ᐅ Electric surface heating (Thermoheld) in a KfW 40 bungalow with 80 sqm?
Created on: 1 Oct 2023 13:42
E
eenuep1
I am currently in contact with a regional house construction company called "Ziegerhaus." This company has been active in house building since 2019 and established a production facility near me in 2022 with an investment of €220 million. It is worth mentioning that the "Ziegler Group," to which the company belongs, also owns subsidiaries for many trades such as woodworking, kitchens, and heating systems.
My focus is on an 80 m² (860 sq ft) timber frame bungalow (L-shaped, 13x10 m (43x33 ft)) with an 18° pitched roof. The standard offering includes a KfW 40 energy efficiency standard with a photovoltaic system and battery storage.
Since I work from home and am basically at home all day, and because I tend to feel cold at 22°C (72°F) in a solid masonry house, I considered installing an air-to-water heat pump with water-based underfloor heating. (However, this concerns a timber frame house.) The company normally provides their electric underfloor heating system "Thermoheld" as standard, which, despite its advertised innovation originating from medical/robotic technology, likely has an efficiency factor of less than 1.
According to the construction company, an air-to-water heat pump in such a small and well-insulated house would only pay off after 36 years. I still need to clarify the exact additional costs for the air-to-water heat pump and the water-based underfloor heating.
I have tried to manually calculate the cost savings from the air-to-water heat pump:
The estimated heating energy demand according to KfW 40:
In a single-person household, my usable energy demand during the heating period would be around 1,000 kWh. Therefore, with an electric heater (efficiency factor 1), I would need to purchase an additional 500 to 1,000 kWh, which means annual costs of approximately €150–300.
With an air-to-water heat pump (efficiency factor 4), the heating energy consumption would be only a quarter, about 1,100 kWh/year. This means I could feed approximately 2,500 kWh back into the grid during the heating season, earning about €205 per year at a feed-in tariff of 8.2 cents/kWh.
The annual savings from the air-to-water heat pump would therefore be between €350 and 500, indicating that the investment (e.g., a €14,000 surcharge over the electric underfloor heating) would pay off after 28–40 years (excluding inflation and opportunity costs). If you consider potential returns from investing in an MSCI World ETF as opportunity costs, the payback time would likely be even longer.
I hope to receive concrete numbers on heating energy demand soon. But I wonder if my assumptions and calculations are correct, or if the company is right, and the electric underfloor heating is the more economical option in this specific case.
Of course, my calculations are based on some assumptions, such as not heating with wood from my own forest, actually maintaining 23°C (73°F) all day, and no changes in electricity prices or feed-in tariffs.
Maybe there is also an error in my estimate of electricity generation during the heating season with the 9.1 kWp system?
My focus is on an 80 m² (860 sq ft) timber frame bungalow (L-shaped, 13x10 m (43x33 ft)) with an 18° pitched roof. The standard offering includes a KfW 40 energy efficiency standard with a photovoltaic system and battery storage.
Since I work from home and am basically at home all day, and because I tend to feel cold at 22°C (72°F) in a solid masonry house, I considered installing an air-to-water heat pump with water-based underfloor heating. (However, this concerns a timber frame house.) The company normally provides their electric underfloor heating system "Thermoheld" as standard, which, despite its advertised innovation originating from medical/robotic technology, likely has an efficiency factor of less than 1.
According to the construction company, an air-to-water heat pump in such a small and well-insulated house would only pay off after 36 years. I still need to clarify the exact additional costs for the air-to-water heat pump and the water-based underfloor heating.
I have tried to manually calculate the cost savings from the air-to-water heat pump:
- Photovoltaic system: 45 m² (484 sq ft) area, approx. 9.1 kWp
- Annual electricity generation at 18° roof pitch: approx. 8,000 kWh
- Electricity generation during heating season (Oct-April): approx. 4,600 kWh (possibly an overestimate?)
The estimated heating energy demand according to KfW 40:
- 15–40 kWh/m² per year
- For 80 m²: 1,200 – 3,200 kWh/year
- With 16 hours of heating per day (instead of 8 hours): +10–20%
- At a desired temperature of 23°C (73°F) instead of the usual 20.5°C (69°F): +15–18%
- This results in an estimated total heating energy demand of about 4,000–4,500 kWh/year.
In a single-person household, my usable energy demand during the heating period would be around 1,000 kWh. Therefore, with an electric heater (efficiency factor 1), I would need to purchase an additional 500 to 1,000 kWh, which means annual costs of approximately €150–300.
