ᐅ Combine an air-to-water heat pump with a wood-burning stove connected to the central heating system
Created on: 29 Mar 2020 14:13
G
GSGauchoHello everyone,
We are currently planning a single-family home built with solid construction to KFW55 standard.
Two full stories, partially basement, without basement about 230m² (2,475 sq ft) of living space for 5 people.
The location is southern Germany at 550m (1,804 ft) above sea level. The shell construction planning is fixed, and the shell and gable roof have already been contracted. Construction start is week 22/2020.
I have also already contracted a 23kWp photovoltaic system on the south/west roof of the house and the south/east roof of the garage.
Due to economic reasons, a battery storage system is currently not an option.
The heating load according to calculation is about 5.5 kW at -15°C (5°F) ambient temperature.
Now it’s time to plan the heating system:
Current status is:
Now I have the first offer for a Stiebel Eltron LWZ 8 cs Premium.
Am I correct to assume that under the above parameters the LWZ 5 cs would also be sufficient?
How can I best integrate the Hoxter stove into the heating system?
As a layperson, I currently see two options:
Option 1
An 800-liter (210-gallon) buffer tank only for heating operation, without domestic hot water.
Domestic hot water is generated by the air-to-water heat pump during daylight; the heat pump runs mainly during the day and stores heat in the screed. From 4 p.m. onward, the Hoxter stove is fired.
The heating circuit would then have to switch to the buffer tank when a certain temperature X is reached in the stove circuit or the heat pump buffer.
Option 2
The air-to-water heat pump always charges the buffer tank with a maximum flow temperature of about 40°C (104°F). If this temperature is exceeded by the stove operation, the heat pump switches off. Also, the heat pump would be programmed to operate only during daytime.
I understand that the combination of air-to-water heat pump with a buffer tank is suboptimal. But a stove without hydronic integration also makes no sense, as it would quickly overheat.
My current bidder is almost unreachable for technical evaluation at the moment, fully booked, so I have no real information about integrating the stove yet.
What do you suggest?
Which other air-to-water heat pump manufacturers would you consider for this configuration?
Thank you very much,
GSGaucho
We are currently planning a single-family home built with solid construction to KFW55 standard.
Two full stories, partially basement, without basement about 230m² (2,475 sq ft) of living space for 5 people.
The location is southern Germany at 550m (1,804 ft) above sea level. The shell construction planning is fixed, and the shell and gable roof have already been contracted. Construction start is week 22/2020.
I have also already contracted a 23kWp photovoltaic system on the south/west roof of the house and the south/east roof of the garage.
Due to economic reasons, a battery storage system is currently not an option.
The heating load according to calculation is about 5.5 kW at -15°C (5°F) ambient temperature.
Now it’s time to plan the heating system:
Current status is:
- Underfloor heating in all rooms except for the pantry and storage/technical room in the basement.
- Central ventilation system combined with an air-to-water heat pump. Air-to-water heat pump installed indoors in the basement.
- A hydronic wood-burning stove from Hoxter with firing from a separate room, i.e., no wood/dirt in the living room. I still have 30rm (cords) of beech wood stored free of charge.
- Due to the high capacity of the photovoltaic system and the stove, I see no sense in a trench collector.
- An 800-liter (210-gallon) buffer tank can be placed almost directly under the Hoxter stove in the basement. The distance to the air-to-water heat pump is about 2.5m (8 feet).
Now I have the first offer for a Stiebel Eltron LWZ 8 cs Premium.
Am I correct to assume that under the above parameters the LWZ 5 cs would also be sufficient?
How can I best integrate the Hoxter stove into the heating system?
As a layperson, I currently see two options:
Option 1
An 800-liter (210-gallon) buffer tank only for heating operation, without domestic hot water.
Domestic hot water is generated by the air-to-water heat pump during daylight; the heat pump runs mainly during the day and stores heat in the screed. From 4 p.m. onward, the Hoxter stove is fired.
The heating circuit would then have to switch to the buffer tank when a certain temperature X is reached in the stove circuit or the heat pump buffer.
