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
I would like to try to manage without technology and I also think it’s great if I can do without a water-based heating system. If I have no other option, a wood gasifier boiler could be considered. However, it’s only suitable as a central heating system to a limited extent since you have to refill it daily during winter. (So, gas plus a fireplace would be preferable after all.)
I fear that a split air-source heat pump won’t be recognized as a heating system and that there will again be issues meeting the KFW 100 standard. Otherwise, that would be a great solution. I’ll look it up right away...
The distribution of heat in the house is something most wood stove owners haven’t planned for. That’s why mine will be centrally located, as described, in the 2-meter (6.5-foot) wide passage between the living room and hallway with stairs leading up.
I now have a similar system in my very old house. The stove is in the kitchen, and the heat flows into the living room through a 1 by 1 meter (3.3 by 3.3 feet) wall opening. It’s important that there are no drops or steps in the ceiling. (Standard doors or door openings are not suitable.) For me, this works great...
I fear that a split air-source heat pump won’t be recognized as a heating system and that there will again be issues meeting the KFW 100 standard. Otherwise, that would be a great solution. I’ll look it up right away...
The distribution of heat in the house is something most wood stove owners haven’t planned for. That’s why mine will be centrally located, as described, in the 2-meter (6.5-foot) wide passage between the living room and hallway with stairs leading up.
I now have a similar system in my very old house. The stove is in the kitchen, and the heat flows into the living room through a 1 by 1 meter (3.3 by 3.3 feet) wall opening. It’s important that there are no drops or steps in the ceiling. (Standard doors or door openings are not suitable.) For me, this works great...
K
Kardionaut7 Nov 2014 17:08Heating with electricity is generally criticized and considered inefficient. However, if the residual heat demand is very low (KfW 40), it can be a reasonable alternative.
Most wood stove owners don’t intend to heat the entire house with it; rather, they want to enjoy a cozy evening by the fireplace.
That’s why the stove should be sized differently than if you were using it as a primary heat source. It may be possible to control heat delivery to the stairwell and living room, but can you distribute it effectively to the other rooms?
One option I could somewhat imagine would be heat distribution via a mechanical ventilation system with heat recovery, but you’re not keen on technology.
Regarding the potential price reduction when reselling, I wouldn’t want to think about that just yet. It’s unlikely a buyer would want a relatively new house heated with coal, as that would require major renovations, which would lower the purchase price. A bank might also view this skeptically.
On the topic of log wood gasifiers:
I quickly checked online and found information suggesting they need to be refueled every 5 days, for example.
That’s why the stove should be sized differently than if you were using it as a primary heat source. It may be possible to control heat delivery to the stairwell and living room, but can you distribute it effectively to the other rooms?
One option I could somewhat imagine would be heat distribution via a mechanical ventilation system with heat recovery, but you’re not keen on technology.
Regarding the potential price reduction when reselling, I wouldn’t want to think about that just yet. It’s unlikely a buyer would want a relatively new house heated with coal, as that would require major renovations, which would lower the purchase price. A bank might also view this skeptically.
On the topic of log wood gasifiers:
I quickly checked online and found information suggesting they need to be refueled every 5 days, for example.
As far as I understand, an air-to-air heat pump produces about 3 kWh of heat from 1 kWh of electricity (which costs around 25 cents), so approximately 8 cents per kilowatt (but only up to about +2°C (36°F)). Connecting it to a photovoltaic system definitely makes sense. The question is whether I could plan a central location for such a device (which apparently produces noise). I would still need radiators in the rooms. So it can only be used as a supplementary system. The issue of dust circulation also remains a concern... However, I would take this idea to the energy consultant.
Hello!
Have you ever thought about what will happen when you get older? Or maybe if you ever need a wheelchair, for example due to an accident? Who will handle the wood then?
What will you do if you go on a longer vacation? Or if you go out in the evening, do you want to leave the children at home alone with the fire?
Have you ever thought about what will happen when you get older? Or maybe if you ever need a wheelchair, for example due to an accident? Who will handle the wood then?
What will you do if you go on a longer vacation? Or if you go out in the evening, do you want to leave the children at home alone with the fire?
When selling, an additional pellet stove could be connected to the chimney. I had already planned a second connection. This must be discussed with the chimney sweep beforehand to ensure that the chimney is designed for it. However, it is true that this could disadvantage the sale.
I only need a standard wood stove of about 10 kW. Most people cannot heat effectively with it because the room to be heated is too small, and they end up overheating. You mainly control the output by the amount of wood you add.
Heat distribution could be an issue, and perhaps the electric heaters in the upper rooms would have to work harder than desired. However, I cannot imagine it being that significant given the insulation, especially since I plan to have air vents near the chimney upstairs to use the waste heat in the bathroom.
@Kardionaut: The problem is that with electric heating, I don’t have a chance to achieve KfW 40 standard, no matter how well I insulate. Or do you have different information?
I only need a standard wood stove of about 10 kW. Most people cannot heat effectively with it because the room to be heated is too small, and they end up overheating. You mainly control the output by the amount of wood you add.
Heat distribution could be an issue, and perhaps the electric heaters in the upper rooms would have to work harder than desired. However, I cannot imagine it being that significant given the insulation, especially since I plan to have air vents near the chimney upstairs to use the waste heat in the bathroom.
@Kardionaut: The problem is that with electric heating, I don’t have a chance to achieve KfW 40 standard, no matter how well I insulate. Or do you have different information?
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