9 Reasons Your Heat Pump Won’t Defrost (+Fix)

There comes a time in winter when your heat pump will ice buildup. The machine will enter the defrosting cycle setting to combat and melt the snow. But, if the mode does not work properly, there is some serious issue with your unit.

Major issues are bad thermostats, poor drainage, a faulty reversing valve, low refrigerant, and electrical issues. Possible fixes include checking the thermostat, clearing the drainage, replacing the reversing valve and refrigerant tube, and fixing the wires. 

You may troubleshoot some issues, but most require expert guidance and help. This article will explore all the possible causes of why the heat pump does not defrost and how to troubleshoot and prevent the problem. 

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What is a defrost cycle, and how does it work?

The ice development process on the outside coils is slow when your heat pump functions.

When the thermostat senses a drastic drop in the temperature, it will shift from a heating to a cooling process with the help of the reversing valve. 

As a result, the defrost mode will start working on unfreezing the ice buildup in your heat pump. 

The evaporator becomes the condenser during the defrost mode.

The heat pump must regularly defrost during the frost, and the cycle should be long enough to melt all the snow and short enough to save energy. 

The defrost relay allows the cycle to keep going for 30 minutes. 

When the heat pump is in cooling mode, the outdoor fan will stop working, and the inside device will stop moving the air inside. 

As a result, it leads to high temperatures and defrosting operations. 

The process may last for 5 to 10 minutes, or even more, based on the thickness of the snow layer. 

The reversing valve and the evaporator will work together, and the high-pressure refrigerant will warm up and defrost all the snow from your unit. 

The thermostat will wait for your unit to return to its normal temperature and stop the defrosting procedure. 

Once the defrost stops, the unit will begin normal heating. 

The freezing will start again if the temperature becomes too low for the unit to handle. 

The old heat pumps also have the defrost mode but do not have thermostats or sensors for signaling. 

Instead, they have timers that allow the defrost to work for a specific time. 

But, the problem with timers is the defrost will work at a specific, with or without ice buildup. 

As a result, this procedure reduces your unit’s efficiency and increases energy bills. 

When does the heat pump start defrosting?

There are situations and times when the heat pump should start defrosting and how. There are those circumstances:

Temperature differences indoor vs. outdoor

When the thermostat or sensor on the outdoor coil’s bottom senses the temperature is very low compared to the outside temperature, the defrost cycle will start immediately. 

It happens if the temperature is as high as 60°F or as low as 30°F. 

Once the outdoor coil’s thermostat reaches close to 60°F, the coil will defrost, and the cycle will stop. 

Air pressure

The space between the coils becomes impenetrable when the coil starts having a layer of ice. 

As a result, the air struggles to pass through the coils and increases the pressure. 

When the air pressure increases, the defrost cycle will start working immediately with the help of a differential switch. 

The switch turns on when the pressure is too high and turns off when the pressure comes back to normal. 

Timer and thermostat

The unit will have a timer and thermostat or sensor that helps turn on the defrost cycle. 

The method is better than the above two. 

The devices measure the temperature every 90 minutes and turn on the defrost mode based on the sensor. 

Why is my heat pump not defrosting?

When the evaporator defrosts, the hot refrigerator is directed to the coil, and the outside fan stops working to speed up the process. 

The compressor moves the refrigerant and increases the pressure. 

You can confirm it by hearing a humming noise when the compressor works, a whooshing noise once the defrost cycle is over, and white steam blowing out of the unit. 

If any methods fail to work properly, the defrost cycle won’t work. 

As a result, the heat pump won’t defrost, and the heating capacity gets compromised. 

Here are some common reasons behind a heat pump not defrosting:

1. The unit is covered with ice and snow.

Ice buildup is common in colder climates. 

If your heat pump is completely covered with snow, it won’t be able to defrost properly. 

You need to melt some ice and let your unit start the defrost cycle. 

Avoid using sharp objects. 

Try running water with a garden hose or pouring hot, warm over the snowy areas. 

2. Drainage issues

Leaves, sticks, snow, and other debris can clog the drainage system and affect the defrost cycle of the unit. 

Due to the blocked drainage system, the water will be accumulated inside the device instead of draining out. 

The water will remain inside your unit, freeze over the other components, and affect their capacity to perform appropriately.

Sometimes, the ice blocks will be too thick for the defrost cycle to break. As a result, the defrost cycle will not work properly. 

Clear everything from your unit and clean the drainage systems to troubleshoot the problem. 

Trim the overhead branches and twigs to stop them from dripping water over your unit. 

The gutters should be empty and in good condition. 

3. Low refrigerant 

The refrigerant or coolant helps in cooling or heating the air of your house. 

The refrigerant gets pumps through the evaporator coils to heat them and defrost the heat pump. 

The heat pump’s defrost cycle depends on this coolant. 

The refrigerant extracts heat from the outdoor air to move it to the evaporator coils and other condensers that require warmth for melting ice. 

If the refrigerant is low, there won’t be enough heat to encourage the defrosting cycle to start. 

Besides this, the pressure and temperature will reduce inside the pump, affecting the defrost cycle negatively. 

The leakage can be caused due to cracks, erosion, and rust. 

You should hire an HVAC expert to deal with it.

4. Bad reversing valve

The reversing valve changes the direction of the refrigerant flow from cooling to heating. 

The valve can malfunction by getting stuck in the heating or cooling position. 

When the reversing valve fails to do its job due to malfunctioning, the heat pump won’t enter the defrost mode. 

It will start building more ice and worsen your unit’s condition. 

You should call an HVAC to deal with the valve. 

