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?
eenuep1 schrieb:
My overall calculation resulted in an energy demand of 5000 kWh/year for the "KfW 40" house with 81 sqm (870 sq ft) of living space. It is possible that I made a miscalculation somewhere. You can’t calculate it like that, or some assumptions are too optimistic.
For transmission losses, always use the exterior surface area.
Add a standard 0.05 W/m²K (0.016 Btu/(h·ft²·°F)) to the wall area for thermal bridges.
For windows, only use the glass U-value of 0.6 (Uw). With the frame, it is usually around 0.8–0.9 (Uf).
For the overall calculation, don’t use average temperature and days.
Look up the heating degree days for your region, assuming 22°C (72°F) indoor temperature and 15°C (59°F) heating limit temperature.
Use these values also for ventilation losses and calculate with an air change rate of 0.4/h–0.5/h for the total volume.
With heat recovery, you can deduct 50% of that. Although the system may have 90% efficiency, you will still have some leakage.
I just did this calculation for another house here recently. I just can’t find it right now.
R
RotorMotor23 Oct 2023 10:09It would be great to share some information from the user here! 🙂