ᐅ Experience with KfW55 solid construction + gas heating + 5 sqm solar panels
Created on: 1 May 2017 20:11
N
Nutshell
Hello everyone,
I wanted to share my experience with my Kfw55 new build from 2013/2014.
We live at just under 300 meters (985 feet) above sea level and built with solid construction.
The house has a rather simple shape.
A standard gable roof without dormers or skylights (upstairs there is one window per room on the gable side).
Styrofoam was not an option for various reasons, so we chose the rather expensive monolithic plan blocks with a lambda value of 0.08.
Size: 111 sqm (1195 sq ft) living space.
Exterior wall: 36.5 cm (14 inches) Ytong blocks // Lambda 0.2 W/m²K
Floor slab insulated with XPS // 0.21 W/m²K
Ceiling on the upper floor has a sloping roof with 24 cm (9.5 inches) // ceiling insulated with 34 cm (13.5 inches) mineral wool // 0.12 W/m²K
Doors and windows are triple-glazed and gas-filled // overall including frames 1.05 W/m²K
Heat generator:
Gas condensing boiler: Viessmann Vitodens 300
System control: Vitotronic 200
Solar thermal system 5 sqm (54 sq ft): Vitosol 200-F
Solar storage tank: Vitocell 100-W 300 liters (79 gallons)
The solar system is used only for domestic hot water (no heating support).
Our gas boiler is switched off from May to September for heating and still consumes about 100 kWh of gas over the summer.
In winter, the solar system barely manages to heat the hot water (2 people), so it usually runs on gas.
Ventilation is handled by decentralized Lunos e² units with 90% heat recovery.
Over 12 months, we had a gas consumption of 8000 kWh.
That currently corresponds to heating costs of about €28 for the house.
We have the temperature set to 23°C (73°F) throughout the house.
(Electricity costs are about €39.)
Overall, we are very satisfied. Of course, the energy certificate shows lower consumption than we actually have.
There is no night setback, and nothing ever switches off or regulates down.
Maybe someone has had similar experiences?
Personally, I’m relaxed about heating costs; even if energy prices rise, paying twice as much would still be less than I used to pay in my smaller 80 sqm (860 sq ft) apartment built in 1990.
I wanted to share my experience with my Kfw55 new build from 2013/2014.
We live at just under 300 meters (985 feet) above sea level and built with solid construction.
The house has a rather simple shape.
A standard gable roof without dormers or skylights (upstairs there is one window per room on the gable side).
Styrofoam was not an option for various reasons, so we chose the rather expensive monolithic plan blocks with a lambda value of 0.08.
Size: 111 sqm (1195 sq ft) living space.
Exterior wall: 36.5 cm (14 inches) Ytong blocks // Lambda 0.2 W/m²K
Floor slab insulated with XPS // 0.21 W/m²K
Ceiling on the upper floor has a sloping roof with 24 cm (9.5 inches) // ceiling insulated with 34 cm (13.5 inches) mineral wool // 0.12 W/m²K
Doors and windows are triple-glazed and gas-filled // overall including frames 1.05 W/m²K
Heat generator:
Gas condensing boiler: Viessmann Vitodens 300
System control: Vitotronic 200
Solar thermal system 5 sqm (54 sq ft): Vitosol 200-F
Solar storage tank: Vitocell 100-W 300 liters (79 gallons)
The solar system is used only for domestic hot water (no heating support).
Our gas boiler is switched off from May to September for heating and still consumes about 100 kWh of gas over the summer.
In winter, the solar system barely manages to heat the hot water (2 people), so it usually runs on gas.
Ventilation is handled by decentralized Lunos e² units with 90% heat recovery.
Over 12 months, we had a gas consumption of 8000 kWh.
That currently corresponds to heating costs of about €28 for the house.
We have the temperature set to 23°C (73°F) throughout the house.
(Electricity costs are about €39.)
Overall, we are very satisfied. Of course, the energy certificate shows lower consumption than we actually have.
There is no night setback, and nothing ever switches off or regulates down.
Maybe someone has had similar experiences?
Personally, I’m relaxed about heating costs; even if energy prices rise, paying twice as much would still be less than I used to pay in my smaller 80 sqm (860 sq ft) apartment built in 1990.
Deliverer schrieb:
The idea itself is good – but the units are chosen too large.
I would go with 2 kW everywhere (if those are available; often 2.5 kW is the minimum), and if you want to be on the safe side, use 2.5 kW in the living room.