Option 2
The air-to-water heat pump always charges the buffer tank with a maximum flow temperature of about 40°C (104°F). If this temperature is exceeded by the stove operation, the heat pump switches off. Also, the heat pump would be programmed to operate only during daytime.
I understand that the combination of air-to-water heat pump with a buffer tank is suboptimal. But a stove without hydronic integration also makes no sense, as it would quickly overheat.
My current bidder is almost unreachable for technical evaluation at the moment, fully booked, so I have no real information about integrating the stove yet.
What do you suggest?
Which other air-to-water heat pump manufacturers would you consider for this configuration?
Thank you very much,
GSGaucho
D
Daniel-Sp29 Mar 2020 14:43Integrating a heat pump into an 800-liter (210-gallon) buffer tank is inefficient. Therefore, solution 2 is out.
Solution 1 would require too much control technology for me...
In my opinion, it should be either one or the other.
Combining a low-temperature system with a high-temperature system is demanding in terms of control technology and usually reduces the efficiency of the heat pump.
Solution 1 would require too much control technology for me...
In my opinion, it should be either one or the other.
Combining a low-temperature system with a high-temperature system is demanding in terms of control technology and usually reduces the efficiency of the heat pump.
Hi,
oh dear. Where to even begin…
1. With proper planning and correct implementation, a trench collector isn’t significantly more expensive than an air source heat pump. In return, you get higher efficiency, no noise issues (and no problems with neighbors), the possibility of very inexpensive passive cooling, and a longer lifespan for the heat generator thanks to indoor installation and gentler operating conditions. For me alone, the higher operating comfort would justify spending a bit more, so I wouldn’t worry about the fan icing up in unfavorable weather. Oh yes, the photovoltaic system hardly generates any power in winter since most of the electricity is already consumed by Hamburg’s demand.
Although I have a very good air source heat pump (not Stiebel) and a large photovoltaic system (22kWp), I would always prefer a ground source heat pump with a trench collector. And yes, I also have a stove.
This practically screams for concrete core activation in the ceilings. At this stage, it’s no problem at all. You get a lot of benefits for little money:
- Greater comfort thanks to radiant heat from the ceiling
- Cooling with a heat pump works much better. Cooling via underfloor heating is less comfortable and less effective.
- Higher efficiency of the heat pump due to reduced supply temperature
That seems unrealistic to me. For my KfW40 house with 240m² (2,583 sq ft) living area, same number of people, and a design outdoor temperature of -14°C (7°F), I have a heating load of almost 7 kW. Therefore, I would recommend a modulating heat pump with somewhat higher capacity. This way, you can use any photovoltaic surplus power during the day more effectively—if there is any—and have the heat pump run less during the night. In the case of an air source heat pump, you can also better use higher daytime temperatures and partially avoid inefficient night operation.
If you do a little research, you will find that this kind of heating is more or less direct electric heating and is quite probably the least efficient system you can install.
The options you describe are basically the worst case for the efficiency of a heat pump, especially an air source one.
1. Instead of a hydronic stove, you can use a masonry heater or small storage stove. This way you avoid nearly all the problems that come with a hydronic stove combined with a heat pump without losing the advantages. Additionally, a masonry heater runs without electricity, unlike a water jacket, so you can save yourself the efficiency killer of a heating buffer tank.
2. If you really want a hydronic stove, you can still hydraulically integrate the heat pump to supply the heating circuits directly without a buffer tank, allowing efficient operation. Operating with a 40°C (104°F) supply temperature into the buffer tank will practically cancel out any savings you might get from wood heating.
3. Wood heating is not cheap. Calculate how much a kWh of heat costs at your wood price. The stove’s efficiency is probably around 70%. Wood heating is fine as a comfort factor, but you won’t save money. You will most likely pay extra.
If you go for an air source heat pump at all, pick one with a calculated seasonal performance factor above 4.5. Brands like Nibe F2120 or IDM come to mind. Search online for seasonal performance factor calculators. This is only possible with supply temperatures below 35°C (95°F). In a new build with concrete core activation, you can easily stay below 30°C (86°F). You may also qualify for BAFA subsidies that should cover the additional costs. Note that a ground source heat pump is often cheaper than a comparable air source heat pump.
