Why Does My Heat Pump Keep Going Into Defrost Mode?

It’s wintertime, and the heat pump has done very well thus far. Heating is good, and there are no noise issues to complain about.

Unfortunately, the defrost cycles never seem to end. It feels like the heat pump is constantly in defrost cycle! Now you’re wondering what this means and whether you should be worried. You’re also wondering if something can be done to remedy the situation.

How the Defrost Cycle Works

Before we explain what could go wrong, potentially causing the heat pump to keep going into defrost cycle, perhaps we should begin by understanding how the purpose of the defrost cycle and how the process works.

The defrost cycle occurs during heating when the heat pump stops pumping warm air indoors and instead uses the warmth to “melt” frost within and outside the heat pump. Heat pump frosting happens naturally during the winter. When temperatures outside dip below a specified point, the moisture inside the heat pump can condense and freeze.

The freezing occurs inside and outside the heat pump in winter, affecting nearly every heat pump component, from the fans (indoor and outdoor) to the heat exchanger and coils (condenser and evaporator).

During the defrost cycle, the heat pump directs the reversing valve to heat outdoors (rather than indoors) to thaw the outdoor coil.

You can tell that your air conditioner is in a defrost cycle if it stops heating and the indoor fan shuts off. Many heat pumps also have a visual indicator on the control panel that shows the unit in a defrost cycle. The outdoor fan also stops running during the defrost cycle while the compressor keeps running.

Defrost Cycle Duration and Frequency

The duration of a defrost cycle varies from one case to the next. In most cases, it takes about 10 minutes. Indeed, it’s set to 10 minutes by default for most new heat pumps. However, the defrost cycle may also take longer.

If you let an HVAC install the unit on your behalf, they’re likely to set the defrost cycle to go on until the temperature sensor on the outdoor coil senses that the coil’s temperature is 75˚F or higher. Others also prefer to let the defrost cycle run until the timer on the circuit board overrides the outdoor sensor.

As such, the defrost cycle can run for as little as 10 minutes or as long as 30 minutes. Very rarely do defrost cycles last more than 30 minutes.

Frequencies also vary. While some heat pumps are designed to enter a defrost cycle every 30 minutes, others are programmed to defrost once per hour. Meanwhile, some heat pumps enter a defrost mode whenever necessary.

In the latter case, the defrost cycle is activated;

  • Outdoor temperatures drop below 31˚F (or a different specified temperature depending on the heat pump) as reported by the temperature sensor in the outdoor unit.
  • The indoor room temperature calls for heat.

The cycle will then run until the end of the specified duration (set to 10 minutes for most heat pumps). When it stops, indoor heating resumes automatically.

More sophisticated heat pumps come with advanced control boards that monitor other operating details such as refrigerant pressure to initiate the defrost cycle only when necessary, thus saving energy.

Factors that Determine Defrost Cycle Frequency

No matter the age of the mini-split, though, whether you’re using an older from the 90s or a modern unit from a few years ago, defrost cycle frequencies depend on the following factors;

  • The heat load: How much work does the heater do on any given day? How many rooms does it serve, and how’s the insulation quality in the rooms? Heat pumps with a bigger load tend to freeze up more frequently as they deliver more heat indoors.
  • The control design: If your unit uses a fixed timer, the defrost cycle will activate when the timer says so, even when the weather outside doesn’t seem too bad. However, if it uses sensors, it will only get into a defrost cycle if a specific temperature is reached, even it happens once every few days.
  • Outdoor temperature: In all heat pumps, the defrost cycle is only activated when the thermostat records lower than 31˚F. Therefore, the frequency of defrost cycles in both timer-based and sensor-based heat pumps depends on how frequently and how long outdoor temperature reaches or stays below 31˚F.
  • Outdoor humidity: If temperatures remain the same, ice will form faster in more humid conditions. Consequently, your heat pump is likely to enter defrost cycles more frequently in humid conditions.
  • Airflow pressure: You may also witness more defrost cycles on days when airflow pressure across the outside coil is lower than usual. Heat pumps usually use that as a sign of icing or frosting on the coil.

8 Reasons Your Heat Pump Keeps Going Into Defrost Mode

Now that you know how frequently a heat pump should enter the defrost cycles and factors that determine defrost frequencies, it’s easy to identify reasons why a heat pump may defrost more frequently.

  1. The unit is undersized: As we’ve seen, the load size is a critical factor in defrost cycling. If the load is too big for the heat pump (i.e., the heat pump is oversized), you may experience more defrost cycles.
  2. Airflow is blocked: In heat pumps with airflow sensors, a significant drop in airflow across the outdoor coils is interpreted as a sign of icing or frosting across the coil. This could lead to more (and unnecessary) defrost cycles.
  3. Dirty filter/fan: A dirty filter or fan can also cause reduced airflow across the coils. Additionally, the heat pump may be overtaxed (increased load). A combination of these two factors can cause more frequent defrost cycles.
  4. Dirty indoor coils: Dirty indoor coils can also block airflow even if the fans and filters are clean. This can cause the heat pump to work harder to meet the heating demands inside the house. The extra load can result in more frequent defrost cycles.
  5. The temperature sensor has failed: Remember that the temperature sensor is responsible for determining the temperature at which the heat pump enters defrost mode in most heating systems. If it malfunctions, the unit may enter the defrost mode even during the summer when there’s no heating.
  6. A failed defrost control board: A wrongly wired or malfunctioned defrost control board or relay timer can cause the heat pump to enter defrost mode unnecessarily.
  7. Refrigerant leakage: Refrigerant leakage can cause a temperature drop across the heat pump, increasing defrost cycles.
  8. Malfunctioned outdoor unit fan: The outdoor fan motor or relay can get damaged. It may also begin to run backward after a lightning strike. When it can’t push heat through the heat pump as needed, there can be a temperature and pressure drop, potentially causing too many defrost cycles.

What to Do

There are only two things you can do to address the issue of too many defrost cycles. The first one is maintenance. Ensure regular maintenance to prevent issues such as blocked filters and dirty fans and condenser coils to prevent over-defrosting in the first place. However, if the issue has already happened, switch off the heat pump and call an HVAC pro to prevent costly damages.