What Is The Heat Pump Defrost Mode?

A heat pump uses a defrost mode when frost builds up on the unit in cold weather and prevents it from working properly. It switches from heat mode to get rid of the frost so it can continue extracting heat from the outdoor air.

The defrost mode isn’t discussed much when it comes to heat pump HVAC systems. But, it’s important for homeowners to understand what it is and how it works, especially since these systems aren’t only used for air conditioning any more.

Now that heat pumps are strong enough to keep homes warm all winter long in Pennsylvania, Maryland, New Jersey, and Delaware, the defrost mode is a more important feature. In this article, I’ll walk you through everything you need to know.

I’ve been the product manager for the Mid-Atlantic Region of Peirce Phelps, a national HVAC distributor, for 14 years. I specialize in high-efficiency equipment like heat pumps and mini splits. And, I work with dozens of HVAC contractors who install thousands of these in homes across the region.

Click below to get more information on specific models and how they work. Or, schedule a free consultation with a certified installer in your area.


Explaining the Heat Pump Defrost Cycle

Heat pumps are an efficient way of heating and cooling homes. They work by transferring heat from the air outside into your home during the winter and vice versa during the summer. However, during the winter months, frost buildup can occur on the outside unit, which can affect the heat transfer process.

What is emergency heat defrost heat pump mode?

Heat pump defrost mode is a feature that allows the heat pump to remove frost buildup from the outdoor unit. This is important because the frost buildup can hinder the heat transfer process. Doing so makes the heat pump less efficient at heating your home.

Why does a heat pump have a defrost cycle?

The defrost cycle is necessary during the winter months because frost build up can cause a reduction in the efficiency of the heat pump. The frost encasing the unit acts as an insulator, preventing the transfer of heat. In extreme cases, the ice can become so thick that it can damage the unit.

How does the defrost cycle work?

The defrost cycle reverses the flow of refrigerant in the heat pump. This causes the outdoor coil to become warm and the outdoor fan melts any frost buildup on the coil. The melted ice drips off the coil and into a drain pan.

The defrost cycle usually lasts between 5-15 minutes. The heat pump switches into this mode every 30, 60, or 90 minutes as needed. It occurs automatically when the heat pump detects frost buildup.

Controlling The Heat Pump Defrost Mode

Time-temperature defrost

Heat pump defrost mode can be controlled in different ways, including time-temperature defrost. This method uses a defrost thermostat to measure the temperature of the outdoor coil.

Once the thermostat reaches a set temperature, usually between 32-36°F (0-2°C), the heat pump enters defrost mode.

The time-temperature defrost method is often used in areas where the temperature remains consistently low.

Demand defrost

Another way to control the heat pump defrost mode is through demand defrost. This method uses defrost relays to determine when a defrost cycle is necessary.

The defrost relays monitor factors such as the length of the run time, the number of cycles, and the amount of energy consumption.

This method is more energy efficient compared to time-temperature defrost because it only initiates a defrost cycle when necessary.

How long should a heat pump run in defrost mode?

The length of time a heat pump should run in defrost mode varies depending on several factors such as the amount of frost buildup, the efficiency of the unit, and the electricity source.

Typically, a defrost cycle lasts between 5-15 minutes. During this time, the fan blows cold air to help felting the ice, and emergency heat may be used to maintain indoor temperature.

However, if the defrost cycle lasts longer than 15 minutes, there may be an issue with the frost thermostat or electrical connections. This can affect energy consumption and efficiency.

Common Causes of Heat Pump Defrost Mode Problems

Here are some common causes of heat pump defrost mode problems:

Faulty Sensors, Reverse Valves, or Thermostat

Malfunctioning internal components like the sensors, reverse valves, or thermostat can cause the heat pump defrost mode to stop working. This can happen due to corrosion, debris, or other factors.

Refrigerant Leaks

Leaky refrigerant can cause the heat transfer process to be less efficient, leading to frost buildup on the coils. The unit may run for longer periods than necessary, and the defrost cycle may not be sufficient to melt the ice.

Electrical Issues

Issues with the electrical connections or the electricity source can cause the heat pump defrost mode to malfunction. For instance, a malfunctioning circuit breaker can trip and prevent the unit from running correctly.

Improper Installation

If the heat pump is not installed correctly, it can cause several problems, including the defrost mode not working. The inner parts of the heat pump may not be connected correctly, or the controls may not be calibrated properly.

Damaged or Broken Coils

The outdoor unit of the heat pump may be damaged due to external factors such as hail, ice, or debris. Damaged or broken coils can cause the unit to freeze and affect the defrost cycle.

Outdoor Unit Getting Clogged

If the outdoor unit of the heat pump is not maintained regularly, it can become clogged with dirt, leaves, or other debris. This interrupts airflow and heat transfer.

How to Prevent A Heat Pump From Freezing Up

Regular maintenance is critical to prevent a heat pump from freezing up. Regularly inspect and clean the outdoor unit and ensure that there is no debris or snow buildup around it.

Check for any obstructions in the outdoor unit, such as fallen leaves, which can restrict airflow. Replace dirty air filters to ensure that warm air can circulate freely.

Additionally, ensure that the refrigerant lines are not leaking and that there are no issues with the sensor, which can cause the heat pump to malfunction.

If you are not sure how to carry out these checks, consult with a professional technician to ensure that your heat pump is well-maintained.

How To Defrost A Heat Pump

When the heat pump goes into defrost mode, there are several ways to defrost it. The first method is to defrost with water. Use a garden hose to spray warm water over the outdoor unit.

The water should be warm, not hot, to avoid damaging the heat pump. The second method is to switch the heat pump to fan-only mode, which can melt the ice buildup on the outdoor unit.

The third method is to manually defrost the heat pump by felting the ice away with a soft brush.

Emergency Heat

If your heat pump is malfunctioning and you need heat, switch to emergency heat. Emergency heat is a backup heating system that reverses the heating process, using electricity to produce warm air.

Emergency heat can be expensive, so it should only be used in emergencies.

Regular maintenance can help prevent issues with the heat pump’s defrost mode. But if the heat pump is malfunctioning, it may need to be defrosted manually.


How do I know if my heat pump is in defrost mode?

You can tell if your heat pump is in defrost mode by checking the outdoor unit. During defrost mode, the outdoor unit will produce steam or a cloud of water vapor. You may also hear a hissing sound as the refrigerant moves through the system.

What happens if a heat pump does not defrost?

If your heat pump does not defrost, it can cause several issues, including reduced efficiency and increased energy consumption. Ice buildup can damage the unit’s components and even lead to a complete breakdown.

What is the lowest temperature a heat pump will work?

Cold climate heat pumps can operate in temperatures as low as -15°C, but their efficiency decreases as the temperature drops.

Are you ready to learn more? Get a free consultation to learn more about expert heat pump installation in Lancaster, PA or anywhere in Pennsylvania, New Jersey, Delaware or Maryland.