing valve to move the refrigerant in a different direction.

  • This allows the condenser coil to temporarily become an evaporator.
  • Using ambient air temperatures for general cooling until the frost melts enough that regular airflow is restored.
  • The outside fan also works in concert with this process by drawing outside air onto a warm outdoor coil. 
  • This provides added thermal energy that helps speed up ice removal from the coil’s fins and tubing.

Turning On the Auxiliary Heat

Auxiliary heat is a supplemental heating system that will provide a boost of additional energy when the outdoor temperature drops and regular heat pumps are unable to draw in enough outside air.

Typically, auxiliary heating systems will kick in during freezing weather or any time the outdoor coil temperature dips below about 30F. 

It ensures consistent comfort indoors by automatically activating when sky-high utility bills might be possible due to the inefficient performance of a regular heat pump.

In addition, it helps activate and maintains an effective defrost cycle as airflow is limited while in this mode.

The defrost cycle ensures anti-freezing throughout all frigid temperatures in order for your HVAC system to function properly, increase efficiency levels, and maintain longevity of the outdoor unit’s components even after years of service.

Activating Defrost Sensors

Heat pump owners should understand the importance of activating defrost sensors in their systems. 

Defrost sensors help thaw any ice buildup that occurs on the outdoor coils when temperatures drop below 32 F to maintain efficiency and extend its lifespan.

This process is carried out through an internal clock, which triggers a signal that sends the unit into a defrost cycle that prevents the outdoor fan from turning on and accelerates temperature increase within the coil itself.

To do this, there are steps you can take to ensure proper activation of your system’s defrost sensor such as,

  • Checking with respected manufacturers for instructions
  • Ensuring there is no build-up of dirt or debris on or around the sensor
  • Testing it if necessary with an electronic meter before exercising voltage checks across it.

All these will prevent full-functioning system failure while keeping energy costs low and maximizing performance over time.

Why Do Heat Pumps Defrost Systems?

Heat pumps have to defrost systems to prevent ice buildup, maintain efficiency and extend their lifespan.

Prevents Ice Buildup

Ice buildup on the outdoor coils of a heat pump is one of the most common issues that can affect its performance. If left unchecked, it can cause more air to be drawn in through the indoor unit which results in higher energy bills and decreased efficiency.

In cold weather, water droplets from humidity freeze when they come into contact with the coil’s icy surface, creating a blanket of thick frost that covers it up and reduces airflow even further.

Maintains Efficiency

Heat pumps have a defrost cycle that is instrumental in maintaining efficiency and optimum performance of the system. 

This cycle helps clear off frost from the outdoor coil which can occur when the ambient temperatures are too low, or due to moisture accumulation on its surface.

When there is standing ice buildup on the outside coil, it can drastically reduce the effectiveness of heat transfer; leading to much lower heating output as required for interior areas of your home.

Furthermore, regular defrosting also helps prevent energy losses thus reducing strain on other components like fan motors inside the unit and helping you achieve optimal HVAC operation even during cold weather.

Extends Lifespan

A well-maintained heat pump defrost cycle can help extend the life of your system by preventing and removing frost build up on the outdoor coils. 

The accumulation of ice on these coils lowers their effectiveness, which in turn reduces the performance and efficiency of the entire heating system.

During a defrost cycle, hot refrigerant is exchanged in order to heat up the evaporator coil as it melts away any frost that has built up on its surface.

This prevents further accumulations and minimizes wear from repeated cycles. 

Regular maintenance for your heat pump should include checking all components of the defrost cycle, especially during cooler months when there is an increased risk for buildup due to lower temperatures outdoors.

A faulty thermostat or low levels of refrigerant could also affect how often a heat pump enters into a defrost mode.

How Often A Heat Pump Should Defrost?

The frequency of a heat pump’s defrost cycle depends largely on the ambient temperatures outside and weather conditions, which can range from every 35 cycles to several times an hour.

What Causes A Heat Pump To Go Into Defrost Mode?

A heat pump defrost cycle can be triggered by a variety of factors, but most commonly it occurs due to ice buildup on the outdoor coil

This happens when ambient temperatures are low and humidity levels are high, creating condensation that freezes on the outdoor coils.

A common sign of a unit in need of a defrost is steam or fog coming from the system’s outdoor unit. 

A “frost thermostat” or “defrost control” will also activate if specifically designed sensors detect frost on the heat pump coils or even non-optimal outside temperatures for long periods at night.

In addition, malfunctioning fan blades caused by debris blocking air flow can indicate an underlying issue with either the motor itself or clogged air filters resulting in inefficient operation causing defrosts cycles too often.

Other culprits include refrigerant leaks leading to decreases in pressure between indoor and outdoor units as well as rodent damage to wiring around fan motors .

How Long Does A Heat Pump Defrost Cycle Last?

