Heat Pump Defrost Mode Explained

The defrost mode is an often-overlooked feature of a heat pump system. It’s when the heat pump switches from heating mode, melting the ice and frost built up on the outdoor unit. It’s essential to keep the system working properly in the winter. But, you may have some questions about what’s happening and why when this happens.

In this article, I’m drawing on my 14 years of experience in the HVAC industry to explain everything you need to know about a heat pump’s defrost mode.

I’ll start off my giving a quick overview of how heat pumps work and discuss the defrost process in great detail. Finally, I’ll give you tips on how to do it manually and avoid needing it at all.

I’ve spend more than a decade staying on top of the latest high-efficiency HVAC systems systems as a product manager for Peirce Phelps. I’ve also worked with dozens of HVAC contractors in the mid-Atlantic region.

They’ve installed thousands of heat pumps and mini splits in Maryland, Delaware, New Jersey, and Pennsylvania homes, and are very familiar with common homeowner questions or concerns.

If you want to learn more about specific models, download our free product guide below. To get answers about your home’s unique needs, schedule a free consultation with a certified local dealer.


A Brief Overview Of Heat Pumps

Heat pumps transfer heat from one place to another using a refrigerant running in a loop between indoor and outdoor units.

They can be used as heating systems by extracting heat from the outside air, ground or water, and sending it inside. Or as air conditioning by removing the heat from a building.

The Significance Of The Heat Pump Defrost Cycle

Frost can build up on a heat pump in cold weather. The frost acts as insulation, preventing heat from entering the system as it’s supposed to do. This reduces the system’s efficiency and performance – you’ll get less heating but pay more as the heat pump uses more electricity.

Eventually, it will also damage the compressor by overworking it.

The emergency heat defrost heat pump system will kick in to prevent these problems. It uses the reversing valve to switch out of heat mode. The refrigerant moves heat from the house out to the outdoor unit. That heat melts the frost built up on the unit.

Heat Pump Defrost Mechanisms

There are two main defrost strategies used in heat pumps: time-temperature defrost and demand defrost.

Time-Temperature Defrost Strategy

This strategy uses a timer to initiate the defrost cycle after a set amount of time has passed. The defrost cycle typically lasts 10 to 15 minutes, This cycle runs every 30, 60, or 90 minutes as necessary.

This method is simple and reliable, but may waste energy if the defrost cycle is used too often.

Demand Defrost Strategy

This strategy uses sensors to detect frost buildup and only initiates the defrost cycle when necessary. It is a more energy-efficient method as it only activates when needed.

Typical Defrost Mode Cycle Time

The defrost mode cycle time depends on various factors, such as the outdoor temperature, humidity, and the frost thermostat’s setting. On average, a defrost cycle takes about 5-15 minutes to complete.

Preventing Heat Pump Freezing

Here are some tips to prevent heat pump freezing when the temperature outside dips below freezing:

Addressing airflow restrictions

Check that the air filter is clean and free from debris. Make sure there are no obstructions around the outdoor unit that could impede airflow.

Resolving technical issues

If the unit has improper installation or malfunctioning indicator lights, it may not be cycling correctly. Leaky refrigerant can also cause freezing. These issues should be addressed by a professional.

Managing excess moisture

Excess moisture can cause heat pump freezing. Clear away any snow or ice buildup around the outdoor unit. Install gutters or diverters to prevent water from pooling near the unit.

Stabilizing the outdoor unit slab

If the outdoor unit is not level or stable, it can cause the refrigerant to pool in one area and freeze. Ensure that the slab the unit is installed on is level and secure.

Regular maintenance

Schedule regular maintenance intervals with a professional to ensure that your heat pump is in good working order. They can check for any internal issues and perform necessary repairs.

Fixing outdoor fan problems

Malfunctioning fan blades or condenser fans can also cause heat pump freezing. If the fan is not spinning properly or is making strange noises, it should be inspected and repaired.

Methods To Defrost A Heat Pump

A heat pump going into defrost mode is a normal part of its operation, but it can be inconvenient for homeowners. Fortunately, there are methods to defrost a heat pump that can prevent it from happening too frequently.

Using water for defrosting

Simply spraying warm water over the outdoor unit can help speed up the defrosting process. However, it is important to avoid using hot water, which can damage the unit’s delicate internal components.

Utilizing the fan-only setting

Another method to defrost a heat pump is to utilize the fan-only setting. This helps to circulate warm air around the unit, which can speed up the defrosting process. However, this method may not work if the unit’s fan motor is slow or malfunctioning.

Manually defrosting the unit

For more severe ice buildup, it may be necessary to manually defrost the heat pump. But, this should  only be done by a professional technician. They can safely remove the ice without damaging the unit’s refrigerant lines or other components.

Frequently-Asked Questions

How often should my heat pump go into defrost?

Heat pumps typically go into defrost mode every 30-90 minutes during colder weather. The frequency depends on the temperature and humidity of the outdoor air.

How much ice is normal on a heat pump?

It’s normal to see some ice buildup on your heat pump during cold weather. Ice more than ¼ inch thick or covering more than 50% of the outdoor coil, can cause problems.

Should my heat pump run constantly in winter?

Heat pumps normally run almost all the time, especially in extreme cold or heat. However, they run in a low-power mode most of the time to maintain the temperature. Call for service if your system is running all the time at full capacity and can’t maintain the temperature you want.

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