Heat Pump Defrost Cycle Is Not Working? How To Fix

Heat pumps naturally freeze during cold weather. It’s not abnormal. However, the units are designed to automatically go into defrost mode to melt the excess ice and keep the appliance running efficiently.

Unfortunately, sometimes the heat might “refuse” to go into defrost mode or keep going in defrost mode. Or, it may get into defrost mode but fail to shed the excess ice.

This can be a significant issue as a thick layer of ice on the AC directly affects heat pump efficiency. Excessively frozen heat pumps can also overwork the fan, which may eventually get damaged. It’s also possible for the outdoor coils to crash, causing the refrigerant to leak out and ruin the compressor.

So, what can cause heat pump defrost failure, and what can you do about it? Let’s begin by understanding how the defrost cycle works.

How the De-Frost Cycle Works

The heat pump defrost cycle is automatic. When the thermostat senses that there’s ice buildup on the outside coils, it instructs the reversing valve to switch from heating to air conditioning mode – yes, air conditioning. Essentially, the evaporator temporarily becomes the condenser.

However, the compressor fan simultaneously shuts down. This means that the condenser can no longer pump air inside the house. This usually leads to higher temperatures inside the condenser, resulting in a rapid defrosting process.

Heat pump defrosts cycles last 5-10 minutes, depending on the degree of ice buildup. However, the thermostat always waits until a specific temperature before suspending the defrosting process and resuming regular heating.

Different heat pumps have different ways of determining when to resume defrosting. Older models rely on mechanical timers that work in conjunction with defrost thermostats to determine the temperature at which to go into defrost mode.

This means that the unit will go into defrost mode whether or not there’s ice buildup as long as the specified temperature is reached. Defrosting is also terminated when a predetermined temperature is reached.

Meanwhile, newer heat pumps tend to rely on solid-state control modules that determine the best time to begin and stop defrosting via sensors.

The even more advanced models use Demand Defrost mechanisms that calculate the best time to start or stop the defrost cycle based on outside air, the refrigerant temperature in the coil, and run time.

Why is the Heat Pump Not Defrosting?

You’ll know that your heat pump is not defrosting if you notice dripping on the heat pump or lack of drainage.

Dripping in the heat pump usually happens because a frozen heat pump accelerates ice-buildup inside the unit. The buildup will become worse both inside and outside the house. So, you’ll likely notice water dripping from the air handler and the compressor.

Meanwhile, lack of drainage often happens when ice blocks are formed inside the heat pump. Excess freezing can cause the condensed water that usually drains out smoothly to freeze up and turn into ice blocks.

Of course, you’ll also know that the defrost cycle isn’t working if you notice that the ice buildup on your heat pump isn’t going away. The following are common reasons why the heat pump defrost cycle may malfunction;

1. Extremely cold conditions

As we’ve mentioned, defrosting requires heat. So, the same heat pump must generate sufficient heat to facilitate the defrost cycle. Unfortunately, external conditions are sometimes too cold for the heat pump to generate enough heat for a proper defrost cycle. This is particularly common in older heat pumps with reduced power.

An easy way to tell that the heat pump isn’t producing enough heat for the defrost cycle is an ice layer on the outdoor condenser that only keeps growing. You may also notice that the heat pump enters a defrost cycle but doesn’t entirely shake off the ice.

2. Drainage blockage

The defrost cycle often involves melting ice inside the heat pump and draining it outside. This helps free the coils and allows the heat pump to resume function.

However, imagine a scenario where the drainage system is blocked! Where would the water go? Nowhere, right? It would be held inside the heat pump. As the cold conditions persist, this water can freeze, forming a block of ice inside the condenser.

Sometimes the block of ice is so big that the defrost cycle cannot break it down. When this happens, you may notice that the defrost cycle runs on and on without shaking off the ice.

