Hello dear forum!
I am planning a new build of 140 m² (KfW 70 heating demand) and wonder if this would be feasible.
A wood-burning stove with 8 to 10 kW (coal + wood) is planned to be centrally located on the ground floor.
It will stand in a 3-meter-wide (10 feet) passageway to the 40 m² (430 square feet) living room and can optionally be separated from the hallway by a ceiling-mounted sliding door (no threshold). This setup allows flexible use of the stove’s heat, which can partially flow through the hallway upstairs and distribute throughout the entire ground floor.
Additionally, electric convector heaters will provide base heat when needed.
Hot Water:
An instantaneous water heater combined with a small pre-storage tank will preheat drinking water in winter, and when excess solar power is available, it will raise the water to the maximum flow temperature of the instantaneous heater.
KfW and the Energy Saving Ordinance 2009
are likely to pose challenges to my plan, although I would only need about 700 kWh of heating electricity from the grid, produce my own electricity, and use coal and wood with an efficiency of at least 85 percent. Perhaps you have ideas on how my plan could still work without losing control of the financial aspects.
If needed, you can find how I arrived at the numbers towards the end of my explanation.
Project:
Electric heating + wood-burning stove (for comparison only)
or
Electric heating + photovoltaic system + wood-burning stove (priority 1)
Calculations:
9000 kWh total demand
6300 kWh heating demand
700 kWh hot water heating (about 2 kWh per day yearly)
2000 kWh household electricity
Heating season approx. October to April: 180 days
(Heating demand: 6300 kWh ÷ 180 days = 35 kWh per day)
Electric heating + photovoltaic system + wood stove:
9000 kWh total consumption
minus 1350 kWh self-consumption during heating period (photovoltaic system)
minus 1000 kWh household self-consumption (photovoltaic system)
9000 kWh − 2350 kWh = 6650 kWh remaining demand
minus 1000 kWh household electricity to buy at $0.25 per kWh = $250
minus 700 kWh × $0.25 (20 days no stove use + water + buffer) = $175
minus 4950 kWh covered by stove (wood is free), coal costs = $270
Costs of about $720 per year + photovoltaic loan repayment $1200 − $300 income =
$1920 total costs − $300 income = $1620 total net costs per year
$135 per month
Gas heating + wood-burning stove
9000 kWh total consumption
2000 kWh household electricity purchased: 2000 × $0.25 = $500
7000 kWh gas × $0.08 = $560
2000 kWh wood × $0.08 = −$160
Annual tank rental = $135
Annual maintenance = $100
$1,135 per year ÷ 12 = $95 per month
For comparison: (which I do not want)
Electric heating + wood stove
9000 kWh total consumption
minus 5000 kWh by stove = coal $270
4000 kWh × $0.25 = $1000
$1270 per year ÷ 12 = $105 per month
Gas heating system €12,000 / electric heating system $4500
$7500 first to be converted into heat.
Gas heating would save $120 per year
$1200 in 10 years
$4800 in 40 years
Result
For 10 years, I pay $40 more per month than with gas and then have paid off the photovoltaic system.
10 years × 12 months × $40 additional cost = $4800
(Loan KfW for $11,000 at 1.55 percent interest)
After 10 years
I assume gas and electricity prices rise equally and keep these numbers.
Ongoing costs for electric heating + photovoltaic per year =
$720 − $300 income − $100 tax = $320 ÷ 12 = $27 per month
Ongoing costs for gas heating: $95 × 12 = $1140 per year
Ongoing costs for electric heating only: $105 × 12 = $1260 per year
Electric + photovoltaic costs $320 per year
Gas costs $820 more per year
Only electric heating costs $940 more per year
Price for electric heating and photovoltaic system: $11,000 + $4500 = $15,500
Price for gas heating with radiators and installation: $12,000
After approx. 18 years, I will start to make a profit. After 10 years, annual ongoing costs are around $820 less.
Questions:
Is an electric heating system as described above realistically and sensibly implementable?
