ᐅ Electric surface heating (Thermoheld) in a KfW 40 bungalow with 80 sqm?

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:

• 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?

Quite a lot of text. ;-)

eenuep1 schrieb:

• Photovoltaic system: 45 sqm (484 sq ft) area, approximately 9.1 kWp
• Electricity generation during the heating season (Oct-Apr): about 4,600 kWh (possibly overestimated?)

According to PVGIS, it's more like 2,000 kWh instead of 4,600.
But it also depends somewhat on location and orientation.
Especially with an 18-degree roof pitch, you can expect very little production in winter.

eenuep1 schrieb:

That means I could feed about 2,500 kWh into the grid during the heating season, which at a feed-in tariff of 8.2 ct/kWh would amount to a

During the heating season, as noted above, there is so little production from the roof that most of it is already consumed on site.
What remains goes to the heating system.

eenuep1 schrieb:

• This results in an estimated total heating energy demand of about 4,000–4,500 kWh per year.

Whether that matches your specific house needs to be calculated.

But assuming this:
You can expect to get a maximum of about 500 kWh from the photovoltaic system for heating and will need to buy the remaining electricity.
So with an air-to-water heat pump, you still need to buy about 500 kWh, and with direct electric heating about 3,500 kWh.
At 30 cents per kWh, that’s €150 versus approximately €1,000.

So, €850 per year higher electricity costs.
With €15,000 higher investment costs, that comes to almost 18 years.
For a house, I don’t find that to be an unreasonably long payback period.

I tend to feel colder at the same temperatures in a solid stone or concrete house compared to a timber-framed house. If I were you, I would try to reduce the primary energy demand even further (Passive House).

My brother is also planning a very small house as a retirement home for himself and his wife. The big cost item “plumbing” is actually a “nuisance” in such a small house, and he is considering a combination of infrared heating, a hot water tank, and a wood stove as a solution.

An alternative would be a small air-to-air heat pump or a split air conditioning system.

That’s why I’m following this discussion here with great interest ;-)

eenuep1 schrieb:

Since I work from home and am at home all day, every day, and because I feel chilly at 22°C (72°F) in a solid construction house, I considered installing an air-to-water heat pump with a hydronic underfloor heating system.

A solid construction house and feeling cold at 22°C (72°F) are unrelated. The temperature feels the same in a timber frame bungalow.

Honestly, for such a small and efficient house, I wouldn’t install a traditional heat pump with underfloor heating at all. I would go straight for a split air conditioning / air-to-air heat pump system. The heating costs are so low (30–35€ per month – excluding photovoltaic electricity credit) that, in my opinion, the investment in a heat pump plus underfloor heating is not worthwhile. Especially since, with the solution I suggested, you can also cool for free in summer (thanks to photovoltaic).

KarstenausNRW schrieb:

To be honest, for such a small and efficient house, I wouldn’t install a traditional heat pump with underfloor heating. I would go straight for a split AC / air-to-air heat pump.
The heating costs are so low (30-35€ per month – without factoring in photovoltaic electricity) that, in my opinion, the investment in a heat pump plus underfloor heating does not pay off. Especially since, with the solution I suggested, you can also cool for free in summer (thanks to photovoltaics).

First, you should clarify the standard outdoor temperature—I’m assuming -12°C (10°F), which is not ideal for an air-to-air heat pump.
I would consider a combination of both systems, but not in all rooms, for example in the office and living room, so that these can also be cooled.
Whether the building is wood-frame or solid construction doesn’t matter much; the heating method is more important. Radiant heat, such as underfloor heating or infrared, has the advantage here.

@RotorMotor: Thanks for correcting the expected photovoltaic yield for winter. The orientation is south, location is Upper Franconia (so the yield is slightly lower). I will update my calculation as soon as I have the heating energy demand from the manufacturer.

@i_b_n_a_n: Interesting, if your brother progresses faster than I do, I’d also be interested from the other side ;-)

@KarstenausNRW: Interesting, I hadn’t considered an air-to-air heat pump / split air conditioning system before; I always thought it would dry out the air, like air conditioners when traveling. But presumably, building technology has improved? How do you arrive at the €30-35/m² heating costs? Regarding summer cooling, I noticed with the manufacturer’s tiny house variant (47 m² (507 square feet), two-story) that the upper floor was unbearable even at the end of September. However, as it was a show home, there was no shading or ventilation, and it had large window areas. For a bungalow, there is relatively more roof and floor area but fewer south-facing windows. I hope it won’t get too warm in summer. The insulation is wood fiber, which is apparently better than mineral wool regarding thermal phase shift.

@Radfahrer: Interesting point about outdoor temperature. I currently live in a renovated old building with water-based underfloor heating in the bathroom (heats up quickly) and low-temperature air heating (along the baseboards) via a gas boiler – but I really can’t stand the air heating. It unfortunately doesn’t warm up the apartment quickly enough, so even when heating from 6 AM to 12 PM, I don’t get above 21°C (70°F) in winter. In contrast, a 400W infrared heating panel on the wall heats the home office temperature quickly enough.