Hello
We are building a KfW 70 house. We have a lot of window area, so the calculations showed that we need to take additional measures to meet the KfW 70 standard.
We decided against a ventilation system, but instead opted for extra panels on the roof, not only to supply hot water via collectors but also to support heating.
However, the 300-liter (79 gallons) tank has now been replaced by a 750-liter (198 gallons) water storage tank. That seems huuuuge! Does the tank’s capacity affect the calculations? I haven’t been able to reach the engineering firm yet to ask, so I wanted to get some input from the forum first.
We are building a KfW 70 house. We have a lot of window area, so the calculations showed that we need to take additional measures to meet the KfW 70 standard.
We decided against a ventilation system, but instead opted for extra panels on the roof, not only to supply hot water via collectors but also to support heating.
However, the 300-liter (79 gallons) tank has now been replaced by a 750-liter (198 gallons) water storage tank. That seems huuuuge! Does the tank’s capacity affect the calculations? I haven’t been able to reach the engineering firm yet to ask, so I wanted to get some input from the forum first.
Additionally, a ventilation system and solar panels to achieve KFW70? Something seems off from the start, doesn’t it? Building without insulation?
And 750 liters (198 gallons) sounds like a lot at first, but it can be reasonable if many solar panels are installed on the roof. I think it’s probably for heating support.
For domestic hot water only, 300 liters (79 gallons) should be completely sufficient.
Who is planning the energy design of your house?
And 750 liters (198 gallons) sounds like a lot at first, but it can be reasonable if many solar panels are installed on the roof. I think it’s probably for heating support.
For domestic hot water only, 300 liters (79 gallons) should be completely sufficient.
Who is planning the energy design of your house?
I think I may have expressed myself unclearly...
Before starting the floor plan design, we planned for:
4.7 sqm (50.6 sq ft) solar collectors on the roof and a 300-liter (79.3-gallon) tank
8 cm (3.1 inches) of sub-slab insulation using Jackodur KF 500
Triple-glazed window glass with a U-value of 0.6 W/m²*K
Windows of the 88 Plus version (88 mm (3.5 inches) installation depth, 7-chamber system)
ETICS (External Thermal Insulation Composite System)
During the planning process, many things changed: the total window area increased significantly and a bay window was added. As a result, the original measures were no longer sufficient, and we were unable to meet the KFW70 standard anymore, so additional measures had to be taken.
According to the engineering firm, the simulation showed that we would approximately meet the KFW70 standard again if the solar collectors on the roof were used not only for hot water generation but also to support the heating system. The calculations showed that the collector area would need to be increased to 11.2 sqm (120.6 sq ft), and heating support would have to be included.
What I don’t understand is this: the 300-liter (79.3-gallon) tank was replaced with a 750-liter (198.1-gallon) tank. The heating installer from our builder argued that this tank size is the standard for this system (11.2 sqm (120.6 sq ft) of solar collectors + hot water generation + heating support). But I don’t get it. Maybe I don’t fully understand the technology. Why is such a large hot water storage tank necessary?
Before starting the floor plan design, we planned for:
4.7 sqm (50.6 sq ft) solar collectors on the roof and a 300-liter (79.3-gallon) tank
8 cm (3.1 inches) of sub-slab insulation using Jackodur KF 500
Triple-glazed window glass with a U-value of 0.6 W/m²*K
Windows of the 88 Plus version (88 mm (3.5 inches) installation depth, 7-chamber system)
ETICS (External Thermal Insulation Composite System)
During the planning process, many things changed: the total window area increased significantly and a bay window was added. As a result, the original measures were no longer sufficient, and we were unable to meet the KFW70 standard anymore, so additional measures had to be taken.
According to the engineering firm, the simulation showed that we would approximately meet the KFW70 standard again if the solar collectors on the roof were used not only for hot water generation but also to support the heating system. The calculations showed that the collector area would need to be increased to 11.2 sqm (120.6 sq ft), and heating support would have to be included.
What I don’t understand is this: the 300-liter (79.3-gallon) tank was replaced with a 750-liter (198.1-gallon) tank. The heating installer from our builder argued that this tank size is the standard for this system (11.2 sqm (120.6 sq ft) of solar collectors + hot water generation + heating support). But I don’t get it. Maybe I don’t fully understand the technology. Why is such a large hot water storage tank necessary?
To ensure effective heating.
Depending on the roof orientation, you have either a west/east-facing roof or a south-facing roof. So, the sun doesn’t shine on it all day.
In our case, it’s west-facing; when I shower in the morning, I use the hot water from the previous evening. If everyone showers at home, no hot water can be used for heating during the day. You simply need a larger hot water storage tank. Also, the 300 liters (80 gallons) heat up very quickly—the more storage you have, the longer you can “use” the solar energy. Once the water in the tank reaches 60°C (140°F), for example, the sun may continue to shine on it for hours, but it doesn’t add any benefit.
Have you questioned the cost-effectiveness? Generally, the opinion is that better insulation is a more affordable way to meet energy efficiency standards like KfW than solar systems.
Depending on the roof orientation, you have either a west/east-facing roof or a south-facing roof. So, the sun doesn’t shine on it all day.
In our case, it’s west-facing; when I shower in the morning, I use the hot water from the previous evening. If everyone showers at home, no hot water can be used for heating during the day. You simply need a larger hot water storage tank. Also, the 300 liters (80 gallons) heat up very quickly—the more storage you have, the longer you can “use” the solar energy. Once the water in the tank reaches 60°C (140°F), for example, the sun may continue to shine on it for hours, but it doesn’t add any benefit.
Have you questioned the cost-effectiveness? Generally, the opinion is that better insulation is a more affordable way to meet energy efficiency standards like KfW than solar systems.
As mentioned, we already have additional underfloor insulation installed, so adding more insulation in this area would not improve energy efficiency. Regardless, the extra cost for the system currently planned as heating support is still reasonable. A ventilation system with heat recovery would have been more expensive, but since we were opposed to ventilation systems from the start, that was not an option for us.
Thank you very much for the feedback.
Thank you very much for the feedback.
Do you have an airtight building envelope, or do you build with a vapor-permeable design?
Everyone has their preference, but a new build without a mechanical ventilation system can really cause problems. I notice it in our home—if I forget to turn it on during the day, the air feels quite "stuffy" in the evening when I come home. I’m just saying diaper pail 🙂
With the system turned on, you barely notice anything 🙂
Everyone has their preference, but a new build without a mechanical ventilation system can really cause problems. I notice it in our home—if I forget to turn it on during the day, the air feels quite "stuffy" in the evening when I come home. I’m just saying diaper pail 🙂
With the system turned on, you barely notice anything 🙂
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