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
Thank you for your detailed explanations!
I know myself that it’s a bit different from a typical concept.
@Bauexperte: I never mentioned underfloor heating but quoted:
Additionally, corn vector heaters (electric) should provide base heat when needed. End of quote.
And in every room (flat wall-mounted radiators).
If I’m not in the mood to heat with the stove, it’s still as warm as I have set it on my smartphone. So much for "back to the roots." I understand what you mean, but I think it’s being overlooked that I want to combine a fully modern electric heating system with smartphone control and a wood stove. I enjoy adding wood and coal. Also because I know it’s a good solution environmentally. With my setup, I would consume about 1000 kWh of electricity but also produce that myself.
My actual question is whether this is even feasible with KfW 40 or 70 standards. Apparently no one here can answer that.
@Kardionaut: I think your KfW 40 standard is based on the air source heat pump. You wouldn’t have met it with electricity alone? Maybe that would be an option. But your technology costs accordingly: photovoltaic system 10,000–13,000 euros, air source heat pump and complete installation probably 15,000 euros, and then your KfW 40 standard with insulation etc. 10,000 euros?
At that point, I might as well stick with KfW 70 using a gas boiler and a modern wood stove.
@Yvonne: I know heat doesn’t pass through closed doors. I myself said that normal door openings are unsuitable for heat distribution.
Anyway, I will ask an energy consultant, because my concern was to find out if I am really rated so poorly with electric heating that I would have to build a KfW 40 house just to meet the permitted primary energy demand of a KfW 100 (minimum standard). With a system efficiency factor of 2.6, my project is basically worthless anyway.
I will probably decide on a KfW 70 with a gas boiler and a water-heated wood stove. Standard.
I know myself that it’s a bit different from a typical concept.
@Bauexperte: I never mentioned underfloor heating but quoted:
Additionally, corn vector heaters (electric) should provide base heat when needed. End of quote.
And in every room (flat wall-mounted radiators).
If I’m not in the mood to heat with the stove, it’s still as warm as I have set it on my smartphone. So much for "back to the roots." I understand what you mean, but I think it’s being overlooked that I want to combine a fully modern electric heating system with smartphone control and a wood stove. I enjoy adding wood and coal. Also because I know it’s a good solution environmentally. With my setup, I would consume about 1000 kWh of electricity but also produce that myself.
My actual question is whether this is even feasible with KfW 40 or 70 standards. Apparently no one here can answer that.
@Kardionaut: I think your KfW 40 standard is based on the air source heat pump. You wouldn’t have met it with electricity alone? Maybe that would be an option. But your technology costs accordingly: photovoltaic system 10,000–13,000 euros, air source heat pump and complete installation probably 15,000 euros, and then your KfW 40 standard with insulation etc. 10,000 euros?
At that point, I might as well stick with KfW 70 using a gas boiler and a modern wood stove.
@Yvonne: I know heat doesn’t pass through closed doors. I myself said that normal door openings are unsuitable for heat distribution.
Anyway, I will ask an energy consultant, because my concern was to find out if I am really rated so poorly with electric heating that I would have to build a KfW 40 house just to meet the permitted primary energy demand of a KfW 100 (minimum standard). With a system efficiency factor of 2.6, my project is basically worthless anyway.
I will probably decide on a KfW 70 with a gas boiler and a water-heated wood stove. Standard.
B
Bauexperte8 Nov 2014 15:15Hello,
To answer that reliably, you need the floor plan, the plot of land of course, a soil expert, a structural engineer, and an MEP planner (mechanical, electrical, and plumbing). Then it also needs to be clarified whether you want to apply for subsidies or if it is enough for you to know that you have calculated to achieve a particular energy-efficiency house standard.
Best regards, Bauexperte
extc2020 schrieb:I understood that correctly, but I agree with you that I should have put "There was also no need for conventional underfloor heating" in quotation marks as well, which might have prevented confusion in two ways.
@Bauexperte : I never mentioned underfloor heating, but said: Quote:
Additionally, convector heaters (electric) should provide base heat when needed. End of quote.
extc2020 schrieb:And that’s exactly how I interpret “back to the roots”. You don’t want to use the fireplace just as a temporary transitional heating during winter to spring or autumn to winter, but as a fully functional heating system for the entire single-family house during the cold season when you “don’t feel like” using anything else. That is as far from high-tech smartphone control as my current location is from Timbuktu.
When I don’t feel like heating with the fireplace, it’s still as warm as I set it via my smartphone. That’s all there is to “back to the roots”.
extc2020 schrieb:That’s definitely a matter open for debate.
Also because I know it’s a good environmental solution.
extc2020 schrieb:No, how could anyone? There are so many factors beyond the new building’s location to consider that a randomly correct answer would be like winning the lottery.
My actual question whether this is even possible with KfW 40 or 70 standards probably nobody here can answer.
To answer that reliably, you need the floor plan, the plot of land of course, a soil expert, a structural engineer, and an MEP planner (mechanical, electrical, and plumbing). Then it also needs to be clarified whether you want to apply for subsidies or if it is enough for you to know that you have calculated to achieve a particular energy-efficiency house standard.
