The recent concern in Germany about heat pumps posing an explosion risk after a power failure stems from a multi-day blackout in southwest Berlin, caused by a suspected arson attack on infrastructure.
Amid freezing temperatures, warnings circulated — especially in media and online – about potential dangers to monoblock heat pumps.
The primary issue is frost damage, not an immediate explosion:
- When power fails for an extended period (days) in sub-zero temperatures, the circulation pump stops.
- Heating water in exposed outdoor pipes or the heat exchanger can freeze, expand, and burst components.
- This can cause expensive total damage to the heat pump (e.g., cracked heat exchangers or pipes).
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From The Blackout- News
Monobloc heat pumps can suffer severe frost damage in the event of a blackout lasting several days because their water-bearing components are located outside the building. As soon as the power fails, the circulation pump stops, and the heating water no longer circulates through the outdoor unit. At sub-zero temperatures, pipes and heat exchangers can freeze. In the process, the ice expands and bursts pipes or components. This is exactly the pattern that threatened in the southwest of Berlin, where around 50,000 households were affected, and persistent cold exacerbates the risk of frost damage (businessinsider: 05.01.26).
This is how frost damage occurs in the outdoor unit – and why Berlin can be particularly affected
Many operators underestimate the frost damage, because the system runs inconspicuously in everyday life. Monobloc units are located completely outside, i.e. including compressors and heat exchangers. That’s why a prolonged power outage hits them harder than systems with more technology in the house. In areas such as Nikolassee, Wannsee, Zehlendorf and Lichterfelde, heat pumps are also likely to be installed more often, because many owners have modernised there.
The problem is in the heating water circuit, because heating water flows from the house to the outdoor unit and back again. As long as there is electricity, the circulation pump keeps the water moving and the water in the pipes remains liquid. If the supply is lost, the water stands still and frost first attacks unprotected, above-ground sections. Then frost damage often occurs abruptly because the ice bursts the components from the inside.
80 percent monobloc – high prevalence increases the risk of frost damage
The Berlin Heating Guild estimates the proportion of monobloc design in air-to-water heat pumps at around 80 percent, which increases the number of potentially endangered systems. The specialist portal IKZ Select has already warned against deceptive security, although many installations appear technically clean. The mechanism of damage remains decisive.
The IKZ formulates this drastically: “The fact is: If the heat exchanger freezes and bursts as a result, this is tantamount to a total loss of the heat pump”.
Such a total loss hits operators twice because, in addition to the costs, there is also a lack of time. Manufacturer services sometimes only come after weeks, and the house remains without a reliable heat source during this phase. After that, discussions with insurers or contractual partners often begin. If you want to avoid frost damage, you therefore need a clear technical strategy.
Insulation, valves, emptying: What helps against frost damage and where the limits lie
IKZ advises solid pipe insulation up to “200 percent insulation”, but this often fails in practice. Many plumbers avoid huge wall openings of 40 or 50 centimeters because they hardly fit structurally. In addition, insulation only brings time, because it “only delays the cooling of the pipes, it cannot be prevented.” With several frost days, even a thick shell can lose, although it protects in the short term.
Technically more robust are shut-off and emptying options near the outdoor unit, because operators can then react in a targeted manner. A classic thermostatic freeze valve opens at a low temperature and drains heating water, leaving too little water in the heat exchanger for destructive pressure. However, the valve costs energy because it remains uninsulated and allows heat to escape. An indoor variant reduces losses, but the operator has to open it manually as soon as a long power outage becomes apparent.
Scandinavian frost protection cycle – less frost damage, but noticeable loss of efficiency
In Scandinavia, many systems solve the risk differently because winters there are long and cold. Operators often use an additional outdoor circuit with antifreeze, while the normal heating water remains in the house. This reduces the risk of frost damage to the water-bearing part, because there is no longer pure heating water outside. The system works more stably, but it costs efficiency.
IKZ gives a concrete parameter for this, because a water/glycol mixture with about 25 percent glycol reduces efficiency by around 15 percent. Nevertheless, many operators accept the disadvantage because they want to avoid failures in frost. In Sweden, the bill is easier because the electricity price there is regularly below 25 cents per kilowatt hour. Germany, on the other hand, is discussing costs more intensively, although resilience in the event of power outages is becoming increasingly important.
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