Heat Pump Defrost Cycle: How and Why It Runs

This guide explains what the defrost cycle does and why it is normal winter behavior, not a fault. Every number and mechanism below comes from a page fetched on the verification date, and each section lists the source record that supports it. Where sources differ on timing or temperature, that is because equipment and controls vary, so the ranges are presented as sourced examples rather than a single universal setting.

Why Frost Forms on the Outdoor Coil

Frost is a byproduct of how a heat pump makes heat in winter. In heating mode the outdoor coil has to be colder than the outdoor air so that heat can flow into it. Energy Vanguard puts it plainly: the heat pump makes the outdoor coil colder than the outdoor air, and moisture in the passing air condenses and freezes on that cold surface.

Because the coil is colder than the surrounding air, frost does not require a freezing forecast. American Standard notes that in late fall and winter the outdoor coil is generally 10 to 20 degrees Fahrenheit colder than the outdoor air, which is why condensation forms on it the same way it forms on a cold beverage can. Energy Vanguard reports observing frost form with the outdoor temperature close to 50 degrees Fahrenheit.

Humidity is the other half of the picture. Energy Vanguard notes that colder air holds less water vapor, so frost behavior shifts with both temperature and moisture in the air. The takeaway is that a light frost or ice film on the outdoor unit during cold, damp weather is expected, and the system is built to clear it on its own.

How the Reversing Valve Runs the Defrost Cycle

The defrost cycle clears the coil by briefly running the system in reverse. American Standard describes it directly: to melt the ice or frost on the outdoor coil, the heat pump's reversing valve reverses the flow of refrigerant to heat up the outdoor coil, essentially putting the heat pump into cooling mode for a few minutes. Energy Vanguard frames the same component as the part that reverses the flow of heat, so defrost brings heat from indoors and dumps it outdoors.

The reversing valve does not act on its own. Snyder AC notes that a defrost control tells the reversing valve when to send hot refrigerant outdoors to thaw the outdoor coil. Fox Family HVAC describes hot refrigerant flowing through the outdoor coil once the system temporarily switches to cooling mode to thaw it.

Because the machine is momentarily cooling the house to heat the coil, defrost is a short, deliberate interruption of heating rather than a breakdown. When it ends, the reversing valve switches back and normal heating resumes.

What Triggers Defrost: Timed vs Demand Control

There is more than one way a heat pump decides to defrost. Fox Family HVAC describes two approaches: timed defrost, where the board counts how long the compressor has been running in heating mode, and demand defrost, where the board looks more closely at coil temperature to decide when defrosting is needed.

Older timed controls work on fixed intervals. InterNACHI notes that many heat pumps use a generic timer that energizes the defrost relays at certain intervals, and that some generic timers energize the defrost cycle every 30, 60, and 90 minutes. InterNACHI also describes a coil temperature trigger: when the outdoor coil temperature drops to around 32 degrees Fahrenheit, the control closes the circuit and the system responds.

Demand systems watch the coil sensor. Fox Family HVAC notes the sensor attached to the outdoor coil dropping to around 26 degrees Fahrenheit before defrost initiates. The practical point is that the trigger you have determines how often you will see defrost, so a timer and a demand board can behave differently in the same weather.

How Long Defrost Lasts and How Often It Runs

Duration is short in every source reviewed here. InterNACHI states a typical defrost cycle might run from 30 seconds to a few minutes, and that the heat pump operates in defrost until the outdoor coil temperature reaches around 57 degrees Fahrenheit. Snyder AC gives a similar bound, noting defrost ordinarily should not take longer than 10 minutes and that the coil usually reaches around 58 degrees Fahrenheit before the cycle ends. Fox Family HVAC describes the cycle ending when the coil warms to roughly 50 degrees Fahrenheit or a maximum time limit is reached.

Newer demand controls tend to run shorter, more targeted cycles. American Standard notes that its demand defrost technology produces cycles that are usually 3 to 5 minutes.

Frequency depends on weather and control type. Snyder AC states that defrosts should be no more than roughly every 35 minutes, while Fox Family HVAC describes cycles occurring every 45 to 90 minutes of run time when it is cold and damp outside. Read these as sourced examples of normal behavior, not as a fixed schedule for every unit.

What You See and Feel During Defrost

Two visible signs are normal during defrost. First, the outdoor fan stops. InterNACHI notes the outdoor fan is prevented from turning on when the heat pump switches over, which accelerates the coil temperature increase, and Fox Family HVAC states that during defrost the outdoor fan usually stops so the coil can warm up faster. Steam or vapor rising off the outdoor unit as the ice melts is part of the same process.

Second, the indoor air can feel cooler for a moment, so the system leans on backup heat. American Standard notes that backup heat comes on, assuming the home has backup electric heat strips, and that the thermostat may display AUX or AUX heat while the heat pump is in defrost mode. Fox Family HVAC similarly describes the heat pump turning on backup heat at the indoor air handler to prevent cool air from entering the home during defrost.

None of this signals a problem on its own. A brief pause in warm airflow, an AUX indicator, a stopped outdoor fan, and visible steam are all consistent with a normal defrost cycle in the sources reviewed here. Persistent icing that never clears, or defrost running far more often than these ranges, is the kind of pattern worth having a technician evaluate.