The duration of a typical heat pump defrost cycle can range between 30 seconds to a few minutes and shouldn’t occur more often than every 35 minutes.

The length of time is likely to differ depending on factors like the type of heat pump, weather conditions, humidity levels and outdoor temperatures. 

How quickly your heat pump goes back into normal operation after defrosting will depend on its design.

Some may take longer than others since they need time for the outdoor fan motor to kick back in once it has been shut off temporarily during deicing operations.

Depends On Weather Conditions

As temperatures drop and outdoor coils become colder, heat pumps can go into defrost cycle more frequently in order to keep the coils clear of ice buildup. 

Heat pump efficiency is at its maximum when the outdoor coil temperature remains above 32ᵒF/0ᵒC.

Once temperatures dip lower than this, frost may begin to form on the outdoor coil which will negatively impact cooling/heating performance as well as cause wear and tear on system components.

This is why frequent or excessive defrost cycles are a sign that preventative maintenance may be necessary for optimally running your heat pump system. 

Common occurrences like heavy snowfalls from winter storms or extreme changes in outside air temperatures could cause higher frequencies of defrost cycles within shorter time periods compared to regular usage where it would only require around 35 minutes per cycle.

Automatic Vs Manual Settings

DescriptionAutomatic settings are set in place to give the heat pump unit optimal performance at times of low temperatures. Manual settings provide a hands-on approach for users who want more control over their heat pumps.
FunctionThe machine is able to analyze weather data, temperature readings within and outside the home.It reduces wear on components as it will not go into defrost mode too often or too late either way. Keeping energy efficient at all times while guarding against any damage from freezing winters.
External or indoor workingThis ensures that frost can be prevented regardless of external or internal conditions. Users have full access to when exactly they want their pump to initiate its defrost cycle, allowing them flexibility with changing outdoor temperatures throughout winter months.

Signs That A Heat Pump Is In Defrost Mode

Be sure to look out for steaming on the outdoor unit, warm air instead of cold, and a hissing sound as potential indicators that your heat pump is in defrost mode.

Outdoor Unit Is Steaming

When the defrost cycle starts, a homeowner may see steam, or smoke, emanating from their heat pump’s outdoor unit. 

Start of defrost cycleEmanating smoke in the first step is completely normal and indicates that the system is working correctly to prevent ice buildup which can impact its efficiency and even cause damage.
Switch off the componentsDuring this process of moving warm air across frozen components of your heat pump outdoors, excess moisture collects on components like filters or condenser coils.It reaches dew point (usually less than 0 degrees Celsius), steam appears with some hissing sound as it escapes from your unit for a few minutes till it goes into cooling mode again.
Transfer of warm airDuring this process of moving warm air across frozen components of your heat pump outdoors, excess moisture collects on components like filters or condenser coils.It reaches dew point (usually less than 0 degree Celsius), steam appears with some hissing sound as it escapes from your unit for a few minutes till it goes into cooling mode again.

Warm Air Instead Of Cold

It’s not uncommon for homeowners with heat pumps to be surprised when their unit starts blowing warm air instead of cold. 

This is a normal part of the defrost cycle and occurs when the reversing valve in the pump is activated and runs the refrigerant backward, extracting heat from inside your home and sending it to the outside coil compressing it.

The process helps outdoor coils that have frozen over due to extremely cold temperatures or heavy snowfall. So that moisture doesn’t build up into ice on them which can affect efficiency and longevity of a heat pump.

Signs like,

  • An outdoor fan motor humming
  • Steaming in winter months
  • Visual indicators 
  • Slight hissing sound 

are surefire ways for owners to know if their heat pump is switching from heating mode to cooling mode during its defrost cycle.

Slight Hissing Sound

During the heat pump’s defrost cycle, reddish-orange hot vapor can sometimes be seen on the outdoor unit. 

This is due to a reversal of flow: for brief periods, refrigerant reverses through the system and exits from the outdoor coils as warm deflation rather than cold deflation.

At this time which is very brief  you may also hear a slight hissing sound coming out of your Unit; this indicates that everything is working correctly.

The hissing noise made by the refrigerant flowing through the system helps in melting any ice buildup quickly so it won’t reduce efficiency or snow up in colder weather.

Common Issues With Heat Pump Defrost Cycles

To ensure your system runs efficiently and keeps your home warm during cold weather, it is important you take the necessary precautions and follow directions from qualified technicians.

Sensor Malfunctions

Sensor malfunctions are one of the main causes of heat pump defrost cycle problems. Heat pumps use sensors to detect when ice has accumulated on their outdoor coil.

When this happens, the heat pump goes into a defrost cycle in order to reduce or eliminate the ice buildup. 

If these sensors fail, it can lead to improper automatic cycling, causing an increased energy consumption and reduced efficiency.

