Should Heat Pump Defrost Cycle Worry You?

Heat pumps have built-in features designed to protect them in freezing weather and maintain their efficiency. When it starts snowing in your area, you might notice that your heating system stops blowing warm air for some time, and its indicator light starts blinking. This might concern you, thinking your heating equipment has given up, but there’s nothing to worry about; it’s completely normal.    

When the outdoor coil starts to freeze, your heat pump goes through a defrost cycle. The process helps unfreeze the outdoor coil to ensure your unit keeps functioning properly.

What exactly is this defrost mode, and how does a heat pump defrost cycle work? Let’s find out!

What Is Defrost Mode on a Heat Pump?

In winter, a heat pump extracts heat from the outside air and expels the warm air indoors. During extremely cold conditions, the moisture in the air freezes on the outdoor unit, resulting in frost on the coil.

When the outdoor coil freezes, it can put a strain on the system’s operation and damage its internal components. To prevent this, your heat pump runs a defrost cycle (sometimes several times) to defrost itself, keeping ice buildup at a minimum using the defrost mode.

How Does a Heat Pump Defrost Cycle Work?

Your heat pump automatically switches to defrost mode when ice starts forming on the outdoor coil. Since the outdoor coil is typically about 10 degrees cooler than the surrounding air, it can freeze even when the air temperature is above 32 degrees. In modern heat pumps, a special thermostat or sensor called a frost thermostat located at the bottom of the outdoor coil detects its temperature. The frost thermostat closes the coil circuit, which causes an internal timer to start. Many heat pumps have a generic timer that activates the defrost relays at certain intervals. A typical heat pump defrost cycle activates every 30, 60, 90, or 120 minutes.

The defrost relays turn on the compressor and switch the heat pump’s reversing valve, making it run in cool mode. The unit has now entered the defrost cycle. You can tell that your heat pump is in a defrost cycle once it stops blowing hot air. Some models have a visual indicator, like a blinking light. 

In a defrost cycle, the heat pump dissipates the heat outdoors, warming up the outdoor coil and melting the ice. When a defrost cycle is initiated, the outdoor fan usually stops while the indoor unit keeps running. The condenser fan remains off until the ice on the heat pump has thawed.

The unit operates in defrost mode until the thermostat senses that the outdoor coil temperature has reached between 55F and 57F. The frost thermostat then opens the circuit, stops the timer, and returns the heat pump to heat mode.

How Long Does a Heat Pump Run In Defrost Mode?

A typical heat pump defrost cycle lasts 5 to 15 minutes and is repeated until the frost has completely melted. Depending on the amount of frost and the intervals, the time it takes to remove the ice from the outdoor heat pump unit will vary.

Types of Heat Pump Defrost Controls

Heat pumps use one of two types of defrost controls: time-temperature or demand defrost. Both systems operate by briefly redirecting heat from your home to the outdoor unit to remove frost or ice.

• Time-Temperature Defrost 

This defrost mode, mostly found in older models, turns on and off at regular intervals. Heat pumps with time-temperature defrost mode do not have a sensor to detect when the unit needs to run a defrost cycle. The cycle can activate even if the coils are not actually frozen, resulting in energy wastage. 

• Demand Defrost 

Demand defrost mode is found in newer heat pumps. It uses a sensor to detect when frost starts forming on the outdoor coil. Once low coil temperature is detected, the pump reverses its mode and starts blowing warm air on the coil. 

Signs Your Defrost Control System Isn’t Working

According to Justin Cornforth, an HVAC expert at Ace Home Co, “Go outside and check your heat pump outdoor coil, fan blades, or casing when the temperature is below freezing. If you notice ice buildup, your defrost mode might not be working.” 

A sudden rise in your bills could also indicate a problem. If the defrost cycle isn’t working properly, the system will work harder and use more energy. 

You can also check for a clear click sound that occurs when the system switches to defrost mode. “If there’s no clicking sound, the valve might be broken,” adds Cornforth.

Why Your Heat Pump Defrost Cycle Might Stop Working

The defrost mode on your heat pump might stop working completely, or it can get stuck in this mode. Moreover, certain issues can also force your heat pump to go into defrost mode frequently. There could be different reasons why that happens; some are easy fixes, while others require professional attention.

1. Faulty Sensors, Reverse Valves, or Thermostat

If the internal thermostat or sensors that detect the coil temperature develop problems, your heat pump’s defrost mode won’t turn on as it is not getting any signals. In this scenario, you can manually switch your programmable or smart thermostat from heat to cool mode. This will cause the unit to gather heat from the inside and send it to the frozen coil outside.

However, this is not a permanent solution; you will need to check the thermostat and sensors thoroughly. If the thermostat is not sensing the temperature accurately, your heat pump can get stuck in defrost mode or start going into defrost mode recurrently. 

There could also be a problem with the reverse valve that switches the unit from heat to cool in defrost mode. You should get it checked by a trained HVAC technician and see if the valve needs to be replaced.

2. Refrigerant Leaks

A refrigerant absorbs or releases heat, depending on the mode your heat pump is operating in. If there is a refrigerant leakage, there may not be enough heat for the heat pump to work properly in defrost mode.

Recharging your refrigerant and fixing the leak will help the defrost cycle to function as usual.

3. Electrical Issues

Over time, with wear and tear, electrical connections can go bad, preventing your heating system from signaling the defrost mode to turn on. Hire a technician to inspect and replace any faulty wires.

