ᐅ Calculating heating demand to determine the size of the ground source heat pump

We are going to build a KfW 55 energy standard house. Solid 42.5 cm (17 inches) exterior walls with a "white tank" basement, which will be insulated. Two full stories. The building footprint is approximately 11.5 x 9.5 meters (38 x 31 feet) with an open space of 3 x 3.5 meters (10 x 11.5 feet). The basement is planned to be partially heated, possibly fully heated. (We are currently debating this with the general contractor because the construction contract states that only part of the basement should be heated, but now they say this is not possible if the KfW 55 standard is to be met.) The heat pump was planned—or rather estimated—with a capacity of around 8 kW, and the Vaillant VWF 88/4 is included in the contract. However, I read in forums that some people manage with 6 kW units for similar house characteristics. Meanwhile, our general contractor says we may need to upgrade to a 10 kW heat pump if the entire basement is to be heated. That would be roughly 291 square meters (3,130 square feet) of living area including the basement. Does anyone have experience with this? Most of the windows (totaling 24 square meters / 258 square feet) face south.

Here is an estimate based on my building. I don’t know your climate zone, but I have a KfW 70 standard (on paper, actually better building technology), with a basement overall (genuine hot water, insulated), although a few square meters (square feet) smaller. The unheated basement rooms, being within the insulated envelope, don’t get colder than 15-16°C (59-61°F) in winter (good for storage – and I don’t have any issues with condensation). The additional heat demand for this should be limited. Our 5.8 kW (6.5 hp) ground source heat pump has always had plenty of capacity, and I’ve set the heating curve to minimum to avoid overheating. With a bit of fine-tuning, the heat pump doesn’t short-cycle. I can’t imagine that a house with better insulation but a few square meters (square feet) larger would have a significantly higher heating load. The south-facing windows don’t really matter since heat demand is especially high during cold winter nights.

I think that if you also use a ground source heat pump and size your probe appropriately, you shouldn’t need more than “one size up” from mine (unless you’re in a particularly cold climate zone). It would definitely be best to have a proper calculation done rather than relying on a “guess” from the general contractor.

We also have an 11.5 x 9.5 m (38 x 31 ft) house with a basement, but a gable roof – which does not affect the heating area.

Our building is almost KfW55 standard, but with a poor Ht-value of 0.29 – we didn’t over-insulate. However, we made a few modifications compared to the energy saving regulation calculation. I might have it recalculated at some point...

Everything is heated – including the hallways. Our heating load is 8.1 kW, just as a reference.

We have 190 sqm (2,045 sq ft) without a basement, with a heat transfer coefficient (HT) of 0.248. Our heating system is 5.3 kW – whether it is 6 or 8 kW should not significantly affect either the price or the heating costs.

What would be a good HT value?

Sebastian79 schrieb:
We also have a 11.5 x 9.5 m (38 x 31 ft) footprint with a basement, but a gable roof – which doesn’t affect the heating area in terms of size.

Our house is close to KfW55 standard, but we have a poor Ht value of 0.29 – our insulation isn’t overdone. We did make a few modifications compared to the energy saving ordinance calculation. I might have it recalculated sometime...

Everything is heated – including the hallways. Our heating load is 8.1 kW, just as a benchmark.

I think your building shape is more complex than mine (mine is a simple rectangle). I’m curious about the actual heating requirement—I’m still wondering why mine is so much lower despite almost the same size. The few unheated square meters in the basement (hallways and other areas are otherwise heated) hardly make a difference since they are within the insulated envelope. They are indirectly heated that way.