Is a 6kW heat pump sufficient for a KfW70 house? Experiences with Dimplex?

Hello, we are building a Kfw 70 house which is to be operated with an air-to-water heat pump. Additionally, a Swedish stove will be installed. Mainly because we have a very good one and we like the "ambiance". The house has a usable area of 177 m². An "energy demand of 6.03 kW/m³" was calculated. Now a Dimplex LA 6TU is to be installed. This can provide a maximum of 6.00 kW/m³. But our heating engineer considers this sufficient because: "as discussed yesterday, we like to rely on the small device – since we always dimension the energy demand for -10 degrees – and during these 3 days we have relief from the wood stove" Do you think this fits? Does anyone have experience with Dimplex? Can this be checked by an expert and what does something like this cost? Thank you very much in advance!

You can't say that so generally. Apart from the fact that without an explicit demand calculation you can't make a reasonable statement anyway, I consider the small pump for 177m² of living or usable space (if fully heated) possibly to be severely undersized.

Just search on Google for "Wärmebedarfsberechnung". There you will find many links where you can at least roughly calculate your heat demand.

Hello,
we have paid money for such a calculation!
Some numbers:
Annual primary energy demand 52.78 kW/m²
Annual final energy demand (absolute): Energy carrier 1 / electricity mix = 3339 kWh
Energy carrier 2 / auxiliary energy (electricity) = 261 kWh
the heated building volume: 6.03 kWh/m³

I can't recalculate it myself anyway.
But I have read here that these calculations often do not match or have little to do with the actual heating costs.
Therefore, my question. I thought "surely a specific user will respond" who usually answers such questions........

..we paid money for such a calculation! Some numbers:...

These numbers are relatively useless for this purpose! They serve, as intended by the legislator, exclusively a primary energy, qualitative comparison. It is inadmissible to dimension heating systems or derive consumption from them. For this, read the fine print in the proof!

....But I have read here that these calculations are often incorrect or have little to do with the actual heating costs.

Exactly! First of all, it should be checked whether an air heat pump is fundamentally sensible. If yes, then a suitable device matching the requirements and necessities must be selected. These are completely unclear here. So first clarify the basics (actual demand (power, energy) for heating and hot water. Only then can it be assessed in context which device is suitable. Air heat pumps have a few peculiarities compared to, for example, brine heat pumps.

Defrosting energy is required for the evaporator. This increases the more a device is oversized. (No power modulation) Air heat pumps are "misused" air conditioning units for heating purposes, providing their highest performance when it is not at all necessary. Therefore, full modulation is almost mandatory. Also, at very low outdoor temperatures, the assistance of an electric heating element is necessary. Through precise planning, dimensioning of the overall system, and device selection, it can be achieved that this energy share amounts to only 0.2..max 2% of the annual heating effort and thus hardly affects the total consumption costs. Air heat pumps have manufacturer-specific, very differentiated heating performance and COP curves. If this is not sufficiently considered, higher consumption costs must be expected. Unlike other heat generators, specifications such as 6 kW output are completely meaningless, as the output depends, among other parameters, on the outdoor temperature.

Well-planned air heat pump systems reach an annual performance factor of ~ 3.5 (heating, hot water) or better. Without sufficient planning, one can be unlucky and have to settle for an annual performance factor of 2.3...2.7. There are plenty of examples of this.

Example: actual energy demand for heating and hot water = 10,000 kWh.

Annual performance factor = 3.5 => consumption 2,857 kWh/a Annual performance factor = 2.3 => consumption 4,348 kWh/a

Additional consumption: 1,491 kWh/a now calculate with the electricity price over 20..25 years. Take into account a reasonable price increase. This quickly amounts to three..four-digit € sums. For this, the costs for precise overall planning are peanuts. If you have questions, send me an e-mail or a private message.

Best regards