However, I will never understand why people choose an air source heat pump when a ground source heat pump would have been possible at nearly the same cost. I’m even a satisfied owner of an air source heat pump myself.
Regards,
Nika
oh dear. Where to even begin…
GSGaucho schrieb:
Because of the high capacity of the photovoltaic system and also the stove, I don’t see any point in a trench collector
1. With proper planning and correct implementation, a trench collector isn’t significantly more expensive than an air source heat pump. In return, you get higher efficiency, no noise issues (and no problems with neighbors), the possibility of very inexpensive passive cooling, and a longer lifespan for the heat generator thanks to indoor installation and gentler operating conditions. For me alone, the higher operating comfort would justify spending a bit more, so I wouldn’t worry about the fan icing up in unfavorable weather. Oh yes, the photovoltaic system hardly generates any power in winter since most of the electricity is already consumed by Hamburg’s demand.
Although I have a very good air source heat pump (not Stiebel) and a large photovoltaic system (22kWp), I would always prefer a ground source heat pump with a trench collector. And yes, I also have a stove.
GSGaucho schrieb:
Single-family house with solid construction, KfW55 standard. Two full stories, partially basement, without basement about 230m² (2,476 sq ft) of living space, 5 people.
This practically screams for concrete core activation in the ceilings. At this stage, it’s no problem at all. You get a lot of benefits for little money:
- Greater comfort thanks to radiant heat from the ceiling
- Cooling with a heat pump works much better. Cooling via underfloor heating is less comfortable and less effective.
- Higher efficiency of the heat pump due to reduced supply temperature
GSGaucho schrieb:
Two full stories, partially basement, without basement about 230m² (2,476 sq ft) of living space, 5 people. [...] Heating load according to calculation is about 5.5 kW at -15°C (5°F) outdoor temperature [...] Am I correct in assuming that under the given parameters the LWZ 5 CS is sufficient?
That seems unrealistic to me. For my KfW40 house with 240m² (2,583 sq ft) living area, same number of people, and a design outdoor temperature of -14°C (7°F), I have a heating load of almost 7 kW. Therefore, I would recommend a modulating heat pump with somewhat higher capacity. This way, you can use any photovoltaic surplus power during the day more effectively—if there is any—and have the heat pump run less during the night. In the case of an air source heat pump, you can also better use higher daytime temperatures and partially avoid inefficient night operation.
GSGaucho schrieb:
Central ventilation system combined with the air-to-water heat pump. Air-to-water heat pump installed indoors in the basement.
If you do a little research, you will find that this kind of heating is more or less direct electric heating and is quite probably the least efficient system you can install.
GSGaucho schrieb:
Option 1
An 800-liter (210-gallon) buffer tank solely for heating operation without domestic hot water.
Domestic hot water is generated via the air-to-water heat pump during daylight; the heat pump operates normally only during the day and charges the screed (floor mass). From 4 p.m., the wood stove is lit.
The heating circuit would need to switch to the buffer tank when a certain temperature X is reached either in the stove circuit or the heat pump buffer.
Option 2
The air-to-water heat pump always charges the buffer tank with a maximum supply temperature of perhaps 40°C (104°F). If this is exceeded by the stove operation, the heat pump switches off. The heat pump is programmed to run only during the daytime.
I’m aware that the combination of air-to-water heat pump with buffer tank is suboptimal. But a stove without hydronic integration also makes no sense, otherwise there is quick overheating.
The options you describe are basically the worst case for the efficiency of a heat pump, especially an air source one.
1. Instead of a hydronic stove, you can use a masonry heater or small storage stove. This way you avoid nearly all the problems that come with a hydronic stove combined with a heat pump without losing the advantages. Additionally, a masonry heater runs without electricity, unlike a water jacket, so you can save yourself the efficiency killer of a heating buffer tank.
2. If you really want a hydronic stove, you can still hydraulically integrate the heat pump to supply the heating circuits directly without a buffer tank, allowing efficient operation. Operating with a 40°C (104°F) supply temperature into the buffer tank will practically cancel out any savings you might get from wood heating.
