How Often Does a Heat Pump Normally Defrost?

Your heat pump just entered defrost mode for like the tenth time in the last few hours. So, you’re now wondering whether something is wrong with it. Do normal heat pumps defrost so frequently? Or is your unit starting to lose its power?

Well, it depends. A typical heat pump defrosts fairly frequently, especially when it’s very cold outside. However, there’s a limit to how many defrost cycles you can have per hour/day. So, you’re right to be worried that your unit is defrosting too often.

Below we discuss everything you need to know about heat pump defrost cycles, including what goes on during the defrost period and how often a heat pump defrosts on average. We’ll also discuss what to do if you think your unit is defrosting too often.

How Often Does a Heat Pump normally Defrost?

Most heat pumps are programmed to enter the defrost cycle after running for a minimum of 35 minutes. This means that the average heat pump cannot defrost more than twice per hour. Each defrost cycle lasts 10 minutes, on average.

What’s the Defrost Cycle?

The defrost mode is a function/feature on most modern heat pumps that momentarily switch the heat pump into air conditioning mode for a while till it thaws frozen condenser coils.

When in heating mode, it reverses the flow of refrigerant to take warmth outside to the frozen condenser coils. Warming up the frozen condenser coils helps thaw the surrounding frost.

When the ambient temperature outside gets very cold, the moisture in the air freezes on the outdoor unit’s heat exchanger as the fan blows the air across it.

Why is the Defrost Cycle Needed?

Because heat pumps freeze quite often. Very few homes go the entire winter season without experiencing frost buildup on the outdoor unit.

The defrost cycle helps thaw any frozen condenser pipes to keep the heating process going. Otherwise, you may experience several heating challenges, including;

  • Reduced heating output: The heat pump unit cannot extract much heat from outdoor air if the condenser coils are covered in frost. This often results in poor heat output, which could leave you in the cold.
  • Reduced heating efficiency: Although the heat pump will still try to keep your home warm, a frost buildup means it has to work extra hard to maintain the thermostat setting. This can often mean increased power consumption, leading to a drop in efficiency.
  • Compressor damage: A layer of frost over your condenser coils spells doom for the compressor unit. If left unchecked, it can damage the unit. For instance, many homeowners have experienced condenser coil bursts in the past, leading to refrigerant leaks.
  • Other damages: Finally, a frost building in your heat pump can also lead to damages elsewhere throughout the AC. For instance, running the fan in a frosted unit can easily leave you with broken fan blades.

How the Defrost Cycle Works

The defrost cycle is activated when the ambient temperature in the outdoor coil temperature drops below 32°F. Many heat pumps begin to form frost at this point.

Extracting heat from the already cold air inside the compressor can lower the temperature of the air surrounding the coils below the freezing point, resulting in frost formation.

When the built-in sensors notice this, they activate the defrost mode. This happens automatically switches to cooling cycle without additional input from the user. Activating the defrost mode does a few things.

First, the heat pump switches from heating mode to air cooling mode. Instead, flowing into the home to heat your rooms, the hot refrigerant will cycle back to the condenser coils to thaw your system.

Secondly, it shuts off the outdoor fan motor. The main reason is to protect the fan motor from damage. Otherwise, the fan may suffer broken blades.

Finally, you may also notice the AUX heat coming on for a few minutes. Auxiliary (AUX) heat is an emergency setting on many heat pumps that temporarily steps in when the heat pump is overpowered to maintain the thermostat setting.

The heat comes from heat strips built into the heat pump. You may see “smoke” from the outdoor unit as it defrosts, which sometimes may seem like the unit is on fire. Don’t worry, it’s not. That’s just steam.

The defrost cycle typically lasts ten minutes, at which point AUX heat goes off and the heat pump resumes normal function.

How Often Does the Heat Pump Defrost?

Generally, the defrost cycle of a heat pump activates every 35 to 40 minutes, depending on the circumstances. This means that your heat pump should defrost once or twice per hour and a maximum of three times every two hours. However, this doesn’t mean that your unit must defrost every 35-40 minutes.

In fact, your heat pump can take much longer between defrost cycles under certain conditions.

1. The Defrost Sensor Must be Closed

The defrost cycle can only begin if the defrost sensor, typically mounted on the condenser, is closed. This sensor only closes when the temperature of the condenser coils is low enough to cause frosting or freezing.

In other words, your heat pump can go the entire day without defrosting if temperatures remain within a good range.

However, once the sensor closes, the heat pump’s control board begins to save accumulated runtime of the compressor.

2. Accumulated Runtime must be Reached

The compressor must reach the accumulated runtime as selected on the jumper on the control board for a defrost cycle to initiate. A typical heat pump will go into defrost every 30, 60, or 90 minutes if the outdoor coil temperature stays below setpoint (~31° F). 

For instance, if the selected runtime is 30, the compressor must reach an accumulated runtime of 30 minutes for the defrost cycle to engage.

If the defrost sensor warms up and opens before the accumulated run time is met, the control board resets the accumulated runtime to zero. The defrost cycle officially begins when both conditions are met, i.e., the defrost sensor is closed and the accumulated runtime is met.