With an air-to-water heat pump (efficiency factor 4), the heating energy consumption would be only a quarter, about 1,100 kWh/year. This means I could feed approximately 2,500 kWh back into the grid during the heating season, earning about €205 per year at a feed-in tariff of 8.2 cents/kWh.
The annual savings from the air-to-water heat pump would therefore be between €350 and 500, indicating that the investment (e.g., a €14,000 surcharge over the electric underfloor heating) would pay off after 28–40 years (excluding inflation and opportunity costs). If you consider potential returns from investing in an MSCI World ETF as opportunity costs, the payback time would likely be even longer.
I hope to receive concrete numbers on heating energy demand soon. But I wonder if my assumptions and calculations are correct, or if the company is right, and the electric underfloor heating is the more economical option in this specific case.
Of course, my calculations are based on some assumptions, such as not heating with wood from my own forest, actually maintaining 23°C (73°F) all day, and no changes in electricity prices or feed-in tariffs.
Maybe there is also an error in my estimate of electricity generation during the heating season with the 9.1 kWp system?
As I mentioned before: I have enough forest area and storage space for firewood, so if allowed, I will install a wood stove and will certainly try to reduce my heating energy demand with it. However, since the house is planned for 30 to 50 years, I don’t want it to be dependent from the start on whether wood heating is permitted, whether I am capable of using it, or whether future tenants would want to use it.
With a wood stove, a heating system that responds quickly would also make more sense than an air-to-water heat pump, as I gathered from the other thread about infrared heating.
With a wood stove, a heating system that responds quickly would also make more sense than an air-to-water heat pump, as I gathered from the other thread about infrared heating.
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RotorMotor2 Oct 2023 10:14eenuep1 schrieb:
Thanks again for the tip about PVGis, it helped to visually see the low energy production in winter.
According to PVGis, a 11.7–15.6 kWp photovoltaic system with a 18° tilt should produce 1,440 - 1,920 kWh for my location during November to February. Great! But keep in mind that the approximately 1,500 kWh in winter won’t be evenly distributed.
On some days you will have a lot of production, and on others, none at all.
So you will still feed in some electricity and have to purchase quite a bit as well.
Just for comparison, my January with a 12 kWp system: It produced 200 kWh, fed 50 kWh into the grid, and I bought 560 kWh.
Also, the question is how well the system manages and actually uses the photovoltaic electricity.
For me, a lot of optimization has gone into running the heat pump especially when the sun is shining.
eenuep1 schrieb:
That means I need about 1,900 kWh energy demand, so I should go for the 15.7 kWp photovoltaic system plus battery storage (27,000 €). A battery storage doesn’t really help you with heating.
eenuep1 schrieb:
Useful energy demand Nov-Feb 500 kWh. That sounds very low.
And how do you generate your domestic hot water?
eenuep1 schrieb:
Also, in the bathroom and everywhere else, I have additional electric ceiling heating panels. Aren’t these electric heating foil panels installed on the floor? Or are they on the ceiling and floor?
RotorMotor schrieb:
Great! But keep in mind that the approximately 1500 kWh (1600 kWh) in winter won’t be evenly distributed.
...
As a comparison, my January with 12 kWp: Generates 200 kWh, feeds 50 kWh into the grid, buys 560 kWh.
Also, the question is how well the system controls and actually uses the photovoltaic power.
For me, I have optimized a lot to run the heat pump mainly when the sun is shining. Thanks for the numbers! That’s true, especially early on I’ll need a lot of heating power to keep the home office warm,
and the photovoltaic system faces south, so I don’t generate electricity in the morning.
RotorMotor schrieb:
Storage really doesn’t help you much with heating. Oh, unfortunately I haven’t dealt with the storage issue yet – it’s just included as standard in the photovoltaic package.
Naively, I thought it might charge at midday and then I could use the electricity from the midday sun in the evening (with losses).
RotorMotor schrieb:
> 1,500 kWh/year useful energy
That already sounds very low.
And how do you produce your hot water? Oops – you’re absolutely right – you can tell I’m just starting out and definitely should have good advice from a modular home provider who hopefully can fix many mistakes I might make. I simply took the electricity usage of a single-person household. But I probably should have realized that the hot water isn’t included in the 2,000 kWh heating energy for a KfW 40 house with 80 m² (860 ft²) 🙂
Thanks for pointing that out.
RotorMotor schrieb:
Aren’t those electric heating films installed on the floor? Or both ceiling and floor? According to the manufacturer: on the floor in the bathroom, otherwise on the ceilings.