In addition, if there is too much frost forming on the outdoor coil due to consistent low temperature readings from a faulty sensor, this can overwork your system by forcing repetitive defrosts or putting excess strain on its compressor.

Heat pump owners should inspect and maintain their systems regularly for signs of sensor malfunctions such as incorrect readings leading up to manual interventions (such as activating emergency heating modes).

Keeping your coils clean and free of debris will also prevent any potential blockages that may affect the operation of your heat pump’s thermostat sensor. 

Dirty Coils

Having clean and well-maintained coils is important for any heat pump to function properly. 

Dirty outdoor coils can fill up with dust, dirt, debris, grass clippings, leaves and other pollutants that reduce the coil’s efficiency by preventing it from releasing the correct amount of thermal energy.

This means the unit has to work harder in order to achieve desired temperatures, thus shortening its lifespan and increasing energy costs. 

A dirty evaporator coil on certain units may cause issues during the defrost cycle known as “Dirty Sock Syndrome” where bad odors are released throughout your home or office from trapped moisture and dust particles inside the coil resulting in potential health risks.

Low Refrigerant Levels

Low refrigerant levels in a heat pump can cause it to struggle to absorb or release the environment’s thermal energy during its defrost cycle. 

Without the right amount of refrigerant, a heat pump won’t be able to take away ambient warmth from outside and transfer it indoors during winter months or remove indoor humidity from inside and expel it outdoors during summer months.

If the system goes into defrost mode, an improperly functioning unit may not be able to switch between modes as quickly as one that is properly maintained leading to dullness in the air coming out of air outlets due to poor performance of the pump’s compressor.

How To Maintain A Heat Pump Defrost System?

Regular cleaning and maintenance of the system, as well as checking for proper airflow and monitoring refrigerant levels create an efficient heat pump defrost cycle that runs smoothly.

Regular Cleaning And Maintenance Of Heat Pump

Properly maintaining a heat pump defrost system is essential for ensuring that it works reliably and efficiently. 

Neglecting regular cleaning and maintenance can lead to costly repairs, decreased efficiency, and higher energy bills.

Not only should air filters be changed regularly, but also the outdoor coil should be cleaned at least once a year to keep the unit running optimally. 

Additionally, it’s important to ensure there is proper airflow over the coils so they can absorb heat or transfer cold effectively if there are any obstructions, these need to be removed as well.

Lastly, checking refrigerant levels can help prevent leaks that will reduce efficiency and cause further issues further down the line.

Checking For Proper Airflow Of Heat Pump

Proper airflow is critical for a heat pump’s defrost system to operate effectively. A lack of airflow can prevent the unit from starting and sustaining the necessary defrost cycle, resulting in reduced output and efficiency.

Insufficient airflow with increased humidity levels will cause frost buildup on piping or outdoor coils that impede efficient operation. 

Monitoring Refrigerant Levels Of Heat Pump

Keeping tabs on refrigerant levels is essential to a correctly functioning heat pump. 

Low or imbalanced refrigerant levels can impact this transfer of heat, leading to a decrease in efficiency that could damage other components of the HVAC system such as compressors, fan motors, or filters.

Therefore, monitoring pressure readings during the defrost cycle helps ensure maximum efficiency while preventing frost buildup from moisture accumulation around indoor coils or outdoor fans caused by low temperatures and high humidity.

Maintenance Of Dirty Coils

To prevent this issue, it is recommended that you regularly inspect both indoor & outdoor coils and schedule routine maintenance visits at least once a year if not twice depending on environmental conditions near your area (higher humidity areas).

Professional technicians have access to specialized tools used specifically for cleaning contaminants on coils so they can remain working efficiently while preserving their quality all throughout every season.


How does my system enter a defrost cycle?

Furnaces with temperature sensors will start the process whenever they detect icing around -4°C or lower. While other models may wait until several cycles of low-temperature operation have been completed in order to conserve energy by preventing frequent starts & stops of components within the unit itself. 

What can I do if my heat pump isn’t entering a defrost cycle properly?

If your heat pump is not entering a valid defrost cycle each time colder temperatures are detected outside then there could potentially be an issue with its thermostat connection or readings taken from sensors detecting lower air temperatures more accurately than expected.

Are there any advantages to having a heat pump defrost cycle?

During their regularly scheduled cycles, fumes produced from burning fossil fuels such as natural gas actually contribute towards global warming.

The users receive benefits both financially (in the form of lower bills) as well as socially because avoiding toxic fumes altogether means fewer pollutants are released in the atmosphere, worsening the situation worldwide even further every year!


Heat pump defrosts cycles are essential for maintaining efficiency, preventing ice buildup, and extending the lifespan of the heat pump unit. The defrost control and sensors regulate when to send hot refrigerant into the outdoor coil during a typical 30-second-long cycle, at regular intervals.

Heat Pump