Is the heat pump running constantly? What is normal and what is not

Anyone switching from an oil or gas boiler knows a certain pattern: the burner starts up, heats vigorously, and goes off again. A modern heat pump often behaves completely differently. It runs quietly for hours on end, in winter sometimes almost without a break. This worries many people in their first winter. Usually without reason.

Long, steady running is usually intentional

Many heat pumps today are speed-controlled, also known as modulating. The klimaaktiv guide describes it like this: they continuously adjust their heating output via the compressor to meet current demand. On a mild day, it runs at low power, on a cold day at higher power.

This is normal and not a fault. According to the guide, modulating units are significantly more efficient in operation than fixed-speed units, which can only switch on and off. A heat pump that runs for a long time at medium power in cold weather is therefore often more economical than one that runs at full speed for a short time and then pauses.

It is not the running time that is decisive, but the electricity consumption. Whether this is appropriate is shown by the performance factor. How you calculate it is explained in the article Checking electricity consumption yourself.

Frequent switching on and off: the real warning sign

More noticeable than continuous running is the opposite: the heat pump starts up, runs for a few minutes, goes off and then comes back on shortly afterwards. This is called cycling.

The most common cause is a system that is too large. klimaaktiv notes that heating systems are frequently oversized, and this has a negative impact on the lifespan and efficiency of the heat pump. Energieberatung Niederösterreich (energy advisory service of Lower Austria) also warns: constant switching on and off leads to high wear and tear and consumption, especially with heat pumps.

Other possible reasons include a heating curve that is set too high, thermostatic valves that constantly restrict the flow, or unfavourable settings on the control system. It is worth taking a look at the article Setting the heating curve.

A quick test: On a mild autumn day, observe how often the system starts for an hour. If it switches on and off several times during this period, speak to your installer about it. Many control systems also display the number of compressor starts and operating hours. The ratio between the two is a good basis for discussion.

Defrosting: steam, silence, whooshing

With air source heat pumps, there is a process in winter that can be startling the first time: defrosting.

The klimaaktiv-Wegweiser (guide) explains the background: the outdoor unit extracts heat from the air. In doing so, moisture from the air condenses on the fins. If the heat exchanger is colder than zero degrees, it freezes. Ice impairs heat transfer, which is why the unit has to defrost regularly. This requires energy, but is still part of normal operation.

What you might notice:

• The fan stops briefly or runs differently than usual.
• A cloud of steam rises from the outdoor unit. This is water vapour, not smoke.
• Meltwater drips and runs underneath the unit.
• There is a brief hissing or whooshing sound when the system switches over.

Afterwards, the unit continues to run normally. In damp weather around freezing point, an air source heat pump defrosts more often than in dry cold. It is important that the meltwater can drain away. Read more about this in the article Maintenance.

Noises: what to expect

A heat pump is not silent. The fan and compressor produce a steady hum or whoosh that changes with the output. In cold weather, when the system has to work harder, it is louder than in autumn.

There are guideline values for the neighbourhood. For residential areas, klimaaktiv specifies planning guideline values according to ÖNORM S 5021 of a maximum of 30 dB(A) at night at the property boundary. You can estimate how loud a unit is at a specific installation location using the noise calculator.

On the other hand, the following are not normal:

• Knocking, banging or rattling
• a new, high-pitched whistling
• vibrations transmitted into the house through walls or floors
• noises that are suddenly significantly louder than before

When to call the installer

You can remain calm if the system runs for long, steady periods, defrosts occasionally and the house is warm.

Contact the installer if:

1. The control system displays an error message. klimaaktiv recommends checking the system regularly for error messages. Note down the code and date.
2. It cycles significantly, i.e. starts and stops at short intervals.
3. The house does not get warm, even though the system is running continuously.
4. Ice piles up under the outdoor unit or the unit remains permanently iced over.
5. Unusual noises occur, as described above.
6. The electricity consumption is significantly higher than what was calculated during planning.

If you have the figures ready, the conversation will be shorter and more productive: meter readings, operating hours, compressor starts, outdoor temperature. Which figures you should record from the very beginning is explained in the article After installation.