Heat Pump Not Defrosting – Troubleshooting Guide

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Is your heat pump not defrosting, under-performing and working with the reduced efficiency? Is there strange sound or white cloud of steam coming out of the unit? Is the outdoor unit frozen, cover with the white frost? All the above including many other symptoms could be due to a vast array of problems, from lack of maintenance to element failure?

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Here’s why it could be happening and what can you do to repair it. Take a look at this troubleshooting guide to learn how to fix your malfunctioning heat pump!

Almost every heat pump or air conditioning system will come to a moment where it will start creating ice on its components. As the ice buildup naturally affects the productivity of the device, the unit automatically enters a state to melt that ice, a mode known as defrosting. Sometimes, however, the device may not start that melting process at all, which could lower the performance significantly and eventually harm your heat pump.

Here we are going to go over everything you need to know when the heat pump is not defrosting, why it is so important, and how you can repair it.

How does a heat pump work?

Before we talk about the defrosting problem, lets first see how does the heat pump work and what are the main components.

The main components of every heat pumps are; compressor, evaporator, condensing coil, reversing valve, sensors, thermostats, fans and other, which can be found in this article.

There are three cycles or modes of the heat pumps; cooling, heating and defrosting.

A heat pump is a device designed for heating, cooling and dehumidifying. The device extracts the heat from air or water, mainly during the hot and warm weather (also during the winter), and exchange it with the refrigerant which is further used for heating.

During the process, and especially in winter, for example, the cold, wet climate and lower temperature could quickly ice up metallic and plastic parts, without problems. If you experience winter-related issues, check out this article.

Sometimes, these heat pumps get covered with the snow and ice, especially in the coil and fan parts which are essential for the air distribution & heating process of the device. When these two parts do not work, the device will undoubtedly have problems heating up the air.

That is when the device starts failing, having damaged elements, working inefficiently, and maybe not even doing its work properly.

However, there’s a process in most heat pumps which fixes the “iced-up” problem within – the “defrost mode.”

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What's the defrost mode?

During the heat pump operation, there is a slow process of developing conditions for the ice buildup on the outside coils. And once it happens, this is where the defrost mode starts – the thermostat senses the drop in the temperature so the reversing valve switches the system from heating to cooling process. The evaporator becomes the condenser temporarily.

The heat pump should defrost regularly when the frosting occurs and should be long enough to melt all of the ice, but short enough to avoid energy waste. The defrosting relays make the cycle occur every 30 min or so.

During the cooling or AC mode, the outside fan stops spinning so the device stops moving air inside, which leads to higher temperatures and rapid defrosting operation.

This process could last from 5 to 10 minutes, or even more, depending on the thickness of the build-up ice. With the combined work of the reversing valve and the evaporator, the high-pressure refrigerant gets warmed up and defrosts all the frozen components.

The thermostat or sensors will wait for the heat pump to come back to a specific temperature and then stop the defrosting process. After the defrost mode stops, the device restarts the normal heating process but also starts icing up again slowly, depending on the environment, temperature, and overall integrity of the heat pump, triggering the defrost process again.

Old heat pump models also have the defrosting mode, but instead of thermostats or sensors, they may use timers instead that would work every certain amount of time. The problem with the older models is that the unit will operate in the defrost cycle no matter is the ice present or not, therefore wasting the energy.

This mode, however, is not perfect, and sometimes may be the reason why your device is not working efficiently. When the heat pump is not defrosting, very likely you will have to repair it before it is too late.

How does a heat pump defrost?

Temperature difference outside/inside

When the thermostat or sensor located on the bottom of the outdoor coil determines that the temperature is too low compared to the outside temperature, the defrost cycle immediately starts. This could happen with temperatures as high as 60ºF or as little as 30ºF depending on the preset in the device.

After the thermostat on the bottom of the outside coil reaches the temperature of close to 60 F, the defrost cycle stops and the coil is free of frost.

Air pressure

When the coil is iced-up, the space between the coils can become tight so the air which is going through the narrow passage works with the higher pressure. Here is when the defrost cycle also starts with a differential switch that turns on when the pressure is too high and turns off when the air pressure lowers to normal levels.


Old heat pump models, typically, don’t have thermostats, sensors or air pressure switches but a timer. Usually, they are set to work every hour or more for a few minutes to defrost the device. This method is usually less effective when defrosting, yet more reliable.

Timer & thermostat

Sometimes, the device can have a thermostat/sensor with a timer at the same time. This is a lot more effective than any of the previous ones alone. It just focuses on measuring the temperature every 90 minutes or so and depending on what the thermostats or sensors say; the device will determine whether to turn the defrost mode ON or not.