By the way, single-split systems are cheaper (shorter pipe runs, less expensive units), more comfortable (because they cycle less frequently), and more redundant than multi-split systems. You just have to find a place for all the outdoor units... Maybe a mix? Interesting approach with the 2.00 kW units.
But according to the datasheet, are they less efficient?
D
Deliverer23 May 2022 19:14I am not familiar with the datasheet, and maybe the specific case of these models needs to be considered. Generally, however, it’s like with heating systems: as large as necessary, as small as possible.
And a 12 m² (130 sq ft) room in a KfW-standard house has a heating load of about 0.5 kW. The “cooling load” tends to be lower because the temperature difference is smaller than when heating.
The larger the unit, the more it cycles on and off. This is inefficient and causes constantly changing noise levels and fluctuating humidity.
Therefore, in a reasonably well-insulated house, I would always recommend installing a single indoor unit with around 3.5 kW capacity at the highest or most central point of the house. It then runs all day (using photovoltaic power) at low output during peak summer, 24 hours a day. With all doors open, the house stays comfortable and, above all, dehumidified. Yes, this setup won’t maintain 20°C (68°F) in the bedroom, but that might not even be necessary.
PS: Wow, I just noticed how old this thread is and what I said back then. 😳 My opinion on insulation standards has changed since. Now, I would only build passive houses. At that time, I didn’t fully understand the financial benefits of omitting underfloor heating. Small wind turbines are still cool but probably almost never economical. That’s why I have invested in a lot more photovoltaic capacity. Anything under 50 kWp is considered small. After that, it’s “medium.” 😎
And a 12 m² (130 sq ft) room in a KfW-standard house has a heating load of about 0.5 kW. The “cooling load” tends to be lower because the temperature difference is smaller than when heating.
The larger the unit, the more it cycles on and off. This is inefficient and causes constantly changing noise levels and fluctuating humidity.
Therefore, in a reasonably well-insulated house, I would always recommend installing a single indoor unit with around 3.5 kW capacity at the highest or most central point of the house. It then runs all day (using photovoltaic power) at low output during peak summer, 24 hours a day. With all doors open, the house stays comfortable and, above all, dehumidified. Yes, this setup won’t maintain 20°C (68°F) in the bedroom, but that might not even be necessary.
PS: Wow, I just noticed how old this thread is and what I said back then. 😳 My opinion on insulation standards has changed since. Now, I would only build passive houses. At that time, I didn’t fully understand the financial benefits of omitting underfloor heating. Small wind turbines are still cool but probably almost never economical. That’s why I have invested in a lot more photovoltaic capacity. Anything under 50 kWp is considered small. After that, it’s “medium.” 😎
Deliverer schrieb:
I’m not familiar with the data sheet, and maybe the specific case of these models needs to be considered. But in general, it’s like with heating systems: as large as necessary, as small as possible.
And a 12 m² (130 sq ft) room in a KfW-certified house typically has a heating load of around 0.5 kW. The “cooling load” tends to be lower since the temperature difference is smaller than when heating.
The larger the unit, the more it cycles on and off. This is inefficient and causes constantly changing noise levels and fluctuating humidity.
Therefore, in a reasonably well-insulated house, I would always recommend installing a single indoor unit with about 3.5 kW capacity at the highest or most central point of the house. In the height of summer, it can run continuously at low power using solar electricity. With all doors open, the house stays comfortable and, importantly, dehumidified. Yes, you won’t get 20°C (68°F) in the bedroom with this, but maybe that’s not even necessary.
PS: Wow, I just noticed how old this thread is and what I said back then. 😳 My view on insulation standards has changed since. Nowadays, I would build only to passive house standards. Back then, I didn’t fully appreciate the financial benefits of skipping underfloor heating. Small wind turbines are still cool but probably almost never cost-effective. That’s why I switched to much more photovoltaic capacity. Anything under 50 kWp is small; after that, it’s “medium.” 😎 Thanks for the idea of a single unit.
Yes, insulation standards really matter. Back then, KfW55 was already very good and not common everywhere 🙂
I paid a flat rate of €28 for heating for years, so investing more money in insulation didn’t make sense at the time. You couldn’t have predicted the war!
D
Deliverer23 May 2022 22:22No. And you still have a decent house. I am doing this with a single device (3.5 kW on 140 m² (1,507 ft²)) in a partially renovated old building. It works well and is cost-effective.