I probably need to build a house with a heating energy demand plus hot water of 5500 kWh (exactly KfW 40).
Is this calculation correct? House with 5500 kWh × eP 2.6 − 4500 kWh solar power ÷ 140 m² (1507 square feet) living space = 70 kWh per m² primary energy and is therefore permitted by KfW 100.
What system performance factor (eP) should I use? (2.6)
Is it possible to achieve a KfW 70 standard with electric heating as described without unrealistic investments?
Foregoing the €5000 grant (KfW 40) and the interest benefits while investing €10,000 more in the KfW 40 standard does not make sense.
Explanations (how I arrived at the numbers)
Photovoltaic system
5 kWp south-facing (50 m²/540 square feet) producing about 4500 kWh annually. Approximate total cost including installation: $11,000.
10-year repayment at 1.55 percent interest is around $100 per month.
The system produces only about 30 percent (1350 kWh) during the heating period, which I fully consume myself.
Additionally, I will use about 1000 kWh per year for household electricity and hot water.
Feed-in to grid: 2500 kWh at $0.12 = $300 income minus 30 percent tax.
Average debt interest of about $110 reduces the income.
Energy Demand
A KfW 70 house consumes about 7000 kWh annually for heating and hot water (2 persons).
A KfW 40 would likely make even more sense because it would require less heating electricity.
The wood stove will be operated with wood (free) and lignite briquettes purchased at a home improvement store. Mornings before leaving and evenings before sleeping, approx. 6 briquettes or more/less as needed. These last about 9 hours and can be refueled immediately with wood or briquettes.
Coal/wood value: 4 kWh minus stove efficiency (−15 percent) = 3.4 kWh per kg.
Coal costs $3 for 10 kg = $0.30 per kg.
1 kWh coal costs $0.088
150 days × 6 kg coal = 900 kg coal × $0.30 = $270
150 days × 5 kg wood = 750 kg wood
900 kg coal × 3.4 kWh = 3060 kWh
750 kg wood × 3.4 kWh = 2550 kWh
Electricity demand:
20 days when the stove cannot be operated (absence)
20 × 35 kWh = 700 kWh × $0.25 = $175 + additional usage days + $25
Since only base temperature is needed when away, this will probably save costs.
X = days when the electric heating starts because the temperature falls below 15°C (59°F) or under 18°C (64°F) in the bedroom.
Other advantages
- Little technical maintenance or repairs needed (if self-repair)
- No heating pipes in the house
- No replacement after 20 or 30 years
- No obsolescence or replacements that are costly if gas/oil prices drop again
- Stove efficiency (coal + wood) at least 85 percent (power plants in Germany approx. 50)
Disadvantages
- Labor-intensive
- Wood must be purchased if no free source or physically not possible
- High ongoing costs without wood and coal
- Likely no KfW subsidy or loan
What do you think about these ideas? Any possibilities or suggestions?
Best regards,
Matthias
I am planning a new build of 140 m² (KfW 70 heating demand) and wonder if this would be feasible.
A wood-burning stove with 8 to 10 kW (coal + wood) is planned to be centrally located on the ground floor.
It will stand in a 3-meter-wide (10 feet) passageway to the 40 m² (430 square feet) living room and can optionally be separated from the hallway by a ceiling-mounted sliding door (no threshold). This setup allows flexible use of the stove’s heat, which can partially flow through the hallway upstairs and distribute throughout the entire ground floor.
Additionally, electric convector heaters will provide base heat when needed.
Hot Water:
An instantaneous water heater combined with a small pre-storage tank will preheat drinking water in winter, and when excess solar power is available, it will raise the water to the maximum flow temperature of the instantaneous heater.
KfW and the Energy Saving Ordinance 2009
are likely to pose challenges to my plan, although I would only need about 700 kWh of heating electricity from the grid, produce my own electricity, and use coal and wood with an efficiency of at least 85 percent. Perhaps you have ideas on how my plan could still work without losing control of the financial aspects.
