Key Takeaways
- A heat pump’s defrost mode temporarily reverses the heating cycle, switching to cooling mode to warm the outdoor coil and melt any accumulated frost.
- The defrost cycle typically lasts between five and 15 minutes—longer or continual cycles indicate bigger problems.
- Pay attention to changes in how your heat pump runs, such as unusual noises, a lack of warm air in the home, or a frozen outdoor unit.
Your heat pump keeps your home comfortable by using refrigerant to transfer heat from the cooler outdoor air into your home during winter. In summer, it reverses this process, removing heat from your indoor air and releasing it outside to keep your home cool.
When temperatures outside drop, defrost mode keeps your heat pump from freezing over. This is an automatic, built-in feature that temporarily reverses the standard cycle to melt frost. By blowing cool air, the outdoor coils heat up, melting the frost.
While defrost mode is usually harmless, it can cause problems if it runs too long, too often, or fails to turn on. In this guide, we cover how defrost mode works and how to spot problems when your heat pump isn’t working correctly.
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How Defrost Mode Works
Defrost mode uses smart sensors and controls to maintain a comfortable temperature in your home, even with cold outside temperatures.
Sensing the Frost
Defrost mode turns on automatically as soon as it senses frost on the coils.
“Even when it’s cold outside, a heat pump extracts heat from the outdoor air to warm your home,” said Danny Pen, president at New Era Plumbing & HVAC, based in Dracut, Massachusetts. “When the weather is damp and extremely cold, moisture in the air collects on the outdoor coils and freezes. Frost on the coils is a completely normal thing for a heat pump in winter, and defrost mode turns on automatically to melt that frost and keep the system running. Just like a refrigerator occasionally runs a defrost cycle to prevent ice buildup inside the freezer, a heat pump does the same.”
Switching to Reverse Cycle
During the defrost cycle, the heat pump warms the outdoor coil to melt accumulated ice. As the ice melts, it often turns to water and then evaporates, which can create visible steam rising from the unit — this is normal and indicates the system is working properly.
Auxiliary Heat Kicks In
While your heat pump switches into cooling mode to defrost itself, don’t worry about your home’s temperature dropping. An efficient heat pump will continue to warm your home, or rely on auxiliary heating sources to keep your home cozy during cold temperatures.
Returning to Heating Mode
After the defrost mode completes, your heat pump will return to its standard heating mode. During freezing temperatures, it will continue to cycle between defrosting and heating modes to keep your home as comfortable as possible while preventing frost buildup.
How Long Should Defrost Mode Last?
Defrost mode typically lasts between five and 15 minutes — just long enough to melt frost on the outdoor coils with a warm refrigerant. During frigid periods, defrost mode will happen every 30 to 90 minutes. However, if you notice that your heat pump system is continually in defrost mode or covered in ice despite entering defrost mode, contact an HVAC professional immediately, as something is likely malfunctioning.
Signs of a Defrost Problem
Wondering if you have a defrost problem? The following signs indicate that you may want to speak with an HVAC professional about your heat pump:
- System is blowing cold air inside for extended periods (or only cold air and no heat)
- Unusually loud hissing or clicking noises during the cycle
- Outdoor unit is frozen solid with no signs of melting
- Defrost mode runs continuously or every 10 to 20 minutes
- Indoor temperature fluctuations
- Sudden higher energy costs
- Heat pump stops working entirely (especially during a cold snap)
What Causes Heat Pump Defrost Issues?
Heat pump defrost issues are caused by a range of factors, including broken parts and clogged coils:
- Faulty defrost sensor or thermostat: If your heat pump isn’t sensing the temperature accurately, it won’t enter defrost mode correctly.
- Broken fan motor: If your heat pump’s fan isn’t engaged or the blades are running in the wrong direction, the system won’t be able to defrost effectively.
- Clogged or dirty coils: Clogged or dirty coils reduce your heat pump’s ability to work and defrost itself. Grass, pet hair, debris, and dirt are often the culprits.
