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
@Jochen: You didn’t read my explanation about the absence. The electric heating would take over.
I am single. With a wheelchair, I have to sell anyway. I have wood stored for 10 years.
Who knows what it will be like in 30 years... Maybe heating will only be needed from December to February.
But you are probably right that a gas heating system is better in old age.
I am single. With a wheelchair, I have to sell anyway. I have wood stored for 10 years.
Who knows what it will be like in 30 years... Maybe heating will only be needed from December to February.
But you are probably right that a gas heating system is better in old age.
Yes, I did indeed overlook that.
What I actually wanted to say is: You should install a heating system, if only for resale value and retirement planning. Which type and how it should be designed is best advised by an energy consultant.
Who in a few years will buy a house without underfloor heating and without radiators (and the necessary piping)? If you want to switch later in life, you will have to redo everything. It’s like buying a 100-year-old house today without heating — a lot of work to retrofit everything.
What I actually wanted to say is: You should install a heating system, if only for resale value and retirement planning. Which type and how it should be designed is best advised by an energy consultant.
Who in a few years will buy a house without underfloor heating and without radiators (and the necessary piping)? If you want to switch later in life, you will have to redo everything. It’s like buying a 100-year-old house today without heating — a lot of work to retrofit everything.
B
Bauexperte8 Nov 2014 11:24Hello Matthias,
Just reading the above takes me back to my childhood, with the only difference being that we wrapped the briquettes in newspaper.
I wonder how anyone (that is, you) seriously plans to carry out such a project? Heating with electricity is not a bad idea in principle; but if so, it should be done consistently with a proper, storage-based photovoltaic system. In that case, you wouldn’t even need a conventional underfloor heating system, as there are excellent marble radiant panels you can mount like pictures on the wall. Heating domestic hot water with an instantaneous water heater is also not a bad idea; in other words, it is better—and above all, cheaper—than its reputation suggests. Consult a qualified expert, and they will confirm this.
Nowadays, I definitely wouldn’t recommend using a wood stove as the primary heat source—especially not an old-fashioned type. It compromises privacy since everything must be open up to the roof to prevent interference with airflow. As a single person, you don’t seem to consider that nowadays the trend is “modern,” not “back to the roots.” From that, one can easily deduce that—should you consider selling or renting out later—few buyers or tenants will welcome your experiment with the same enthusiasm you have.
In my opinion, a wood stove should be considered a “nice to have” for transitional seasons—there are still homeowners who shut off their main heating systems during summer—and for creating a cozy atmosphere. Anything beyond that is costly and labor-intensive but not very convincing, even if you store an entire forest behind your house.
Visit the local energy agency and discuss your heating ideas with them. I am almost certain their advice will align with the thoughts expressed above.
Regards,
Bauexperte
extc2020 schrieb:
I operate the wood stove using wood (free) and lignite briquettes from the hardware store. In the morning before leaving and in the evening before going to bed, about 6 briquettes or more or less as needed. These last about 9 hours, and you can immediately add more wood and briquettes.
Just reading the above takes me back to my childhood, with the only difference being that we wrapped the briquettes in newspaper.
I wonder how anyone (that is, you) seriously plans to carry out such a project? Heating with electricity is not a bad idea in principle; but if so, it should be done consistently with a proper, storage-based photovoltaic system. In that case, you wouldn’t even need a conventional underfloor heating system, as there are excellent marble radiant panels you can mount like pictures on the wall. Heating domestic hot water with an instantaneous water heater is also not a bad idea; in other words, it is better—and above all, cheaper—than its reputation suggests. Consult a qualified expert, and they will confirm this.
Nowadays, I definitely wouldn’t recommend using a wood stove as the primary heat source—especially not an old-fashioned type. It compromises privacy since everything must be open up to the roof to prevent interference with airflow. As a single person, you don’t seem to consider that nowadays the trend is “modern,” not “back to the roots.” From that, one can easily deduce that—should you consider selling or renting out later—few buyers or tenants will welcome your experiment with the same enthusiasm you have.
In my opinion, a wood stove should be considered a “nice to have” for transitional seasons—there are still homeowners who shut off their main heating systems during summer—and for creating a cozy atmosphere. Anything beyond that is costly and labor-intensive but not very convincing, even if you store an entire forest behind your house.
Visit the local energy agency and discuss your heating ideas with them. I am almost certain their advice will align with the thoughts expressed above.
Regards,
Bauexperte
Bauexperte schrieb:
....What you don’t seem to consider at all as a single person is that nowadays, “modern” is in trend, not “back to the roots.” From this it can be easily inferred that—if you consider a future sale or even renting out—there will hardly be buyers or tenants who will welcome your experiment with the same enthusiasm as you do. ...Haha, I have to correct you a little, but I myself thought of “back to the roots” while reading this thread.
It’s not about creating something modern—the modern style, like the country style, is just a matter of current taste.
It’s more about building a contemporary house (in times of environmental awareness, energy-saving regulations, so in the third millennium).
@ Matthias: Honestly, what is a house without heating worth?
I think what you have in mind would be nice to implement in a holiday home.
We have an additional wood-burning stove that currently provides us with cozy extra warmth. And yes: the heat, as long as the open floor plan allows, also rises to the upper floors.
But don’t expect that a guest toilet or any other room on the ground floor that can be closed off by a door will receive any of that warmth.
Best regards, Yvonne
K
Kardionaut8 Nov 2014 12:06We have a KfW 40 standard house and heat exclusively with electricity. We use ceiling-mounted radiators (radiant heat) and electric underfloor heating grids beneath the tiles in all tiled rooms. The insulation is high-quality, the windows are triple-glazed, and there is a ventilation system with heat recovery. Domestic hot water is provided by an air-source heat pump. Additionally, there is a photovoltaic system on the roof. The remaining heat demand compensated by electricity is about 20 Watts per square meter (without guarantee, but I believe that’s correct).
Best regards
Best regards
K
Kardionaut8 Nov 2014 12:20I think the concepts contradict each other. I understand that the fireplace is supposed to be your primary heat source, but I would like to point out something.
Heating with electricity only works if you have a very well-insulated house. And I’m not just talking about external wall insulation systems, but the full package; otherwise, using electricity won’t be cost-effective. I’m not sure to what extent a fireplace, chimney, etc., might conflict with this approach, due to thermal bridges, connections to the outside, and so on. People even advise against exhaust air systems (like in the kitchen) to avoid interrupting the insulated building envelope, and then you want to add a chimney—hmm…
But I must emphasize again, I’m an amateur…
Heating with electricity only works if you have a very well-insulated house. And I’m not just talking about external wall insulation systems, but the full package; otherwise, using electricity won’t be cost-effective. I’m not sure to what extent a fireplace, chimney, etc., might conflict with this approach, due to thermal bridges, connections to the outside, and so on. People even advise against exhaust air systems (like in the kitchen) to avoid interrupting the insulated building envelope, and then you want to add a chimney—hmm…
But I must emphasize again, I’m an amateur…
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