Best regards, Bauexperte
Well, there are certain values I have to meet at a minimum to achieve a KfW 70 or 40 standard. One of these is the annual primary energy demand, which is calculated from the heating demand (kWh) plus the hot water demand (kWh) multiplied by the system efficiency factor. You then divide this result by the square meters and need to reach approximately 74 kWh per square meter (KfW 100) to be allowed to build at all.
(The question was whether this is realistically achievable with electric heating.)
Furthermore, the insulation values or heat loss of the house must meet a minimum standard of 0.4 W/m²K (watts per square meter per kelvin), with KfW 70 requiring 85 percent of this value and KfW 40 requiring 55 percent.
These values have nothing to do with the location or the plot of land but are related to the construction method. I go to the construction company and say I want to meet at least these values because I want the KfW 40 subsidy and the low-interest loan. However, if I build a house that meets the KfW 40 insulation values, the electric heating is calculated with a system efficiency factor of 2.6, and the wood stove is not taken into account. This results in a primary energy demand that just meets the KfW 100 standard (if self-generated electricity can be deducted). With KfW 70 insulation values and electric heating, I wouldn’t be allowed to build at all. That’s how I understood it, and if this is correct, it doesn’t make sense to use electric heating, since I already invest about 10,000 euros more for a house with KfW 40 insulation values, but the electric heating ruins the 5,000 euro benefit for KfW 40 and I wouldn’t get a KfW 70 loan either.
I think that’s how it is, but I wanted to either get confirmation or a correction. I understand that not everyone wants to use a wood stove so extensively. For me, it’s about the theoretical and legal feasibility, which also needs to be financially reasonable for me.
So, I don’t know if that came across clearly... The electric heating could theoretically heat the whole house (radiators in every room), and I can control it via smartphone. And every kilowatt-hour I produce with the stove obviously saves electricity costs...
Thanks for your reply.
(The question was whether this is realistically achievable with electric heating.)
Furthermore, the insulation values or heat loss of the house must meet a minimum standard of 0.4 W/m²K (watts per square meter per kelvin), with KfW 70 requiring 85 percent of this value and KfW 40 requiring 55 percent.
These values have nothing to do with the location or the plot of land but are related to the construction method. I go to the construction company and say I want to meet at least these values because I want the KfW 40 subsidy and the low-interest loan. However, if I build a house that meets the KfW 40 insulation values, the electric heating is calculated with a system efficiency factor of 2.6, and the wood stove is not taken into account. This results in a primary energy demand that just meets the KfW 100 standard (if self-generated electricity can be deducted). With KfW 70 insulation values and electric heating, I wouldn’t be allowed to build at all. That’s how I understood it, and if this is correct, it doesn’t make sense to use electric heating, since I already invest about 10,000 euros more for a house with KfW 40 insulation values, but the electric heating ruins the 5,000 euro benefit for KfW 40 and I wouldn’t get a KfW 70 loan either.
I think that’s how it is, but I wanted to either get confirmation or a correction. I understand that not everyone wants to use a wood stove so extensively. For me, it’s about the theoretical and legal feasibility, which also needs to be financially reasonable for me.
So, I don’t know if that came across clearly... The electric heating could theoretically heat the whole house (radiators in every room), and I can control it via smartphone. And every kilowatt-hour I produce with the stove obviously saves electricity costs...
Thanks for your reply.
D
DerBjoern10 Nov 2014 10:48The location is important because the annual heating degree days naturally affect the heating energy demand. Information about the plot is interesting on the one hand because the orientation of the house influences the solar gains through the windows, and on the other hand, soil information affects the efficiency of, for example, a ground source heat pump.
D
DerBjoern10 Nov 2014 10:52Also, I’m concerned that the total financial effort you need to invest to ensure your electric heating system complies with the primary energy requirements of the energy-saving regulations will be significantly higher than if you install a hydronic heating system.
Another issue is that if you insulate your house extremely well (KfW40 or better), where will the waste heat from the stove go? Stoves start at around 5 kW heating output. Even at -15°C (5°F), 5 kW is far too much heat for a house like that.
For this project, you really need a professional planner.
Another issue is that if you insulate your house extremely well (KfW40 or better), where will the waste heat from the stove go? Stoves start at around 5 kW heating output. Even at -15°C (5°F), 5 kW is far too much heat for a house like that.
For this project, you really need a professional planner.
B
Bauexperte10 Nov 2014 11:04Hello,
Regards, Bauexperte
extc2020 schrieb:Of course, the location does matter. To give a striking example for better understanding: a house on the Zugspitze requires different insulation than a house in the mild Rhineland. And what if, for example, the soil survey shows that concrete supports need to be installed within the masonry in some areas, or in the worst case, the foundation slab must be fully or partially reinforced? Then your calculation is off because these thermal bridges (more steel/reinforcement) are not taken into account in your simple calculation. That is why every reputable builder will give you the same answer—first a soil survey, structural analysis, and calculation of the heat demand, and then consider which heating system can be used efficiently and, above all, economically. Basically, every offer should include a note along the lines of: "Compliance as an energy-efficient house xyz cannot be guaranteed, as the building’s orientation with respect to the cardinal directions and the general location of the plot are significant factors in calculating the required energy performance certificate."
These values have nothing to do with the location or the plot, but with the construction method. I go to the builder and say I want to meet these values at least because I want the KfW 40 subsidy and the low-interest loan.
Regards, Bauexperte
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