Update,
The air conditioning system has now been in use for 12 months.
The house was delivered turnkey in summer 2014 with a gas consumption of 1692 kWh due to screed drying.
How have gas prices developed?
2017–2018: 4.74 cents / kWh
2022–2023: 16.51 cents / kWh
2023–2024: 9.7 cents / kWh
How have electricity prices developed?
2017–2018: 22.91 cents / kWh
2022–2023: 29.71 cents / kWh
2023–2024: 26.60 cents / kWh
Installed is a gas heating system, as well as domestic hot water production with a 300-liter (79-gallon) solar storage tank + 2 collectors for hot water without heating support.
Gas meter readings converted to kWh (1 m³ = 11.28 kWh)
Summer 2014: 1692 kWh
Summer 2015: 11426 kWh = Consumption 9734
Summer 2016: 19356 kWh = Consumption 7930
Summer 2017: 27636 kWh = Consumption 8280
Summer 2018: 34979 kWh = Consumption 7343
Summer 2019: 41758 kWh = Consumption 6779
Summer 2020: 48300 kWh = Consumption 6542
Summer 2021: 56490 kWh = Consumption 8190
Summer 2022: 63089 kWh = Consumption 6599
Summer 2023: 65875 kWh = Consumption 2786
In summer 2022, the mentioned air-to-air heat pump was added.
Electricity meter
Summer 2017: 7,123 kWh
Summer 2018: 9,023 kWh = Consumption 1,900
Summer 2019: 11,010 kWh = Consumption 1,987
Summer 2020: 13,266 kWh = Consumption 2,256
Summer 2021: 15,721 kWh = Consumption 2,455
Summer 2022: 18,265 kWh = Consumption 2,544
Summer 2023: 22,512 kWh = Consumption 4,247
The Viessmann heating controller shows the following regarding solar systems – domestic hot water:
39,247 kWh produced via solar collectors, period 2014–2023.
The following consumption figures are according to the heat pump app:
Jul 22 consumption 21 kWh
Aug 22 consumption 62 kWh
Sep 22 consumption 33 kWh
Oct 22 consumption 90 kWh
Nov 22 consumption 243 kWh
Dec 22 consumption 385 kWh
Jan 23 consumption 197 kWh
Feb 23 consumption 332 kWh
Mar 23 consumption 87 kWh
Apr 23 consumption 189 kWh
May 23 consumption 45 kWh
Jun 23 consumption 46 kWh
12 months total: 1,730 kWh
Heating over the last 12 months:
I tried to mainly heat using the heat pump. The heat pump was switched off only at very low temperatures, and the gas underfloor heating was activated.
Gas consumption for this heating support was 1,617 kWh.
Domestic hot water over the last 12 months:
In November and April, there was little contribution from the solar system, but also no gas heating.
Gas consumption for domestic hot water was 115 kWh and 120 kWh respectively.
In December, January, February, and March, the gas heating was on, but I tried to estimate gas consumption for hot water.
In May, with more sunshine, consumption dropped to 70 kWh. In June, 18 kWh of gas was used for hot water, now again completely supplied via collectors.
I estimate a total gas consumption of 1,170 kWh for hot water from summer 2022 to summer 2023.
Normally, we had an overall annual gas consumption of around 6,500 to 8,300 kWh.
Winter 2022–2023 was relatively cold compared to usual, so let's estimate about 7,700 kWh.
7,700 - 1,617 = 6,083 kWh, which is the estimated heating output provided by the heat pump.
According to the app, the heat pump consumed 1,556 kWh for heating.
The resulting "COP" value based on these assumptions is therefore 6,083 / 1,556 = 3.9
Thoughts on the achieved COP value:
Compared to the gas heating, the heat pump was activated on demand room-by-room.
Gas heating was always run 24/7 in the past at a setpoint temperature of 23°C (73°F).
Additionally, at least two indoor units were programmed to operate simultaneously when useful, as I was told that higher load increases efficiency.
Another factor that should positively influence the COP value is the fact that the heat pump was turned off at low temperatures and the gas heating switched on instead.
What’s next?
Gas prices have fallen significantly again: -41.2%
For the next winter, my wife convinced me to set a base temperature on the gas underfloor heating and use the air conditioning system on demand to quickly raise room temperatures.
I’m not entirely convinced if this makes sense. The gas heating will likely cycle more frequently and run less efficiently. Whether this will really save energy remains to be seen.
