ᐅ What capacity does the heat pump need to have?

Hello everyone,

We are currently building a two-family log house from Fullwood and are in the process of sizing a heat pump. Since we don't have much expertise in this area and received different advice from various specialists, we are a bit uncertain.

The heated area is 200m² (2,153 sq ft) and the heat pump only needs to supply the underfloor heating. We had a heating load calculation done according to DIN EN 12831, which resulted in a standard building heat load of 5.7 kW.

We have also received an offer for a Panasonic Monoblock Aquarea system with either 7 kW or 9 kW capacity, each with a 300-liter (79 gallons) buffer tank.

The system should neither be too small nor too large. What capacity would you recommend, or are there additional parameters that should be considered?

Thanks in advance for your response.

Regards, Dieter

For 5.x kW, I would definitely take the risk with the 5 kW monoblock; if it’s not enough, you can add a second one. If it regularly runs at full load, pay attention to acoustic insulation towards the neighbors.

For our house, 4 kW was calculated without hot water; in reality, 3 kW including hot water is sufficient.

Use an inverter-controlled one... it only draws as much power as needed and doesn’t make that much noise.

The Panasonic unit is an example of this. At our 3kW heating load, it operates very quietly, but when running at 5kW it becomes noticeably louder. Therefore, it’s important to note that with a tight system design, acoustics become more critical since the unit will run at high power more frequently.

By the way, the 7kW heat pump is definitely not a good choice, because unlike the popular 5kW model, it loses capacity as the temperature drops and cannot deliver the full 7kW output. At higher outdoor temperatures, the 7kW capacity is not needed anyway.

And please do not use a buffer tank, although this is probably already discussed in the HTD forum.

Thank you all for your responses. You have provided us with really helpful additional information. The topic of heat pumps is quite complex, and during a consultation, we were already recommended a much larger unit (16kW) with a buffer tank at a price that was no longer acceptable. We will forgo the buffer tank. The system is located 4.5m (15 feet) from the neighboring house, so acoustics are not entirely unimportant. Does it make sense to use one heat pump as we currently do with our two-family house (two stories), or would it be better to install two smaller units, each serving one condominium separately?

The idea of using two smaller heat pumps for each household is no longer relevant, as the smaller units start at 5 kW (5.5 hp) and that would not be practical in our case. It was just a thought.