If needed, you can find how I arrived at the numbers towards the end of my explanation.
Project:
Electric heating + wood-burning stove (for comparison only)
or
Electric heating + photovoltaic system + wood-burning stove (priority 1)
Calculations:
9000 kWh total demand
6300 kWh heating demand
700 kWh hot water heating (about 2 kWh per day yearly)
2000 kWh household electricity
Heating season approx. October to April: 180 days
(Heating demand: 6300 kWh ÷ 180 days = 35 kWh per day)
Electric heating + photovoltaic system + wood stove:
9000 kWh total consumption
minus 1350 kWh self-consumption during heating period (photovoltaic system)
minus 1000 kWh household self-consumption (photovoltaic system)
9000 kWh − 2350 kWh = 6650 kWh remaining demand
minus 1000 kWh household electricity to buy at $0.25 per kWh = $250
minus 700 kWh × $0.25 (20 days no stove use + water + buffer) = $175
minus 4950 kWh covered by stove (wood is free), coal costs = $270
Costs of about $720 per year + photovoltaic loan repayment $1200 − $300 income =
$1920 total costs − $300 income = $1620 total net costs per year
$135 per month
Gas heating + wood-burning stove
9000 kWh total consumption
2000 kWh household electricity purchased: 2000 × $0.25 = $500
7000 kWh gas × $0.08 = $560
2000 kWh wood × $0.08 = −$160
Annual tank rental = $135
Annual maintenance = $100
$1,135 per year ÷ 12 = $95 per month
For comparison: (which I do not want)
Electric heating + wood stove
9000 kWh total consumption
minus 5000 kWh by stove = coal $270
4000 kWh × $0.25 = $1000
$1270 per year ÷ 12 = $105 per month
Gas heating system €12,000 / electric heating system $4500
$7500 first to be converted into heat.
Gas heating would save $120 per year
$1200 in 10 years
$4800 in 40 years
Result
For 10 years, I pay $40 more per month than with gas and then have paid off the photovoltaic system.
10 years × 12 months × $40 additional cost = $4800
(Loan KfW for $11,000 at 1.55 percent interest)
After 10 years
I assume gas and electricity prices rise equally and keep these numbers.
Ongoing costs for electric heating + photovoltaic per year =
$720 − $300 income − $100 tax = $320 ÷ 12 = $27 per month
Ongoing costs for gas heating: $95 × 12 = $1140 per year
Ongoing costs for electric heating only: $105 × 12 = $1260 per year
Electric + photovoltaic costs $320 per year
Gas costs $820 more per year
Only electric heating costs $940 more per year
Price for electric heating and photovoltaic system: $11,000 + $4500 = $15,500
Price for gas heating with radiators and installation: $12,000
After approx. 18 years, I will start to make a profit. After 10 years, annual ongoing costs are around $820 less.
Questions:
Is an electric heating system as described above realistically and sensibly implementable?
I probably need to build a house with a heating energy demand plus hot water of 5500 kWh (exactly KfW 40).
Is this calculation correct? House with 5500 kWh × eP 2.6 − 4500 kWh solar power ÷ 140 m² (1507 square feet) living space = 70 kWh per m² primary energy and is therefore permitted by KfW 100.
What system performance factor (eP) should I use? (2.6)
Is it possible to achieve a KfW 70 standard with electric heating as described without unrealistic investments?
Foregoing the €5000 grant (KfW 40) and the interest benefits while investing €10,000 more in the KfW 40 standard does not make sense.
Explanations (how I arrived at the numbers)
Photovoltaic system
5 kWp south-facing (50 m²/540 square feet) producing about 4500 kWh annually. Approximate total cost including installation: $11,000.
10-year repayment at 1.55 percent interest is around $100 per month.
The system produces only about 30 percent (1350 kWh) during the heating period, which I fully consume myself.
Additionally, I will use about 1000 kWh per year for household electricity and hot water.
Feed-in to grid: 2500 kWh at $0.12 = $300 income minus 30 percent tax.