The air conditioning system has now been in use for 12 months.
The house was delivered turnkey in summer 2014 with a gas consumption of 1692 kWh due to screed drying.
How have gas prices developed?
2017–2018: 4.74 cents / kWh
2022–2023: 16.51 cents / kWh
2023–2024: 9.7 cents / kWh
How have electricity prices developed?
2017–2018: 22.91 cents / kWh
2022–2023: 29.71 cents / kWh
2023–2024: 26.60 cents / kWh
Installed is a gas heating system, as well as domestic hot water production with a 300-liter (79-gallon) solar storage tank + 2 collectors for hot water without heating support.
Gas meter readings converted to kWh (1 m³ = 11.28 kWh)
Summer 2014: 1692 kWh
Summer 2015: 11426 kWh = Consumption 9734
Summer 2016: 19356 kWh = Consumption 7930
Summer 2017: 27636 kWh = Consumption 8280
Summer 2018: 34979 kWh = Consumption 7343
Summer 2019: 41758 kWh = Consumption 6779
Summer 2020: 48300 kWh = Consumption 6542
Summer 2021: 56490 kWh = Consumption 8190
Summer 2022: 63089 kWh = Consumption 6599
Summer 2023: 65875 kWh = Consumption 2786
In summer 2022, the mentioned air-to-air heat pump was added.
Electricity meter
Summer 2017: 7,123 kWh
Summer 2018: 9,023 kWh = Consumption 1,900
Summer 2019: 11,010 kWh = Consumption 1,987
Summer 2020: 13,266 kWh = Consumption 2,256
Summer 2021: 15,721 kWh = Consumption 2,455
Summer 2022: 18,265 kWh = Consumption 2,544
Summer 2023: 22,512 kWh = Consumption 4,247
The Viessmann heating controller shows the following regarding solar systems – domestic hot water:
39,247 kWh produced via solar collectors, period 2014–2023.
The following consumption figures are according to the heat pump app:
Jul 22 consumption 21 kWh
Aug 22 consumption 62 kWh
Sep 22 consumption 33 kWh
Oct 22 consumption 90 kWh
Nov 22 consumption 243 kWh
Dec 22 consumption 385 kWh
Jan 23 consumption 197 kWh
Feb 23 consumption 332 kWh
Mar 23 consumption 87 kWh
Apr 23 consumption 189 kWh
May 23 consumption 45 kWh
Jun 23 consumption 46 kWh
12 months total: 1,730 kWh
Heating over the last 12 months:
I tried to mainly heat using the heat pump. The heat pump was switched off only at very low temperatures, and the gas underfloor heating was activated.
Gas consumption for this heating support was 1,617 kWh.
Domestic hot water over the last 12 months:
In November and April, there was little contribution from the solar system, but also no gas heating.
Gas consumption for domestic hot water was 115 kWh and 120 kWh respectively.
In December, January, February, and March, the gas heating was on, but I tried to estimate gas consumption for hot water.
In May, with more sunshine, consumption dropped to 70 kWh. In June, 18 kWh of gas was used for hot water, now again completely supplied via collectors.
I estimate a total gas consumption of 1,170 kWh for hot water from summer 2022 to summer 2023.
Normally, we had an overall annual gas consumption of around 6,500 to 8,300 kWh.
Winter 2022–2023 was relatively cold compared to usual, so let's estimate about 7,700 kWh.
7,700 - 1,617 = 6,083 kWh, which is the estimated heating output provided by the heat pump.
According to the app, the heat pump consumed 1,556 kWh for heating.
The resulting "COP" value based on these assumptions is therefore 6,083 / 1,556 = 3.9
Thoughts on the achieved COP value:
Compared to the gas heating, the heat pump was activated on demand room-by-room.
Gas heating was always run 24/7 in the past at a setpoint temperature of 23°C (73°F).
Additionally, at least two indoor units were programmed to operate simultaneously when useful, as I was told that higher load increases efficiency.
Another factor that should positively influence the COP value is the fact that the heat pump was turned off at low temperatures and the gas heating switched on instead.
What’s next?
Gas prices have fallen significantly again: -41.2%
For the next winter, my wife convinced me to set a base temperature on the gas underfloor heating and use the air conditioning system on demand to quickly raise room temperatures.
I’m not entirely convinced if this makes sense. The gas heating will likely cycle more frequently and run less efficiently. Whether this will really save energy remains to be seen.
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