Hello,
the sizing of the heat pump is not quite straightforward. In the past, the usual consensus was to size the heat pump rather too small than too large. This was to prevent the machine from short cycling (frequent switching on and off of the compressor) at moderate temperatures.
However, heat pumps have since evolved. Nowadays, modulating – i.e. compressor speed-variable – heat pumps are standard. They adjust their output to the required heating demand. Therefore, a moderate oversizing of a heat pump is uncritical.
On the other hand, an undersizing of an air-water heat pump has a long-neglected disadvantage. An air-water heat pump must defrost regularly because the compressor freezes. This is normal and physically unavoidable. During the defrosting process, heat energy is withdrawn from the heating circuit / buffer storage, and the heat pump is "reversed". Under unfavorable weather conditions with high humidity and outdoor temperatures around the freezing point, these defrosting cycles can occur at short intervals. Depending on the machine, defrosting every 30 minutes is not uncommon.
To compensate for the energy loss, the heat pump must generate a higher heat output between defrosting cycles. A heat pump that is actually correctly or minimally undersized can run into trouble here and, even at the highest modulation level, no longer produce enough heat to compensate for the defrosting losses. As a result, the house will slowly cool down. Unfortunately, this disadvantage, or this important background information, is not considered either in the common calculation methods for sizing or in the manufacturers’ specifications.
Best regards