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WilderSueden2 Oct 2023 19:30eenuep1 schrieb:
I had naively thought that it might charge around noon and then I could use the electricity from the midday sun in the evening (accounting for losses). That basically works like that, but: In practice, the sun is low in the sky during winter, the days are short, and it’s often overcast. So, not much energy comes in around noon, and at the same time your heating is running and consuming part of it.
If we take RotorMotor’s example, your storage can only hold 50 kWh to begin with.
Thank you again for the extensive feedback. As a layperson who is "finally" planning to carry out my own building project, it has been extremely helpful.
I have now had the first sales meeting and received the detailed scope of work along with the U-values for the wall and window surfaces. I initially estimated the U-values for the roof and floor based on the wall construction and the minimum KfW requirements. In particular, I found that even with 120 cm (47 inches) of concrete for the floor, I only achieved a U-value of 0.27 instead of the required 0.25.
My overall calculation resulted in an energy demand of 5000 kWh/year for the "KfW 40" house with 81 m² (872 ft²) of living area. It is possible that I made a mistake somewhere. If anyone with more expertise wants to take a detailed look at my calculations, here they are:
Based on this data, I have now requested price quotes for an air-to-water heat pump and an air-source heat pump, stating that the “simple” heating system is a deal-breaker for me. The air-to-water heat pump would also offer savings for domestic hot water preparation. Both heat pumps would save me the challenge of constantly trying to maintain the “just comfortable” temperature. Especially working from home, I notice that after about 4 hours of sitting, I start to feel cold.
I have now had the first sales meeting and received the detailed scope of work along with the U-values for the wall and window surfaces. I initially estimated the U-values for the roof and floor based on the wall construction and the minimum KfW requirements. In particular, I found that even with 120 cm (47 inches) of concrete for the floor, I only achieved a U-value of 0.27 instead of the required 0.25.
My overall calculation resulted in an energy demand of 5000 kWh/year for the "KfW 40" house with 81 m² (872 ft²) of living area. It is possible that I made a mistake somewhere. If anyone with more expertise wants to take a detailed look at my calculations, here they are:
- Exterior walls including openings: 46 m (151 ft), assumed height: 2.5 m (8.2 ft)
- Total exterior wall area: 115 m² (1238 ft²)
- Window and front door details:
- 6.7 m (22 ft) are front door/floor-to-ceiling windows (assumed height: 2.1 m (6.9 ft))
- 1.6 m (5.2 ft) window height: 1 m (3.3 ft)
- 1.6 m (5.2 ft) window height: 0.7 m (2.3 ft)
- Total window/front door area: 15.72 m² (169 ft²)
- Exterior wall area without openings: 99.28 m² (1069 ft²)
- U-values:
- Walls: 0.13 W/m²K
- Windows/front door: 0.6 W/m²K
- Roof: 0.104 W/m²K (estimated from construction)
- Floor: 0.25 W/m²K (according to KfW 40 minimum requirement)
- Temperature difference: 20.5 K (22°C (72°F) indoor, 1.5°C (35°F) outdoor in winter, estimated for northern Bavaria)
- Heat loads:
- Walls: 262.82 W
- Openings (windows/doors): 193.3 W
- Roof: 232.284 W
- Floor: 562.125 W
- Total heat load without ventilation: 1250.53 W or kWh/day
- House footprint (L-shaped): 109.5 m² (1178 ft²)
- Ventilation heat loss without heat recovery (heat recovery): 1250.53 W (equal to total heat load; possibly a wrong assumption?)
- Using a decentralized ventilation system with 80% heat recovery: heat loss 250.1 W
- Total heat load including heat recovery: 1500.635 W
- Heating period (Oct-Apr): 210 days at 16 hours of heating per day, resulting in an energy demand of 5042.1 kWh.
Based on this data, I have now requested price quotes for an air-to-water heat pump and an air-source heat pump, stating that the “simple” heating system is a deal-breaker for me. The air-to-water heat pump would also offer savings for domestic hot water preparation. Both heat pumps would save me the challenge of constantly trying to maintain the “just comfortable” temperature. Especially working from home, I notice that after about 4 hours of sitting, I start to feel cold.
I believe the seller’s information was: approximately 3,000 kWh heating energy demand. However, this might include factors like 20.5°C (69°F), different outdoor temperatures (they are further south, where it is warmer), only 8 hours at home, etc.
That said, I probably made a calculation error above: being at home for 16 hours doesn’t mean double the heating energy demand, rather about 10–20% more energy?
That said, I probably made a calculation error above: being at home for 16 hours doesn’t mean double the heating energy demand, rather about 10–20% more energy?
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