Understanding the Heat Pump Defrost Cycle
TL;DR (Too Long, Didn’t Read) Quick Summary
Wondering what that steam or humming noise from your outdoor unit is on a chilly Phoenix morning? It might just be your heat pump defrosting. Here’s the short version:
- ❄️ A heat pump defrost cycle melts frost buildup on the outdoor coil to keep the system efficient
- 🔁 Most units defrost automatically using temperature sensors (no input needed from you)
- ⏱️ Typical defrost cycle lasts 10–20 minutes, once or twice per hour on very cold days
- 🔍 If your outdoor fan stops but the compressor is still running, it’s likely in defrost mode
- 🧠 Don’t worry—this is normal, unless it keeps happening for more than 30 minutes
- ⚠️ Pro tip: Don’t go climbing on your roof to check if your rooftop unit is defrosting
Ready for a deeper dive? Keep reading …
What Is a Heat Pump Defrost Cycle?
If you’ve read my article How Does a Heat Pump Work, you know that in heating mode, heat pumps reverse the AC process using a component called a reversing valve. When this happens, the outdoor unit starts expelling cold air—colder than the outdoor air temperature.
Here in Phoenix, where we use our heat pumps for heating more than folks up north might think, this reversed cycle causes moisture in the air to condense and freeze on the outdoor coils. Over time, this frost can block airflow, which impacts performance and efficiency. That’s where the defrost cycle comes in.
Why Defrost Is Necessary
- ❄ Frost builds on outdoor coils during heating mode
- 🧊 Ice reduces airflow and heat transfer
- 🔥 The defrost cycle melts frost before it becomes a problem
How the Heat Pump Knows to Defrost
Most modern heat pumps use electronic temperature sensors to monitor the coil temperature. Once it drops below a preset threshold, the system automatically initiates a defrost cycle—no need for homeowner intervention.
Each manufacturer has its own engineering approach. Here’s how a few of the major brands differ:
| Manufacturer | Trigger Temperature | Defrost Method | Average Cycle Time |
|---|---|---|---|
| Bosch | Below 62°F | Sensor-based | ~11 minutes |
| Lennox | Below 58°F + Timer | Time & temperature | Up to 20 minutes |
| Gree | Varies by model | Sensor-based | ~16 minutes |
So if you’re wondering, “Why does my neighbor’s heat pump sound different than mine at night?”—now you know.
How Often Does the Defrost Cycle Happen?
Frequency varies by outdoor conditions and the system design. On very cold days (yes, we get those in the Phoenix valley), expect a defrost cycle:
- Once or twice per hour on freezing mornings
- Once every 1–2 hours on mildly cold days
- Typically during early morning or late-night hours
We get dozens of calls every winter when folks think their unit has stopped working. Often, it’s just the defrost cycle doing its job. Our Magic Touch Mechanical CSRs are trained to explain what’s happening and when a service call is (or isn’t) necessary.
How Long Should a Heat Pump Defrost Cycle Last?
Generally, defrost cycles last between 10 and 20 minutes. Most units finish closer to the 12-minute mark. If your system is still not heating after 30 minutes, that’s when it’s time to call a pro.
Can I Tell If My Heat Pump Is in Defrost Mode?
Yes, and you don’t need tools or technical training. Here’s how to tell:
✅ You might notice:
- Outdoor fan not spinning, but compressor is running
- Steam rising from the top of the unit
- Frost melting on the coil through the grill
⚠️ If your unit is on the roof:
Let it be. If there’s frost on the coil, there’s frost on the roof. Wait it out and avoid a trip to the ER. If the system doesn’t resume heating in 30 minutes, call us.
✅ Check your thermostat:
Some smart thermostats (like Ecobee or Nest) may show a “Defrost” or “Aux Heat” message when it’s happening.
Thermostat-Related Defrost Issues
While your thermostat shouldn’t interfere with defrosting, we’ve seen cases where it does—particularly with smart thermostats.
Ecobee Quirk (and How We Fixed It)
We recently ran into an issue where both the heat pump and Ecobee were trying to control defrost timing—sending mixed signals that confused the system. Neither manual warned us. We figured it out the old-fashioned way: trial, error, and experience.
We wrote a separate guide about how to fix it: Ecobee Thermostat Heat Pump Setting
(Link to your Ecobee article here when live)
📌 Pro Tip: If your thermostat has defrost settings, let your heat pump handle it instead. Call a local HVAC pro if you’re unsure what to adjust.
When the Defrost Cycle Isn’t Working Properly
If your heat pump is not defrosting, or defrosts constantly, you might be looking at one of these issues:
Common Heat Pump Defrost Failures
- 🔧 Faulty defrost control board
- ❌ Bad temperature sensor
- 🔌 Low-voltage wiring problems
- 🧠 Thermostat or sub-base miscommunication
- 🥶 Low refrigerant charge
- 🧼 Dirty outdoor coils
These aren’t DIY repairs—best to let a trained tech handle them. Luckily, that’s where Magic Touch comes in.
Staying Comfortable During Defrost
Here’s how to plan ahead and stay cozy even when your system defrosts in the middle of the night:
Pro Tips for Staying Warm
- 🕓 Set your thermostat to your morning temp early—let it defrost before you get up
- 🛏️ Avoid turning off your heat overnight—starting from zero means instant defrost
- 🔥 Consider adding strip heat (electric resistance heating) for backup during defrost
Important Note on Strip Heat:
Electric strip heat needs a dedicated circuit from your panel. Costs vary based on your home and unit location—it could be a few hundred bucks or a few thousand. But it makes a big difference on cold nights.
When to Call Magic Touch Mechanical
If your heat pump is doing any of the following, it’s time to get it checked:
- It’s not heating after 30 minutes of defrost
- You hear loud or unusual noises during defrost
- The unit is constantly freezing over
- You’ve got an error message on the thermostat
We’ve been helping homeowners in Mesa, Chandler, Gilbert, Queen Creek, Phoenix, Scottsdale, and beyond for 28+ years. If something doesn’t seem right with your heat pump, give us a call—or better yet, schedule online.
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