3. Defrost Continues Until the Defrost Sensor Opens

The defrost sensor only opens when the condenser coils are warm enough to prevent freezing, typically a 75°F. Also, it only opens when all the previously existing frost has been removed.

This typically takes a short time because turning off the condenser fan motor allows the coils to heat up fast.

But, what if the defrost sensor fails for some reason or the defrost continues for longer than anticipated?

That’s where the 10-minute limit applies. Most heat pump control boards are programmed to terminate the defrost process after 10 minutes.

Setting the Defrost Cycle Timer

You don’t need to worry about setting the defrost cycle timer as that’s the job of the heat pump installer. However, you may want to know a few things about the settings.

A standard heat pump control circuit has three settings –30, 60, and 90. These settings indicate the amount of time in minutes between defrost cycles for your heat pump.

For instance, if the unit is set to 30, then your heat pump will defrost every 30 minutes, as long as other conditions are met. 

Can You Shorten the Timer Setting?

Yes, you can shorten the period to just a few seconds. But, you need to be very careful and observe standard safety procedures. With that done;

  1. Find and remove the defrost sensor connection from the defrost control boards and place a jumper across them. This simulates a closed defrost sensor.
  2. Check the board for two pins labelled “speed up.”
  3. Place a jumper or carefully short the two pins
  4. This will speed up the defrost cycle to just a few seconds.

Factors Affecting the Frequency of Heat Pump Defrost Cycles

As we’ve seen, the standard cycling frequency is every 35 minutes. However, the cycles can be longer or shorter depending on how frequently the defrost sensor closes.

The following are several factors that typically influence how often the defrost sensor closes;

Heating Load

The bigger the load, the faster the condenser coils “shed” heat. This means the coils may freeze more frequently, resulting in more frequent defrost cycles.

Outdoor Temperatures

You’ll rarely need to defrost the heat pump in fair weather. However, the extremely cold weather, typically below 30°F, may necessitate frequent defrost cycles because it puts more weight on the heat pump’s shoulders.

Outdoor Humidity

A higher outdoor humidity accelerates frost formation because more humidity essentially means more moisture turns into frost.

Other Factors

Other factors that may impact the frequency of frost formation on your heat pump during winter include the age of the appliance and quality of maintenance.

Generally, older heat pumps are weaker and thus struggle with the slightest hint of increased load.

Remember that older appliances also pump refrigerant slightly slower. These issues often necessitate more frequent defrost cycles. 

When Does the Defrost Cycle Stop?

Generally, the defrost cycle naturally stops when the defrost sensor opens. This can be several seconds to a few minutes after defrosting starts.

Otherwise, the control circuit will force it to stop after ten minutes. In other words, it cannot run for more than ten minutes.

However, a few sophisticated defrost systems are designed with additional features to stop or prevent defrost cycles based on pre-defined conditions, often to prevent compressor motor damage. for instance,

Abnormal Cycling? Signs to Watch Out For

Abnormal defrost cycles aren’t too uncommon and can be very costly. The following are a few signs to watch out for;

  • Outdoor component runs when it shouldn’t: This is usually a sign of temperature sensor or condenser control circuit error. You need to call a professional to repair or replace the two parts.
  • Outdoor unit defrosts too frequently: The main reasons for too frequent defrost cycles are an under-sized compressor, airflow challenges (blockages), dirty air filter/blower fan, and dirty condenser coils.
  • Outdoor unit doesn’t defrost – no defrost cycles: This is normal if the weather isn’t too cold. However, if the outdoor unit won’t defrost when temperatures drop below 32°F, there’s a problem. Either the reversing valve is bad or the defrost controls (sensor or circuitry) is bad.
  • Indoor unit stops when defrosting cycle stops: This is normal. The heat pump’s control panel “disconnects” the outdoor compressor’s fan during the defrost cycle, until normal heating resumes. If the indoor air handler air filters or blower fan are dirty then the indoor airflow and heat delivery are reduced and the heat pump may be over-taxed 

How Often Does a Heat Pump Normally Defrost? FAQs

Why does my Heat Pump Keep Defrosting?

If your heat pump seems to defrost nonstop, either it’s extremely cold outside, the unit is undersized, or you have airflow challenges because of dirty filters or filthy condenser coils.

How Long does a Heat Pump Normally Defrost?

Generally, heat pumps defrost for 10 minutes or less. The exact defrost time depends on whether your heat pump follows the timer or compressor-mounted sensor. However, it cannot exceed ten minutes.

How Much Frost is Normal on a Heat Pump?

Any frost buildup that doesn’t compromise airflow into the outdoor condenser is considered normal. However, there’s a problem if the frost lasts more than two hours. You need to call your HVAC technician. Similarly, never allow the frost buildup to cover the entire outdoor unit.

Summary

Heat pumps typically defrost every 35 minutes, though the defrost period can be shorter or longer depending on various factors, including the prevailing weather and the load.

However, beware that too frequent defrost cycles can be a sign of underlying heat pump issues. So, don’t hesitate to contact an HVAC pro if you feel that the unit is defrosting too often.

The same applies if your heat pump refuses to defrost even when outdoor temperatures drop below 32 degrees Fahrenheit