What is heat pump defrost mode?

HVAC Logo IconBy HVAC.com Staff WriterAugust 1, 2023
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The heat pump defrost mode is a self-preservation feature that keeps the unit running properly when the unit’s coil temperature dips below freezing. In most instances, a system running in defrost mode isn’t a heating and cooling emergency.

heat pump defrost snowed in

What is defrost mode for a heat pump?

When a heat pump goes into defrost mode, it temporarily reverses the heat transfer process to warm the outdoor coil and melt frost. Heat pumps use refrigerant, or coolant, to transfer warm thermal energy from outdoor air into your home to provide heat.

The outdoor coil of a heat pump is about 10 degrees cooler than the outdoor air at any time. So, defrost mode typically occurs during cold weather, but it doesn’t have to be below 32 degrees outside for the coil to freeze.

The unit can be programmed to check whether a defrost cycle is needed every 30, 60, 90, or 120 minutes. Because the heat pump can’t tell whether the coil is frozen, the unit’s sensors read the coil temperature and outdoor ambient air temperature. Time and temperature readings initiate defrost when needed.

During defrost mode, the unit reverses the heating process and begins pulling warm energy from inside the home to the outdoor coil. Models may differ, but typically the heat pump will operate in defrost mode until the coil reaches 55 degrees or a maximum of 15 minutes, whichever comes first.

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Heat pump defrost controls

Heat pumps will have one of two defrost controls: time-temperature or demand defrost. Both methods work by temporarily redirecting heat from your home to your outdoor unit.

One heat pump defrost cycle takes anywhere from 5 to 15 minutes. In winter, cycles tend to be 30 to 90 minutes apart.

No matter which defrost mode your unit uses, the outdoor fan will stop running during the cycle. That’s completely normal, and there’s no need to call an HVAC pro when the heat pump fan stops during defrost mode.

Let’s break down how the defrost modes differ.

Time-temperature defrost

Time-temperature defrost control occurs on a set schedule. Defrost mode turns on and shuts off on consistent, timed intervals.

Time-temperature defrost mode activates regardless of whether your heat pump or coil is actually frozen. This makes time-temperature controls much less efficient than demand defrost.

Heat pumps with time-temperature defrost often waste energy. Since there’s no sensor to detect if your heat pump truly needs defrosting, the setting may cut on unnecessarily.

Time-temperature defrost controls are more common in older and value heat pump models.

cold family heat pump defrost

Demand defrost

Demand defrost is a more common feature in new heat pumps. Heat pumps with demand defrost have a sensor that can tell when frost is forming on the coil.

When the sensor notices the frost buildup, it sends hot refrigerant from the indoor air handler to the outdoor coil. This mechanism is the same as time-temperature defrost systems.

While demand defrost is much more efficient than time-temperature defrost, defrost mode can still drain your system if it’s malfunctioning or coming on too often.

Defrost mode transfers the hot refrigerant that would ordinarily heat your home to the outdoor coil. To keep from a drastic temperature drop indoors, your heat pump temporarily switches to emergency heat for your home.

Emergency heat won’t tank your energy efficiency if used occasionally. However, a malfunctioning defrost mode could use too much emergency heat and take a toll on your energy consumption.

Common causes for your heat pump’s defrost mode

If your defrost mode works properly, it should either defrost on a regular schedule or when frost builds up on your outdoor coil or unit.

However, defrost modes can malfunction for a variety of reasons. Here’s why your heat pump may be defrosting too often or not at all.

  • Leaky refrigerant
  • Dirty or damaged outdoor coil
  • Slow fan motor
  • Malfunctioning fan blades

Call a professional technician for help. Defrost mode issues are usually related to inner parts and can become system-wide issues if left unchecked.

You may not be totally out of luck, though. If your heat pump is frozen over, check the “how to defrost a heat pump” section for some temporary, at-home solutions.

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How to prevent heat pump freeze up

Defrost mode prevents heat pumps from freezing over. A unit that continuously freezes over may be a sign your heat pump’s defrost mode isn’t working properly.

Regular maintenance is the easiest way to prevent a heat pump from freezing. A contractor can diagnose any issues with your defrost mode and fix it before the cold season hits.

Extremely cold temperatures can also cause a frozen heat pump. If your climate causes your heat pump to freeze over often, an alternate heating solution, like a furnace, may better suit you.

How to defrost a heat pump

A fully frosted heat pump can lead to frustration…and shivers, no doubt. Thankfully, you may be able to solve the issue without hiring a technician.

Here are some DIY techniques to get your heat pump thawed out and ready to run. If none of these options work out, your best bet is to call a professional.

Defrost with water

First, turn off your heat pump at the electricity source. This is either a circuit breaker or an on-off switch. Switching the thermostat “off” won’t kill the electrical current, so it poses a safety hazard.

Head outside to your outdoor unit. Using a garden hose, water your heat pump like you would a plant. The ice may start to melt.

This method only works in milder climates. If your air is so cold that garden hose water freezes up, try the fan-only method below.

Fan-only mode

A quick fix for defrosting a heat pump is turning on fan-only mode. Most mid-range and premium heat pumps have this feature.

If it’s working, you should see improvement in about an hour. This solution works best for mild frosting on the heat pump’s exterior. Unfortunately, it’s not as effective in more extreme cold climates.

Manual defrosting

If your heat pump’s defrost mode isn’t automatically switching on, you may have another option. Some models come with a manual defrost heat pump feature.

This feature’s location on your heat pump varies by brand. You may have to dig through product literature or dial your local contractor.

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