Even in summer, these systems should work efficiently. However, sometimes this doesn’t happen which leads to the frosted heat pumps. If any of these previous methods don’t happen, very likely you will have to look further for the cause of your heat pump not working.


The defrost relay is in charge to turn the compressor on, switch the reversing valve and stop the fan from spinning.

Causes & symptoms of heat pump not defrosting 

When the evaporator or condenser defrosts, the refrigerant (hot) is redirected into the coil while the outside fan stops working to speed up the process. The compressor is the element which moves the refrigerant and increases the pressure/temperature. The process is followed by the humming noise when the compressor is operating, “whooshing” sound once the defrost cycle is over and white steam that blows out the top of the unit.

But even though this process seems pretty efficient, it could work deficiently. When any of the previous methods for defrosting is not being efficiently processed, the device will stay frosted and thus harm the heating capacity of the heat pump.

Some of the causes & symptoms are:

Dripping on the heat pump

When there is water dripping on the heat pump evaporator or condenser, the ice build-up process during the cold weather accelerates.

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Lack of drainage

When devices are located in places where drainage is stopped or clogged, the device will store the water inside, reducing the temperature and creating blocks of ice.

Low refrigerant levels

The less refrigerant there is in the heat pump, the more likely you will experience iced-up and frosted components. The refrigerant is the one that heats up the iced-up components, without it you won’t be able to defrost any part of it. Leaks may also cause coolant to work slower and to diminish with time.

Wrong installation

When there is no airflow, no proper drainage, lack of flow inside the heat pump, position too low in the ground, or even just a bad angle in which the refrigerant doesn’t pass through the device as it should, could cause the defrost mode not to work correctly.

Bad reversing valve

When the valve that reverses the heating process to cooling process doesn’t work, the device automatically creates more ice than expected and never enters the defrost mode.

Wiring problems

Wires are the ones that pass current and which eventually make specific components do their work correctly. If some cables are damaged, the connection between components may not function and thus defrost never starts.

Bad thermostat/sensor/timer

If the device that measures the temperature, air pressure, or overall iced-up state of the heat pump is not working correctly, the device will never enter the defrost mode. It could be a sensing problem or just a problem with communication with the rest of the device.

Damaged outdoor coil

When the outdoor coil doesn’t work correctly, air can’t pass through the device efficiently, and then the ice is created faster inside. This could also damage how to defrost mode starts, making it stop due to the inefficiency of the outdoor coil.

Inefficient fan power

If the fan doesn’t turn on when it is needed to help the defrost mode begin or stop, then the heat pump will immediately start having problems. When the fan works backward, this process also gets stuck, and the defrost mode never begins either.

Low RPM motor

When the fan or the motor are not working efficiently enough, it is very likely that the ice created is due to a problem with refrigerant temperature or with the evaporator/condenser not doing their work correctly.

Troubleshooting when a heat pump is not defrosting

Do the following to prevent or reduce the risk of heat pump not defrosting issue:

Remove leaves and debris from the outdoor coil

This is an easy work for any homeowner - make sure you maintain your heat pump’s outdoor coil clean at all times, especially during the coldest months. Keep the leaves, branches, and snow away from the device.

Check the internal area

A bad defrost thermostat, sensor, timer, a bad relay, a stuck reversing valve, a bad motor, an inefficient fan, a stuck fan, or just anything of the like could be harming your heat pump defrosting process. Check the internal area to make sure these components are in good state.

Schedule maintenance

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Call an HVAC professional every six months or before winter comes. This way you will ensure the device is well enough in every aspect that could make it not defrost, including the reverse valve, the refrigerant, and any other problem it may have.

Install an EDIDS 

An Energy Docs Intelligent Defrost System, EDIDS, is a sensor, timer, and thermostat system that induces defrost faster, more efficiently, and with the proper delivery to help your heat pump work better.

It adds the pressure sensor to the thermostat and timer, so every 90 minutes or so, when the timer tells the thermostat to measure temperature, the pressure sensor will do as well. If the temperature and pressure drop in the device is way too high, the defrost might start immediately and until both (temperature and pressure) come to a decent level.

With the EDIDS, the defrost mode will only start when required.

Need to fix your heat pump not defrosting?

As you see, the causes for a heat pump not defrosting can be many. These problems often occur when there is low maintenance done to your device, so take that into account as the first and most crucial step.

If there is a component that needs repair, check it out first, as that could be your problem. No matter what is happening when the heat pump is not defrosting, calling an expert will always be the best idea.

Keep in mind that during the cold and humid weather, the frosting of the outside coil is normal. It shouldn’t be seen during summer. If it happens, most likely there is a problem.

What is making icing even worst are rain, dirty coils and the pile of snow. Defrosting is normal. But if it doesn’t work properly, the operation cost gets higher and there is more stress on the components leading to damages and expensive service work later on.

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