5. Bad thermostat, sensor, or timer

The defrost cycle startup depends on the thermostat or the heat pump’s timer that signals the unit to start the cycle. 

The thermostat or the timer also decides the running time of the defrost mode. 

If the thermostat or the timer malfunctions, the unit won’t enter the defrost mode to start the cycle. 

Even if the unit enters the mode, it will fail to defrost the ice. 

The thermostat or timer malfunction occurs due to poor sensors or communications. 

You should hire an HVAC expert to fix the thermostat and timers. 

6. Damaged or blocked outdoor coil

The outdoor unit has a condenser coil to enable the heat exchanger. 

In the cooling mode, the condenser coil transfers the warm air outside your house. 

In the heating mode, it transfers the warm air to the coolant, which changes it to cold air and circulates it inside your house. 

The coils have enough space for the maximum heat exchangers. 

If the coils get damaged due to rust or get blocked due to dirt and debris, it can lead to a refrigerant leak. 

As a result, the defrost mode will not work properly. 

Check the coil for signs of damage. 

Since the coils are difficult to access, call your HVAC team to check them and clean or replace them. 

7. Improper installation 

You can expect an improper unit installation when a heat pump does not defrost properly. 

You should install the heat pumps by taking help from the heating and cooling company excelling in heat pumps. 

Some common installation mistakes include:

  • Refrigerant is poorly charged
  • Kink in the refrigerant line due to carelessness
  • A place with insufficient airflow
  • Installed over a flat or low surface

You should call a professional to look into your unit if you suspect improper installation.  

8. Electrical or wire issues

Wires are the electrical issues through which current passes to the heat pumps and other components. 

Suppose the wires are damaged, loose, or frayed. In that case, the connection between the components will damage, and the defrost cycle will never start. 

You should hire an electrician to fix the wires. 

9. Insufficient fan power

The heat pump fan helps start or end the defrost cycle. 

If the fan does not start or its power is insufficient, the heat pump will start several issues. 

Defrost not working is one of them. 

The defrost cycle gets stuck when the fan works backward and never starts again. 

Call an HVAC expert to fix the fan. 

What to do when the defrost does not work in the heat pump?

You can resolve the problems that lead to the defrost issue by yourself or by seeking help from the HVAC expert team. 

To prevent the problem from further occurring, follow the following steps:

Remove leaves and debris from the outdoor coil.

Make sure to clean the surrounding of the outdoor coil always, especially during the snowy winter and after windy weather. 

Keep the leaves, twigs, snow, and branches away from the unit. 

Clean the internal areas.

Check the internal components like the timer, relay, reversing valve, motor, and other things to ensure they are in good shape. 

Call an HVAC if you suspect anything.

Install an EDIDS

EDIDS stands for Energy Docs Intelligent Defrost System. 

It is a sensor, timer, and thermostat system that generates faster defrost cycles and produces adequate delivery to help your unit work efficiently. 

It also adds a pressure sensor to the thermostat and the timer to measure the temperature whenever the timer tells the thermostat to do so. 

Whenever the pressure and temperature drop low, the defrost will start immediately and keep running until both come to a proper level. 

With EDIDS, the defrost cycle will start when it is required. 

The original system is built only on time and temperature. 

The EDIDS will make your heat pump better and more intuitive. 

Give the unit some time.

Sometimes, it is not a problem, and not resolving, or prevention is required. 

Your heat pump requires some time to deliver the results.

Generally, the unit takes several minutes after you notice the ice layer. 

The older heat pumps will need more time to start the defrost cycle. 

Run the fan

Running the fan can help the heat pump to melt the ice if the defrost refuses to start. 

Running the fan with the heating in off mode will thaw the coil within one hour. 

Set your fan to exhaust mode if the outdoor temperature is too low. 

Thawing will not solve the frost issue completely, but it should help your heat pump to run back again. 

Check the thermostat 

The defrost setting should be correct in the thermostat. 

The defrost cycle mostly starts when the outdoor temperature goes below 31°F. 

If the setting in the thermostat is lower, adjust it properly. 

Defrost manually

The energy-saving settings may interfere with the defrost cycle and prevent the automatic defrost cycle from starting. 

Most modern heat pumps have a manual defrost system where you can run the defrost cycle by pressing a button. 

If your unit has such a system, press the defrost button to run the cycle manually. 

Maintain regular maintenance

Maintain routine maintenance and servicing by the HVAC team every year. 

The team will inspect the condition of your heat pump and its components and inform you if any repair or replacement is required. 

The members can fix if any repair is needed and help in the smooth functioning of your unit.

Final thoughts

Since a heat pump is a machine, it won’t come without a flaw. 

If your heat pump does not defrost, you should inspect your unit. 

Cross-check with every reason explained in this guide to find out why the heat pump refuses to defrost. 

Do not use any sharp or hard objects to remove the ice buildup. 

The fan coils are delicate, and using hard objects will damage them. 

Check for the thermostat, sensors, drainage, refrigerant level, outdoor units, reversing valves, and electrical parts. 

These are some common grounds where malfunctioning will negatively affect the defrost cycle.

Eventually, call an HVAC if you face complications or hesitation in inspecting the unit. 

How do I know if my heat pump is in defrost mode?

When the indoor fan stops, the heating halts, and the indicator light blinks, it means the heat pump is in defrost mode. 

Can I force my heat pump to go to defrost mode?

The outdoor fan will also stop, but the compressor will run. 
If your heat pump’s parts are fine, you can force your unit to defrost by setting a low temperature and adjusting the thermostat to the cooling mode. 
Once the ice has defrosted, switch it back to the heat setting.

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Arthur Smith

Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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