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?
K
KarstenausNRW1 Oct 2023 19:05Radfahrer schrieb:
First, you should clarify the standard outdoor temperature—I’m assuming around -12°C (10°F)—which isn’t ideal for an air-to-air heat pump. Yes, the efficiency isn’t outstanding. But considering the size of the house and its overall efficiency, it’s not a big issue. The extra investment of several tens of thousands of euros for a heat pump with underfloor heating will never pay off.
eenuep1 schrieb:
Interesting, I hadn’t thought about air-to-air heat pumps / split air conditioning systems before; I always assumed they dry out the air, like air conditioners when traveling. But I guess building technology has advanced since then? How do you arrive at heating costs of €30–35 per square meter? The house size combined with KfW 40 efficiency standard results in electricity consumption somewhere between 1,500 and a maximum of 2,000 kWh. At the current electricity price of around €0.25 per kWh, that’s between €375 and a maximum of €500, not considering any photovoltaic yields.
That’s why I argue that the choice of heating technology doesn’t make much financial difference. A 20% increase in consumption—around €100 per year—adds up to only €2,000 over 20 years (without factoring in electricity price increases). That’s bearable if the heat pump with underfloor heating costs roughly €20,000 more than an air-to-air heat pump.
You also mentioned that water-based underfloor heating heats up quickly. That’s a misconception. Water-based underfloor heating embedded in screed is very slow to respond. You won’t notice changes in room temperature until several hours later or overnight, especially if your supply water temperature maxes out at 30°C (86°F) in the coldest winter months. It is the slowest heating system you can choose. In a KfW 40 house, however, you’d only lose about 1°C (2°F) overnight if the heating fails.
KarstenausNRW schrieb:
House size combined with KfW 40 standard will result in an electricity consumption somewhere between 1,500 and a maximum of 2,000 kWh. With current electricity prices of around 25 cents per kWh, that equals between €375 and a maximum of €500. Yields from photovoltaic systems are not taken into account.Thank you for the calculation details!
I now need to research the costs of an air-to-water heat pump and, if possible, an air-to-air heat pump, as well as the expected energy demand.
As mentioned, the supplier usually installs only their own electric surface heating systems (see image) and typically uses hydronic underfloor heating only if district heating is available (possibly due to the low energy demand).
Their heating system would be included in the price, so I need to consider whether it makes sense to pay extra just for an air-to-air heat pump, especially since the system may not be much more efficient.
Regardless, I have no experience with air-to-air heat pumps or split air conditioning units, so I cannot assess whether I would find the system comfortable in terms of indoor air quality (e.g., dryness or drafts). I am rather sensitive in both respects (mucous membranes and eyes).
In terms of noise, the infrared/electric panel heating would of course be an advantage.
Regardless, I have no experience with air-to-air heat pumps or split air conditioning units, so I cannot assess whether I would find the system comfortable in terms of indoor air quality (e.g., dryness or drafts). I am rather sensitive in both respects (mucous membranes and eyes).
In terms of noise, the infrared/electric panel heating would of course be an advantage.
I just had another thought: besides options A (air-to-water heat pump) and B (air-to-air heat pump), there is also
C (electric underfloor heating, included in the price) but with a larger photovoltaic system:
- 3,000 kWh energy demand during the heating season (2,000 kWh heating energy demand + 1,000 kWh usable energy demand)
- South-facing roof area of the house: 58-77 sqm (625-830 sq ft), resulting in 2,500-3,400 kWh yield during the heating season
- Costs according to the home builder for photovoltaic + storage with 11.7 and 15.6 kWp photovoltaic systems: 22,500 and 27,000 euros respectively
That means instead of choosing an upgrade to a different heating system, I could simply pay the approximately 7,000 euro additional cost for the 9.1 kWp system and cover my entire heating energy demand myself, without issues related to potentially dry air or drafts (air-to-air heat pump?), and without the noise emissions of the air-to-water heat pump, for a total smaller additional investment.
As is well known, it always makes sense to stay “close” to the builder’s standard equipment; anything else is charged at a premium since the builder cannot spread planning costs across a large number of customers (modular construction).
C (electric underfloor heating, included in the price) but with a larger photovoltaic system:
- 3,000 kWh energy demand during the heating season (2,000 kWh heating energy demand + 1,000 kWh usable energy demand)
- South-facing roof area of the house: 58-77 sqm (625-830 sq ft), resulting in 2,500-3,400 kWh yield during the heating season
- Costs according to the home builder for photovoltaic + storage with 11.7 and 15.6 kWp photovoltaic systems: 22,500 and 27,000 euros respectively
That means instead of choosing an upgrade to a different heating system, I could simply pay the approximately 7,000 euro additional cost for the 9.1 kWp system and cover my entire heating energy demand myself, without issues related to potentially dry air or drafts (air-to-air heat pump?), and without the noise emissions of the air-to-water heat pump, for a total smaller additional investment.
As is well known, it always makes sense to stay “close” to the builder’s standard equipment; anything else is charged at a premium since the builder cannot spread planning costs across a large number of customers (modular construction).
R
RotorMotor1 Oct 2023 22:17A larger photovoltaic system is often a good idea. However, the fundamental issue remains that the heating usually operates when the sun is not shining (well). You can also simulate this yourself using PVGIS. So even with a large photovoltaic system, a heat pump can still offer advantages!
RotorMotor schrieb:
However, the basic problem remains that the heating usually runs when the sun isn’t shining (well).
You can also simulate this with PVGIS.
So even with a large photovoltaic system, a heat pump can still provide advantages!Thanks again for the reminder about PVGIS; it helped visualize the low energy production in winter. According to PVGIS, a photovoltaic system of 11.7–15.6 kWp should generate 1,440–1,920 kWh for my location + 18° tilt during the period from November to February.
Heating energy demand for November to February, KfW 40 standard: about 1,400 of 2,000 kWh, useful energy demand for November to February: 500 kWh.
That means I need about 1,900 kWh energy demand and should therefore consider a 15.7 kWp photovoltaic system with battery storage (27,000 euros).
And after one year, once I have real data, I could still install an air conditioning system (air-to-air heat pump / split AC) later on, in case my photovoltaic yield isn’t enough to cover both heating and useful energy, right?
The good thing is: with this floor plan (see attachment), an air conditioning system would probably be sufficient to heat the living area and bathroom (with the door open), I think—plus I have electric ceiling panel heating in the bathroom and everywhere else as a backup.
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