ᐅ KfW (a German development bank), heating demand, heat pumps, consumption
Created on: 14 Jan 2020 13:34
S
Specki
Hello,
we are now starting to take a closer look at the topic of house construction.
We keep encountering different opinions about whether KFW funding is worthwhile or not.
The current subsidies are indeed quite substantial—18,000 euros for KFW55 and 24,000 euros for KFW40.
In our case, we definitely want a centralized mechanical ventilation system with heat recovery and a heat pump. Gas is not an option for us, so a heat pump seems like the most sensible choice.
A photovoltaic system will definitely be installed on the roof (unless it becomes completely uneconomical due to political conditions).
Now I’m trying to find out how much heating energy I can save with a KFW-standard house. (I will only know the additional costs once we speak with the construction companies).
We are assuming a house with 145 sqm (1560 sq ft) of living space, without a basement. One full storey and an extended attic with a rather low knee wall of 50 cm (20 inches) due to a restrictive zoning plan.
Now I’m starting to do some calculations....
This results in the following heating energy requirements for 145 sqm (1560 sq ft):
This is the heating energy required to heat the house.
Now, assuming a heat pump with a seasonal performance factor (SPF) of 3.5, I calculate the following electricity consumption to heat each house:
Let’s assume electricity costs 30 cents per kWh.
(Yes, with photovoltaic you can produce electricity at a lower price, but most of the heating electricity will be needed when the PV system generates the least, so I want to be conservative here and calculate only with electricity from the grid.)
So, compared to the Energy Saving Ordinance (EnEV), KFW40 saves about 250 €/a, and KFW55 about 125 €/a.
Is this calculation correct, or is there an error somewhere?
With this, I could relatively easily calculate after how many years the higher cost for the respective KFW standard would pay off (of course, considering the subsidies first).
Is this a reasonably useful approach, or do you see any major mistakes in my reasoning or calculations?
Thanks in advance!
Best regards,
Specki
we are now starting to take a closer look at the topic of house construction.
We keep encountering different opinions about whether KFW funding is worthwhile or not.
The current subsidies are indeed quite substantial—18,000 euros for KFW55 and 24,000 euros for KFW40.
In our case, we definitely want a centralized mechanical ventilation system with heat recovery and a heat pump. Gas is not an option for us, so a heat pump seems like the most sensible choice.
A photovoltaic system will definitely be installed on the roof (unless it becomes completely uneconomical due to political conditions).
Now I’m trying to find out how much heating energy I can save with a KFW-standard house. (I will only know the additional costs once we speak with the construction companies).
We are assuming a house with 145 sqm (1560 sq ft) of living space, without a basement. One full storey and an extended attic with a rather low knee wall of 50 cm (20 inches) due to a restrictive zoning plan.
Now I’m starting to do some calculations....
| KFW Efficiency House 70 | ≤ 45 kWh/(m²·a) | |||
| KFW Efficiency House 55 | ≤ 35 kWh/(m²·a) | |||
| KFW Efficiency House 40 (Plus) | ≤ 25 kWh/(m²·a) |
This results in the following heating energy requirements for 145 sqm (1560 sq ft):
| KFW Efficiency House 70 | 6525 kWh/a | |||
| KFW Efficiency House 55 | 5075 kWh/a | |||
| KFW Efficiency House 40 (Plus) | 3625 kWh/a |
This is the heating energy required to heat the house.
Now, assuming a heat pump with a seasonal performance factor (SPF) of 3.5, I calculate the following electricity consumption to heat each house:
| KFW Efficiency House 70 | 1864 kWh/a | |||
| KFW Efficiency House 55 | 1450 kWh/a | |||
| KFW Efficiency House 40 (Plus) | 1036 kWh/a |
Let’s assume electricity costs 30 cents per kWh.
(Yes, with photovoltaic you can produce electricity at a lower price, but most of the heating electricity will be needed when the PV system generates the least, so I want to be conservative here and calculate only with electricity from the grid.)
| KFW Efficiency House 70 | 559 €/a | |||
| KFW Efficiency House 55 | 435 €/a | |||
| KFW Efficiency House 40 (Plus) | 310 €/a |
So, compared to the Energy Saving Ordinance (EnEV), KFW40 saves about 250 €/a, and KFW55 about 125 €/a.
Is this calculation correct, or is there an error somewhere?
With this, I could relatively easily calculate after how many years the higher cost for the respective KFW standard would pay off (of course, considering the subsidies first).
Is this a reasonably useful approach, or do you see any major mistakes in my reasoning or calculations?
Thanks in advance!
Best regards,
Specki
N
nordanney14 Jan 2020 14:32Specki schrieb:
How am I supposed to compare in the end whether it’s worth it.You can’t. You can only decide if you feel good about it. Maybe also with the thought that the resale value might be a bit higher (unless in 100 years everyone has a mini fusion reactor in their basement and the heat pump is just gathering dust).B
boxandroof14 Jan 2020 15:31You will only know the actual heating energy demand after moving in. However, you can still use the calculation method described to estimate the cost-effectiveness of additional insulation. For the annual performance factor, I would assume 4–4.5 for an air-to-water heat pump and 5–5.5 for geothermal: good performance figures come at no extra cost, unlike insulation. Then factor in photovoltaics covering 20–30% of your electricity needs and include the feed-in tariff costs. This changes your calculations and makes insulation less cost-effective.
Much more crucial than insulation is the planning of the technical systems. Apart from your time, this costs little to nothing. Often, you even save money by purchasing more appropriate components. This way, you can operate your energy saving ordinance house more economically than your neighbor does their passive house with poor planning.
If I were to build today, I would do the following:
- Plan the heat pump’s surface heating system very well (e.g. horizontal collector or geothermal heat pump)
- Design the roof to accommodate a large photovoltaic system
- Insulation roughly at the level of KfW 55, slightly better or worse depending on costs
- Many south-facing windows with good shading, few north-facing windows
Much more crucial than insulation is the planning of the technical systems. Apart from your time, this costs little to nothing. Often, you even save money by purchasing more appropriate components. This way, you can operate your energy saving ordinance house more economically than your neighbor does their passive house with poor planning.
If I were to build today, I would do the following:
- Plan the heat pump’s surface heating system very well (e.g. horizontal collector or geothermal heat pump)
- Design the roof to accommodate a large photovoltaic system
- Insulation roughly at the level of KfW 55, slightly better or worse depending on costs
- Many south-facing windows with good shading, few north-facing windows
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