Average debt interest of about $110 reduces the income.
Energy Demand
A KfW 70 house consumes about 7000 kWh annually for heating and hot water (2 persons).
A KfW 40 would likely make even more sense because it would require less heating electricity.
The wood stove will be operated with wood (free) and lignite briquettes purchased at a home improvement store. Mornings before leaving and evenings before sleeping, approx. 6 briquettes or more/less as needed. These last about 9 hours and can be refueled immediately with wood or briquettes.
Coal/wood value: 4 kWh minus stove efficiency (−15 percent) = 3.4 kWh per kg.
Coal costs $3 for 10 kg = $0.30 per kg.
1 kWh coal costs $0.088
150 days × 6 kg coal = 900 kg coal × $0.30 = $270
150 days × 5 kg wood = 750 kg wood
900 kg coal × 3.4 kWh = 3060 kWh
750 kg wood × 3.4 kWh = 2550 kWh
Electricity demand:
20 days when the stove cannot be operated (absence)
20 × 35 kWh = 700 kWh × $0.25 = $175 + additional usage days + $25
Since only base temperature is needed when away, this will probably save costs.
X = days when the electric heating starts because the temperature falls below 15°C (59°F) or under 18°C (64°F) in the bedroom.
Other advantages
- Little technical maintenance or repairs needed (if self-repair)
- No heating pipes in the house
- No replacement after 20 or 30 years
- No obsolescence or replacements that are costly if gas/oil prices drop again
- Stove efficiency (coal + wood) at least 85 percent (power plants in Germany approx. 50)
Disadvantages
- Labor-intensive
- Wood must be purchased if no free source or physically not possible
- High ongoing costs without wood and coal
- Likely no KfW subsidy or loan
What do you think about these ideas? Any possibilities or suggestions?
Best regards,
Matthias
If you are going to use electricity, at least use a heat pump. If you don’t want underfloor heating, I would choose a domestic hot water heat pump along with a multi-split air conditioning system with several indoor units. These also blow out warm air for the basic heating, but they produce 3 kWh of heat from 1 kWh of electricity, not just “one” like infrared heaters and similar systems. In other words, you need three times less electricity for the same amount of heat.
We currently heat this way: wood stove and split air conditioning unit. Hot water is currently provided by an instantaneous water heater, which isn’t ideal and will be changed. A pellet boiler is already installed, and underfloor heating will be added soon. We also have a photovoltaic system.
We currently heat this way: wood stove and split air conditioning unit. Hot water is currently provided by an instantaneous water heater, which isn’t ideal and will be changed. A pellet boiler is already installed, and underfloor heating will be added soon. We also have a photovoltaic system.
I have considered that option as well. Wood is my primary fuel, which I can obtain for free. As far as I know, a heat pump would not be cost-effective since I don’t want to cover my remaining heat demand of about 700 kWh or, with less coal usage, maybe 3000 kWh with an extremely expensive heat pump. Around 12,000 euros for the cheapest air-source heat pump that draws electricity from the grid. With the photovoltaic system, I at least earn money in the summer and have electricity to use. The heat pump only heats water, which I hardly need. In that case, I might as well get a gas boiler. I prefer to burn wood visibly in the living room.
Thanks for the contribution.
Best regards, Matthias
Thanks for the contribution.
Best regards, Matthias
I hardly believe anyone here in the forum can answer that for you. Most of the forum members are homeowners. The majority of them have chosen more traditional heating systems (gas heating or heat pumps).
Your problem will probably be the distribution of heat. The living room might become uncomfortably warm, while the rest of the house remains rather chilly.
The Elina climate split unit does not operate through traditional radiators or underfloor heating but is a simple blower with heat pump technology, costing only a few hundred euros.
Your problem will probably be the distribution of heat. The living room might become uncomfortably warm, while the rest of the house remains rather chilly.
The Elina climate split unit does not operate through traditional radiators or underfloor heating but is a simple blower with heat pump technology, costing only a few hundred euros.
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