3. Low refrigerant levels

The defrost cycle, just like regular heating, relies on the refrigerant to transfer heat where it’s needed. In defrosting, the refrigerant extracts heat from outdoor air and transfer it to the coils and other condenser areas that need the warmth to melt ice.

If the level of refrigerant is too low, two things will happen. First, there won’t be enough heat to facilitate the defrost cycle. Secondly, there will be reduced pressure and a drop in temperatures inside the heat pump. Lower temperatures can negatively impact the defrost cycle.

4. A bad reversing valve

The reversing valve is critical to the defrost cycle. It’s the component that changes the direction of refrigerant flow. Reversing the flow of refrigerant changes the mini-split from an air conditioner to a heat pump.

Although it doesn’t happen too often, this valve can malfunction. For instance, it can get stuck in a heating or cooling position. It can also leak internally. Or the coil can become defective.

When the reversing valve fails, forget about defrosting. The heat pump will not enter defrost mode. It will also produce more ice, which can worsen the icing issue.

5. A bad thermostat/timer

The defrost cycle depends on the thermostat or a special timer that calls for the heat pump to switch into defrost mode to melt ice. The thermostat or timer also determines how long the heat pump stays in defrost mode. If the thermostat or timer malfunctions, the unit may enter the defrost mode unnecessarily or fail to defrost when necessary.

Thermostat and timer problems often originate from sensors or poor communication. If one of the sensors malfunctions, it may fail to call for defrosting even when it’s time to defrost.

6. Damaged/blocked outdoor coil

The outdoor compressor unit contains condenser coils that facilitate heat exchange. In air conditioner mode, it transfers warm air outside the house, while in heating mode, it transfers warm air to the coolant material. The coils have a large surface area for maximum heat exchange.

However, the coils can become blocked or damaged, thus compromising the defrost cycle. For instance, hail damage has been reported in many parts of the country. The coils can also become blocked from debris, stones, and other elements.

What to Do if your Heat Pump Defrost Cycle is not working

Dealing with a heat pump that won’t defrost is a delicate matter. If you’re impatient, you may cause even more damage. Here’s what to do;

Give it time

Sometimes the defrost cycle takes a while to deliver results. This is especially true for older heat pumps. It can take several minutes before you notice the layer of ice receding. It may take even longer for the unit to complete the defrost cycle.

Check for debris

Is there something blocking airflow into or out of the air conditioner? Are the filters clean and the vents free? If you notice leaves and other debris next to the outdoor unit, remove them. Also, check the vents closely and remove any stones and twigs that maybe be blocking airflow.

Run the fan

Running the fan is one of the best solutions to a heat pump that has refused to defrost. Running the fan, with heating off, in most cases thaws the coils within 60 minutes. If outdoor temperatures are extremely low, set the fan to exhaust mode. Thawing won’t solve the problem permanently but should get your heat pump back running.

Check the thermostat

First, make sure the defrost temperature is set appropriately. In many cases, the defrost cycle starts when outdoor temperatures dip below 31˚F. If your setting was lower, adjust it appropriately. If that doesn’t solve the problem, verify whether the thermostat is functioning normally or if some parts or wiring are damaged.

Defrost it manually

Many modern heat pumps have a manual defrost button that you can press to run the defrost cycle. If your unit has this button, you can press it to run the defrost cycle manually. Perhaps your power-saving settings are interfering with the defrost cycle, impeding automatic defrosting.

Ensure regular maintenance

As you can tell by now, most causes of defrosting failure are related to poor maintenance. Issues such as dirty filters and condenser coils, bent coils, and dirty fans also result from poor maintenance. Therefore, you can significantly reduce the chances of heat pump defrost failure by scheduling regular professional maintenance.

Don’t Wait Until It’s Too Late

Heat pump defrost failure can be a costly problem. If it happens when you’re away, you may only come back to a wrecked compressor unit. Moreover, freezing problems can spread to the indoor air handler with ease. If this happens, you may lose the mini-split. Even water damage is possible.