Electric surface heating (Thermoheld) in KFW 40 bungalow with 80 sqm

As I have written before: I have enough forest and storage space for firewood, so I will install a wood stove (if permitted) and will certainly try to reduce my heating energy demand with it. But I do not want a house that is planned for 30-50 years to be dependent from the start on whether wood heating is allowed or whether I am able to do so / or if I ever rent it out and the tenants are interested in it.

With a wood stove, a "quickly" responsive heating system would make more sense than an air-to-water heat pump, as far as I have read in the other thread about infrared heating.

Thank you for the renewed reference to PVGis, that helped to visually see the low energy generation in winter. According to PVGis, for the period Nov-Feb an 11.7-15.6 kWp photovoltaic system with 18° tilt should deliver 1,440 - 1,920 kWh for me.

Top! But also consider that the approx. 1500 kWh in winter do not come evenly distributed. Rather, on some days much and on some days nothing at all. So you will still feed in some electricity and still have to buy quite a bit. As a comparison, my January with 12 kWp: Generates 200 kWh, feeds in 50 kWh, buys 560 kWh. Also, the question is how well the system regulates and actually uses the photovoltaic power. For me, a lot of optimization has already gone into running the heat pump especially when the sun is shining.

That means I need 1,900 energy demand, so I should go for a 15.7 kWp photovoltaic system + storage (27k €).

Storage actually doesn’t bring you anything for heating.

Useful energy demand Nov-Feb 500 kWh.

That already sounds very little. And how do you actually produce hot water?

Or rather, in the bathroom and everywhere I additionally have the electric ceiling surface heating.

Aren’t these electric heating foils on the floor? Or ceiling and floor?

Great! But also keep in mind that the approx. 1500 kWh in winter do not come evenly distributed.
...
Just for comparison, my January with 12 kWp: Produced 200 kWh, fed in 50 kWh, bought 560 kWh.

Also, the question is how well the system regulates and really uses the photovoltaic power.
For me, a lot of optimization has already gone into running the heat pump especially when the sun is shining.

Thanks for the numbers! True, especially early on I will need a lot of heating power so that it’s warm enough for the home office,
at the same time, the photovoltaic faces south, meaning I don’t generate electricity in the morning.

Storage doesn’t really help you with heating.

Oh, unfortunately I haven’t dealt with the storage topic yet – it’s simply included as standard with the photovoltaic package.
I naively thought that it might charge up at midday and then I could use the power from the midday sun in the evening (with losses).

> 1,500 kWh / year usable energy
That already sounds very little.
And how do you actually produce hot water?

Oops – you’re absolutely right – you can tell I’m still at the beginning of this and definitely well advised by a prefabricated house provider who hopefully corrects many mistakes I would make. I had simply taken the electricity consumption of a one-person household. But that the hot water is not included in the 2,000 kWh heating energy for KFW 40 at 80 sqm, I actually should have guessed :)
Thanks for the hint.

Aren’t these electric heating foils on the floor? Or ceiling and floor?

According to the manufacturer: in the bathroom on the floor, otherwise on the ceilings.

I naively thought that it might charge up at midday and I could then use the electricity from the midday sun in the evening (with losses).

Basically, it works like that too, but: In practice, in winter the sun is low, the days are short, and it is usually overcast. Not much comes in at midday, and then your heating is still running and consumes some of it. If we now take the example of RotorMotor, your storage can only store 50 kWh at all.

Thanks again for the numerous feedback. As a layperson who "finally" wants to realize his own construction project, this was enormously helpful.

I have now had the first sales talk and received the construction service description along with U-values for the wall and window surfaces. I initially determined the U-values for the roof and floor based on the wall structure and the KfW minimum requirements. Especially for the floor value, I had to find that even with 120 cm of concrete, I only got a U-value of 0.27 instead of the required 0.25.

My overall calculation resulted in a energy demand of 5000 kWh/year for the "KfW 40" house with 81 sqm living area. It is possible that I made a miscalculation somewhere. If anyone is more experienced, they are welcome to take a detailed look at my calculation:

[*
Exterior walls including openings: 46 m, assumed height: 2.5 m
[*]Total area of exterior walls: 115 sqm
[*]Window and front door details:
[LIST]
[*]6.7 m are front door/floor-to-ceiling windows (assumed height: 2.1 m)
[*]1.6 m window height: 1 m
[*]1.6 m window height: 70 cm


[*]Total window/front door area: 15.72 sqm
[*]Exterior wall area without openings: 99.28 sqm
[*]U-values:


[*]Walls: 0.13 W/m²K
[*]Windows/front door: 0.6 W/m²K
[*]Roof: 0.104 W/m²K (estimated based on structure)
[*]Floor: 0.25 W/m²K (according to KfW 40 minimum requirement)


[*]Temperature difference: 20.5 K (22° indoor, 1.5° outdoor temperature in winter, estimated for northern Bavaria)
[*]Heating loads:


[*]Walls: 262.82 W
[*]Openings (windows/doors): 193.3 W
[*]Roof: 232.284 W
[*]Floor: 562.125 W


[*]Total heating load without ventilation: 1250.53 W or kWh/day
[*]Ground area of the house (L-shaped): 109.5 sqm
[*]Ventilation heat loss without heat recovery (heat recovery): 1250.53 W (= total heating load. maybe this assumption is wrong?)
[*]When using a decentralized ventilation system with 80% heat recovery: heat loss 250.1 W
[*]Total heating load including heat recovery: 1500.635 W
[*]Heating period (Oct-Apr): 210 days with 16 hours heating time per day, resulting in an energy demand of 5042.1 kWh.

Based on these data I have now requested the surcharges for air-to-water heat pump and air heat pump and mentioned that the "simple" heating system is a disqualifying criterion for me. The air-to-water heat pump would also bring savings for water heating. Both heat pumps save me the challenge of constantly finding the "barely comfortable temperature." Especially in the home office, I notice that I start to feel cold after sitting for 4 hours.

I believe the seller's specification was: approx. 3,000 kWh heating energy demand However, it could also include 20.5°, other outside temperatures (these are further south where it is warmer), only 8 hours at home, etc.

Although I probably have a calculation error above: 16h at home does not mean double the heating energy demand, rather 10-20% more energy?