ᐅ Is a 6 kW heat pump sufficient for a KfW 70 energy standard house? Experiences with Dimplex?

Hello,
we are building a KfW 70 house that will be heated using an air-to-water heat pump.
Additionally, a wood-burning stove will be installed, mainly because we already have a very good one and like the “ambience” it creates.
The house has a usable floor area of 177 m² (1905 sq ft).
The calculated energy demand is 6.03 kW/m³.
Now, a Dimplex LA 6TU is planned for installation.
It can provide a maximum of 6.00 kW/m³.
However, our heating installer considers this sufficient because:
"as discussed yesterday, we prefer to rely on the smaller unit – since we always design the energy demand for -10 degrees Celsius (14°F) – and for those 3 days we have the wood stove to reduce the load"
Do you think this will work?
Does anyone have experience with Dimplex?
Is it possible to have this checked by an expert, and what would the cost be?

Thank you in advance!

You can’t really make a general statement about that. Besides the fact that you can’t make a reliable assessment without an explicit heat demand calculation, I think the small pump might be significantly undersized for 177m² (1905 sq ft) of living or usable space (assuming it is fully heated).

Try searching on Google for “heat demand calculation.” You’ll find many links where you can at least roughly estimate your heating requirements.

Hello,
we have paid for such a calculation!
Some figures:
Annual primary energy demand: 52.78 kW/m² (16.1 kW/ft²)
Annual final energy demand (absolute): Energy source 1 / electricity mix = 3339 kWh (11,407,000 BTU)
Energy source 2 / auxiliary energy (electricity) = 261 kWh (891,000 BTU)
Heated building volume: 6.03 kWh/m³ (0.17 kWh/ft³)

I can’t recalculate it myself anyway.
However, I have read here that these calculations are often inaccurate or have little to do with actual heating costs.
Therefore, my question. I thought, surely a particular user who usually answers such questions would respond...

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

These numbers are relatively useless for this purpose! They serve, as required by law, exclusively for a primary energy-based, qualitative comparison. It is not permissible to size heating systems or derive consumption from them. For this, please read the fine print in the report!

hawken schrieb:
....I have read here that these calculations often do not match or have little to do with actual heating costs.

Exactly!
First, it should be checked whether an air-source heat pump is generally suitable. If yes, then a sensible unit must be selected according to the requirements and necessities. These are completely unclear here. So first clarify the basics (actual demand (capacity, energy) for heating and domestic hot water). Only then can you assess, in context, which unit is appropriate.
Air-source heat pumps have some specific characteristics compared to, for example, ground-source heat pumps.

Defrosting energy is required for the evaporator. This increases the more oversized a unit is (no capacity modulation).
Air-source heat pumps are essentially "misused" air conditioners for heating purposes, delivering their highest output exactly when it is not needed at all. Therefore, full modulation is almost mandatory.
Also, at very low outdoor temperatures, auxiliary electric heating is necessary. Through precise planning, dimensioning of the overall system, and equipment selection, it is possible to keep this energy share to just 0.2 to max. 2% of the annual heating demand, thus having little impact on overall consumption costs.
Air-source heat pumps have manufacturer-specific, highly differentiated heating capacity and COP curves. If this is not sufficiently considered, higher consumption costs can be expected.
Unlike other heat generators, specifications such as “6 kW capacity” are completely meaningless because capacity depends, among other factors, on the outdoor temperature.

Well-designed air-source heat pump systems achieve an annual performance factor (APF) of about 3.5 (heating, domestic hot water) or better. Without adequate planning, you may be unlucky and have to accept an APF of 2.3 to 2.7. There are many examples of this.

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

APF = 3.5 => consumption 2,857 kWh/year
APF = 2.3 => consumption 4,348 kWh/year

Extra consumption: 1,491 kWh/year — now calculate this with electricity prices over 20 to 25 years, considering a reasonable price increase. This can easily add up to a three- or four-digit amount (€). In comparison, the costs for precise overall planning are minimal.
If you have questions, send me an e-mail or a private message.

Best regards,