Understanding Your Heat Pump’s Defrosting Cycle

There’s something a little unsettling about standing outside on a frosty Seattle morning, watching your heat pump suddenly go quiet, or hearing it hum unusually, especially if you’re relying on it to keep your home cozy. If your system seems to stop blowing warm air for a few minutes, don’t panic. More than likely, you’re just witnessing a normal part of its wintertime operation: the defrosting cycle.

For many homeowners, understanding a heat pump’s defrosting cycle helps clear up a lot of confusion and prevents unnecessary service calls. So, if you’ve ever wondered why your heat pump seems to stop heating temporarily when it’s cold out, this one’s for you. We’ll explain what’s happening, when and why it happens, how long it should last, and what’s normal vs. what’s not. We’ll also go into how to support your system during this process to keep your heating as reliable and energy-efficient as possible.

What’s Going On When a Heat Pump “Defrosts”?

To understand the defrosting process, it helps to quickly revisit how a heat pump works during the colder months. When it’s in heating mode, your heat pump’s job is to extract heat from the outside air and transfer that heat energy into your home. Even when it’s cold outside, there’s still thermal energy to be captured. This is what makes heat pumps so efficient—they simply move heat from one place to another instead of creating it from scratch.

But when the outdoor temperature drops, especially below 40°F, things start to get a little icy—literally. As the fan blows cold, moist air across the outdoor unit’s heat exchanger, water vapor in the air can freeze on contact. This frost buildup on the outdoor coil reduces the system’s ability to grab heat from the outside, forcing it to work harder than it should.

To avoid major performance loss, the system will enter defrost mode, which temporarily reverses operation to melt frost off the coil and protect the outdoor unit.

How the Defrost Cycle Works

When a heat pump detects that frost is forming on the outdoor coil, it switches into a temporary defrost cycle. This involves a few noticeable changes. The reversing valve flips the system from heating back to cooling for a few minutes—only this time, the heat is redirected to the outdoor coil instead of your living space.

This means your outdoor unit will push hot refrigerant to warm up the coil and clear off the ice. During this process, the outdoor fan is typically stopped to help the coil warm up faster. Meanwhile, inside your home, the indoor unit may stop delivering warm air briefly, or may rely on emergency heat (also called electric heat) to keep indoor temperatures steady until the system flips back to heating mode.

A properly working defrost control will monitor coil temperature and ensure the system exits defrost mode once the ice has melted. This usually happens once the coil reaches around 58°F.

How Often Will the Defrost Cycle Happen?

Many heat pumps will run a defrost cycle at timed intervals during winter, especially when frost conditions occur—typically every 30 to 90 minutes, depending on the model. But newer, smarter systems are equipped with sensors and logic boards that activate the defrost cycle only when needed.

That said, the frequency and duration of a defrost cycle will depend on a few key factors:

• The ambient temperature
• Outdoor humidity levels
• The amount of heat load your home requires
• The condition of your heat pump system

When everything’s working right, the defrost cycle shouldn’t last more than 5 to 10 minutes and should only happen as needed. If your unit switches to defrost too often—or not at all—that could indicate a problem, which we’ll get into below.

What It Looks and Sounds Like

From indoors, you might notice a slight pause in heating or feel a wave of cool air if emergency heat doesn’t kick in right away. From outside, you’ll often hear the compressor running, but the outdoor fan has stopped. You might also spot steam rising from the outdoor unit, which is just condensation evaporating as the coil warms up.

Some systems also have a blinking light on the thermostat or indoor unit to indicate when it’s in defrost mode. That said, if your heat pump keeps going into defrost every 10–15 minutes, or the cycle lasts more than 10 minutes at a time, it could be time for a service call.

Signs Your Heat Pump Might Have a Defrost Problem

So how do you know when something’s actually wrong and not just a normal part of the cycle?

• The system never seems to resume heating: If your home isn’t warming back up after the cycle, there may be a deeper issue with the reversing valve, thermostat, or defrost control.
• Frequent, back-to-back defrost cycles: This could be due to blocked airflow, dirty air filters, or a refrigerant problem.
• Outdoor unit completely iced over: If the outdoor coil is encased in solid ice, your system might not be initiating defrost properly, or it may be trying and failing.
• Strange noises during or after the cycle, like loud banging or rattling, may point to compressor stress or other mechanical issues.

In any of these cases, it’s a good idea to contact a professional HVAC technician for inspection and repair.

Keeping Your Heat Pump Happy

Keep the Outdoor Unit Clear

Your heat pump needs breathing room. If it’s boxed in by snow, leaves, or other debris, it can’t do its job. During the colder months, make it a habit to check your outdoor unit weekly. Brush off any snow or frost, and trim back any vegetation crowding the unit. This keeps airflow steady and helps prevent unnecessary frost buildup.

Schedule Regular Maintenance

Seattle’s winters may be mild, but that doesn’t mean your heating system should go unchecked. Regular maintenance ensures that your system stays in peak shape, especially during those stretches of back-to-back rainy days and the occasional cold snap. Our techs at Fox Plumbing, Heating & Cooling will inspect coils, refrigerant levels, air filters, and electrical connections, making sure your heat pump system can shift in and out of defrost mode without a hitch.

Does Defrost Mode Affect Energy Efficiency?

That brief moment where your system switches to cooling and stops heating your home might seem like a step in the wrong direction. But in reality, it’s doing exactly what it should. The defrost cycle is designed to protect your system from long-term inefficiency and damage.

If the outdoor coil acts like an evaporator and gets too iced up, it can no longer absorb heat from the outside air. That forces your system to work overtime, driving up your energy usage and shortening equipment life. So yes, while the unit switches to defrost may momentarily lower energy efficiency, it’s far better than letting your heating system choke on ice.

Can the Defrost Cycle Cause a Sudden Blast of Cold Air?

It can—especially if your system doesn’t have a good electric heat backup or if the indoor fan continues running during the defrost cycle. Some heat pumps are built to keep the fan running even while the system is in cooling mode, which can push a bit of cold air into the home. Others will shut the indoor fan off to minimize this.

If you’re frequently noticing chilly gusts during wintertime operation, ask your technician if your thermostat supports fan delay settings or if your system can be configured to avoid this behavior.

Is Your Heat Pump Ready for Seattle’s Winter?

Seattle’s winters are usually mild, but they’re also wet, and moisture is a major trigger for frost on outdoor coils. If your system hasn’t been checked in a while, or you’re unsure whether your defrost control is working the way it should, don’t wait for problems to show up during the next cold snap.

Fox Plumbing, Heating & Cooling has been helping Seattle homeowners stay warm through the grey, drizzly months for decades. Our team understands how Pacific Northwest weather interacts with your heat pump system—and we know what to look for when something’s not working quite right.

We offer repair, maintenance, and full installations of high-performance, energy-efficient heat pumps and other HVAC systems built for the Seattle climate. If your heat pump is cycling too often, not melting ice fast enough, or delivering inconsistent warm air, give us a call. We’ll get your system tuned up, cleaned up, and ready for whatever winter brings.

Need heat pump help? Fox can handle it. Call us today at (206) 654-4986 to schedule your service or book online in just a few clicks.