- Low refrigerant levels: Refrigerant flows through the heat pump, heating and cooling the air and maintaining the desired temperature in your home. If you notice that your home isn’t warm or cool enough, low refrigerant might be the reason.
If you suspect your heat pump is experiencing any of these issues, schedule an inspection with an HVAC professional to have your system evaluated and repaired. Any unusual behavior means you should contact a pro for your family’s comfort and safety.
Preventing Defrost Cycle Problems
To prevent problems with your heat pump’s defrost cycle, start by clearing the outdoor unit of leaves, snow, and debris. Don’t cover your heat pump in the winter — this prevents proper airflow for heat extraction. Instead, shield it from snow and water by placing it under the eaves, under a snow shield, or beneath a deck.
Schedule annual maintenance to check sensors, refrigerant, and coil cleanliness. Likewise, address any issues quickly. Don’t wait for small repairs or issues to become bigger, warranting a full-blown HVAC installation. You can also speak with your HVAC technician about cold-climate upgrades, such as dual-fuel systems or better controls, to keep your heat pump working efficiently in freezing temperatures.
We recommend maintenance every one to two years, but check your pump’s manufacturer specifications.
Lastly, you should prepare your heat pump for each season. Check your manufacturer’s guide to ensure the current settings are optimal for the time of year. The guide may recommend certain maintenance tasks for different seasons.
Is Heat Pump Defrost Mode Bad for Efficiency?
A heat pump’s defrost mode isn’t bad for efficiency overall. It may cause a slight dip in efficiency, but it’s a necessary part of proper operation.
“Heat pump defrost mode is necessary but can temporarily reduce efficiency because the system switches to a backup heat source (often electric resistance) to prevent freezing,” said Omero Flores, CEO of American AC & Heating, based in Harlingen, Texas. “However, it’s still more efficient than older oil or gas systems, especially when combined with a modern thermostat and proper maintenance.”
Next Steps
Your heat pump defrost mode is a normal, essential function that prevents your heat pump from freezing over during cold weather. However, if it fails to activate or runs continuously, this points to a deeper issue, such as broken fans or faulty temperature sensors.
Start by checking the outside unit and removing any snow or plant debris. Then, examine the heat pump and listen and look for anything out of the ordinary, such as unusual sounds. If the coils are frozen over, contact a HVAC professional immediately to have your heat pump and HVAC system examined.
Speak with your HVAC professional about potential tax credit savings when upgrading, repairing, or installing a new HVAC system.
An HVAC technician can inspect the sensors, coils, and refrigerant levels. Taking care of a minor problem now can save you lots of frustration and money down the line, so don’t wait to call in a professional.
FAQs About Heat Pump Defrost
How often should my heat pump go into defrost mode?
Your heat pump should enter defrost mode when the outdoor coils drop below 32°F. Condensation freezes on the coils, forming a layer of frost. Your heat pump will automatically enter defrost mode to melt the ice on the coils with a warm refrigerant.
Why is my heat pump blowing cold air in winter?
Your heat pump may blow cold air in the winter, even when it’s in heating mode, during the defrost cycle. The defrost cycle prevents frost buildup on the outdoor coils. While it temporarily reduces efficiency, it’s better for the overall comfort of your home.
If your heat pump continually blows cold air, consult an HVAC technician, as it may need repairs.
Can I manually trigger or disable defrost mode?
Depending on your heat pump, you may be able to trigger defrost mode. However, we recommend that you have a HVAC service technician look at your heat pump and trigger defrost mode for diagnostic purposes first to ensure nothing is wrong.
Is it bad if frost is building up on my heat pump?
If frost is building up on your heat pump, it may be malfunctioning due to wiring issues, low refrigerant levels, or faulty temperature sensors. Consult with an HVAC contractor to have your heat pump assessed and repaired, and don’t use your heat pump if it’s heavily frozen.
What should I do if my outdoor unit is frozen solid?
Immediately stop using your heat pump if the outdoor unit is frozen solid. Ensure you have a backup heating system in place to keep your home warm. Then, call and schedule an appointment with an HVAC professional to have your heat pump assessed.