4. Improper Installation

Proper HVAC installation by a qualified technician is crucial. If your HVAC unit is not installed according to guidelines, it can cause problems in the heat pump defrost cycle.

If your heat pump isn’t positioned correctly, it can result in impeded airflow and, eventually, frozen coils. Improper installation can also cause twists in the refrigerant lines, resulting in a leak.

5. Damaged or Broken Coils

Moisture from the air and debris can accumulate on the coils, leading to corrosion in the long run. The problem is exacerbated when you neglect coil cleaning. Damaged coils can also cause refrigerant leaks, potentially disrupting the defrost function. 

6. Outdoor Unit Getting Clogged

Leaves, snow, dust, and debris can clog your outdoor unit. Since the airflow is restricted in a clogged unit, warm air will not reach the coils, and the defrost mode will not work at all.

Related: How to Clean an Outdoor Unit

What Causes Your Heat Pump to Freeze Unnecessarily?

Apart from low outside temperatures, your heat pump can also freeze for numerous other reasons. When that happens, your heat pump goes into defrost mode frequently, which is not a healthy sign. So, whatever is causing your outdoor coil to freeze unnecessarily warrants a thorough checkup to avoid any severe damage.

1. Restricted Airflow

In case of a restricted air supply, the air does not pass over your heat pump’s outer evaporator sufficiently. This results in ice formation on your heat pump in winter.

Restrictive airflow can be caused by debris buildup inside the outdoor unit or dirty air filters clogging up the unit. Moreover, if you have planted shrubs to hide your outdoor unit but haven’t maintained the necessary distance, it can suffocate the system.

Related: How to Clean Your AC Filter for Optimal Performance

You might be blocking your heat pump vents unknowingly by putting furniture pieces in front of them. This compromises the airflow, leaving you cold and uncomfortable. Remove any furniture or any other obstruction to avoid HVAC airflow issues. 

2. Technical Problems

As heat pumps work day and night to keep you comfortable, they are exposed to harsh elements that may cause certain issues.

For example, refrigerant leak, malfunctioning defrost cycle, faulty thermostat sensors, damaged fan motor, and other problems can lead to ice on the heat pump. Also, if your heat pump gets stuck in defrost mode, the frost on the coil won’t melt.

Related: Heat Pump Troubleshooting: 11 Common Heat Pump Issues and Their Fixes

3. Excess Moisture

Excess moisture from snow or rain gets trapped inside your heat pump, increasing the chances of freezing. Water falling on your unit due to a damaged roof gutter is also a culprit.

When your roof gutters become clogged with leaves and debris, water can directly fall onto your heat pump, causing it to freeze up.

Fix broken gutters that are over your heat pump, and if you have no gutters, try installing them so that water does not run onto your HVAC unit.

4. Outdoor Unit Slab Settling in the Ground

The heat pump’s outdoor unit is usually installed on a slab for leveling. Over time, the slab can settle in the ground. Even an inch of settling can restrict airflow and block the normal drainage path. This can lead to moisture accumulation and, subsequently, ice buildup on your heating unit. 

Heat pump leveling is highly important to prevent moisture accumulation and to avoid damage to internal parts. The heat pump slab should be a few inches above the ground. You can also place patio blocks under the unit to level it.

5. Lack of Maintenance

Heat pumps need routine maintenance to stay in top-notch condition. If you ignore the regular tune-ups, such as changing air filters and removing debris from the outdoor unit, it can result in many issues, including your unit freezing up unnecessarily.

Preventative HVAC maintenance can go a long way toward eliminating problems that can contribute to heat pump freezing. Choose the type of filter best suited for your home and regularly clean them. Also, regularly clean the coils and trim back the vegetation around the outdoor unit. In addition, keep an eye on snow, branches, and debris from building up around your outdoor unit.

Also, schedule a professional inspection at least once a year to determine whether everything’s working properly and to extend the life of your unit.

6. Outdoor Fan Issue

The outdoor fan blows air over the coil, which is essential in the heat transfer process. If the fan becomes defective, there is no airflow, which can cause your outdoor unit to freeze. The issue could also be related to a capacitor that stores electrical charge to give the fan a boost for starting. 

Contact an HVAC professional and ask them to fully inspect the outdoor fan and the capacitor. 

How to Manually Defrost Your Heat Pump

Here are some DIY techniques that you can use to thaw your heat pump manually when the defrost cycle on your heat pump isn’t working.

1. Run Your Heat Pump In Fan Mode

If your defrost mode isn’t working, you can try running your unit in fan mode. This helps in case your coils are not heavily covered in ice, and you will see results in half an hour. 

2. Defrost With Water 

Start by turning off the power to your heat pump at the electrical source, either through the circuit breaker or the main switch.

Head outside to your outdoor unit. Use a garden hose to gently spray water over the unit. This can help melt the ice. Note that it will work in mild weather only and won’t work if it’s too cold.

3. Use Manual Defrost Mode

Some heat pumps have a manual defrost feature. You will have to refer to your manual to see how to use it. 

The Takeaway

If your heat pump switches to defrost mode in cooler months, there’s nothing to worry about. The heat pump defrost cycle is designed to maintain your unit’s efficiency. In the defrost cycle, the unit dissipates heat on the outdoor coils, thus helping to melt the ice and protecting the unit. 

To prevent your HVAC system from freezing unnecessarily, keep your system well maintained. Trimming the plants around the outdoor units, cleaning the coils, and changing the filters are easy measures that you can take to keep your system running smoothly.