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Glossary · Energy industry & energy transition

Defrost cycle

Defrost cycle heat pump air-to-water heat pump Efficiency heat Operation

A defrost cycle is an automatically initiated operating period of a heat pump during which frost and ice are removed from the air-side evaporator. It particularly affects air-to-water and air-to-air heat pumps in heating mode. The evaporator is the heat exchanger through which the unit absorbs heat from the ambient air. Defrosting is part of normal operation: a temporary layer of frost alone does not indicate a fault.

Why the evaporator ices up

For heat to transfer from the air to the refrigerant, the evaporator must be colder than the outdoor air. If its surface falls below freezing, moisture in the air can accumulate on it as frost or first condense and then freeze. Icing is therefore possible even when outdoor temperatures are above freezing. Humid weather near zero degrees Celsius is often particularly unfavourable. In dry, much colder air, the need for defrosting may be lower.

A growing layer of ice impairs heat transfer and narrows the air passages between the fins. This reduces useful heating output and operating efficiency. However, precautionary defrosting when there is insufficient need is also unfavourable. The control system must therefore find an appropriate time between excessive icing and unnecessary defrost cycles.

How defrosting works

In many air-to-water heat pumps, the refrigerant circuit is temporarily reversed. The outdoor heat exchanger then releases heat instead of absorbing it, and the ice melts. Some of the required heat comes from the heating water. During this time, normal heat delivery to the building is interrupted or reduced. The compressor continues to run and consumes electricity.

The heating system must provide sufficient usable heat and the water circulation required by the manufacturer. A buffer tank can help fulfil this requirement, but is not universally required regardless of the hydraulic design. An electric immersion heater may be involved, but is not necessarily required. Manufacturers sometimes specify different minimum water volumes for systems with supplementary heating enabled and disabled.

Different methods are used to initiate defrosting: fixed run-time intervals, time- and temperature-dependent methods, and demand-based controls. The latter can, for example, evaluate temperature differences as an indication of increasing icing. The termination temperature, maximum defrost duration and drip time are also part of the control strategy. A universal rule such as “ten minutes every hour” is therefore not a reliable description of all heat pumps.

Energy demand and accurate energy accounting

Defrosting consumes energy, but also prevents the losses associated with a permanently iced-up system. Evaluation therefore requires complete operating periods, including icing, defrosting and the return to heating mode. Neither “defrosting is insignificant in energy terms” nor “every defrost cycle is a malfunction” accurately describes this relationship.

Electrical energy and heat extracted from the heating system must not simply be added together as if both were electricity consumption. A purely illustrative calculation: if a compressor runs for six minutes at an average electrical power input of 2 kW, it consumes 0.2 kWh of electricity. If 0.5 kWh of heat is also extracted from the heating water, this is not an additional 0.5 kWh of electricity. The heat must be replaced later; the electricity required depends on subsequent operation. These freely chosen values are not typical equipment data.

The coefficient of performance (COP) describes the ratio of heating output to electrical power input under specified conditions. By contrast, the seasonal performance factor relates heat output to electrical energy over a year. For comparisons, the system boundary must be consistent: are supplementary heating, circulation pumps and other auxiliary drives included? A calculated seasonal performance factor must also be distinguished from a yearly value actually measured.

The direction of measurement also matters: if the heat pump extracts heat from the heating circuit during defrosting, a meaningful net heat balance must account for this return flow. Otherwise, a meter that totals only positive heat delivery can produce an overly favourable performance factor. What matters, therefore, is not just whether a heat meter is installed, but how the measuring device and the evaluation handle negative heat flows.

Laboratory values do not automatically exclude defrosting

The blanket claim that laboratory COP values never account for defrosting is false. For example, manufacturers explicitly publish coefficients of performance according to EN 14511 “with defrosting” for the A2/W35 operating point. This denotes outdoor air at 2 °C and heating water leaving at 35 °C. Nevertheless, such a test point does not represent all the weather, part-load and installation conditions of a building. Neither the annual defrost loss nor the seasonal performance factor can therefore be read directly from a single COP.

Normal operation or reason for inspection?

Clouds of mist around the outdoor unit can be normal during defrosting. However, drainage of the resulting water is just as important as melting the ice. The drain must be protected against freezing in accordance with the installation arrangement and manufacturer’s instructions. Water that freezes again can impair the unit or form hazardous ice patches on walkways.

  • In particular, heavy icing that remains after defrost cycles, recurring fault messages or a significant decline in heating performance warrant inspection. Do not chip off ice or remove it with hot water; doing so can damage components.

  • For a professional diagnosis, weather conditions, the duration and spacing of cycles, complete ice removal and energy consumption should be considered together. Defrost parameters should not be changed without knowledge of the unit-specific control system.

Defrosting is also not the same as cycling: during cycling, the compressor alternates between running and stopping; during defrosting, ice is deliberately removed. A brief drop in heating-water temperature therefore no more proves undersizing than an indication that supplementary heating is active. A diagnosis requires operating data and an understanding of the system design.

Implications for buildings and the electricity system

The additional energy input and temporarily interrupted heat supply mean that defrosting can also affect the electrical load profile. However, this does not necessarily imply a simultaneous load peak from many units: similar weather does not mean identical start times or control strategies. Nor does defrosting categorically rule out flexible heat pump operation. Planning for such operation simply needs to account for the technical operating requirements of the specific system.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 06:38

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