3. Wood heating is not cheap. Calculate how much a kWh of heat costs at your wood price. The stove’s efficiency is probably around 70%. Wood heating is fine as a comfort factor, but you won’t save money. You will most likely pay extra.
GSGaucho schrieb:
Which other manufacturer of air-to-water heat pumps would you consider for this configuration?
If you go for an air source heat pump at all, pick one with a calculated seasonal performance factor above 4.5. Brands like Nibe F2120 or IDM come to mind. Search online for seasonal performance factor calculators. This is only possible with supply temperatures below 35°C (95°F). In a new build with concrete core activation, you can easily stay below 30°C (86°F). You may also qualify for BAFA subsidies that should cover the additional costs. Note that a ground source heat pump is often cheaper than a comparable air source heat pump.
However, I will never understand why people choose an air source heat pump when a ground source heat pump would have been possible at nearly the same cost. I’m even a satisfied owner of an air source heat pump myself.
Regards,
Nika
Thank you for the prompt responses.
Regarding the horizontal ground heat exchanger versus the air-to-water heat pump: The Stiebel model offered does not have an external heat exchanger. In my opinion, there is no risk of icing in that case.
The photovoltaic system at our location is forecasted to generate 24 kWh per day in January. According to our own recorded actual values, we need about 14-16 kWh of household electricity per day in January, so about 10-12 kWh during the daytime. Therefore, at least 12 kWh should be available for operating heating/domestic hot water and ventilation.
About the ceiling heating: The ceiling on the upper floor, adjacent to the cold attic space, is a wooden beam ceiling with exposed beams in some areas. We do not want to install two systems there, meaning a floor heating system is fixed.
I have wood for free, so that is not a factor.
A good storage heater or masonry stove costs more than 20k. I know people who have one and advise against it because it tends to cause overheating, especially when the sun shines during the day. Our house also has large window areas.
Stiebel promotes this air-to-water heat pump as "can be combined with solar thermal." I probably should contact them to clarify how the hydraulic integration is supposed to work. Or perhaps that only applies to summer operation. ops:
What I will look into is the horizontal ground heat exchanger and the option of connecting the heat pump directly to the heating circuit and buffer tank in parallel.
One big advantage of my solution is that if it gets really cold, I can fully rely on wood heating.
Regarding the horizontal ground heat exchanger versus the air-to-water heat pump: The Stiebel model offered does not have an external heat exchanger. In my opinion, there is no risk of icing in that case.
The photovoltaic system at our location is forecasted to generate 24 kWh per day in January. According to our own recorded actual values, we need about 14-16 kWh of household electricity per day in January, so about 10-12 kWh during the daytime. Therefore, at least 12 kWh should be available for operating heating/domestic hot water and ventilation.
About the ceiling heating: The ceiling on the upper floor, adjacent to the cold attic space, is a wooden beam ceiling with exposed beams in some areas. We do not want to install two systems there, meaning a floor heating system is fixed.
I have wood for free, so that is not a factor.
A good storage heater or masonry stove costs more than 20k. I know people who have one and advise against it because it tends to cause overheating, especially when the sun shines during the day. Our house also has large window areas.
Stiebel promotes this air-to-water heat pump as "can be combined with solar thermal." I probably should contact them to clarify how the hydraulic integration is supposed to work. Or perhaps that only applies to summer operation. ops:
What I will look into is the horizontal ground heat exchanger and the option of connecting the heat pump directly to the heating circuit and buffer tank in parallel.
One big advantage of my solution is that if it gets really cold, I can fully rely on wood heating.
GSGaucho schrieb:
The photovoltaic system at our location has a forecasted yield of 24 kWh per day in January. According to our own recorded actual values, we need about 16 kWh of household electricity per day in January. So, around 8 kWh should be available for heating and ventilation. You shouldn't confuse average values with actual values. What use is the yield if most of it is generated on 5-10 days in January and on the other days there isn’t even enough for household use? My yield this January was 470 kWh, the majority of which was generated on 7 days. January is still quite good. December is more interesting in this regard. If your heat pump is also undersized, it will have to run at night without photovoltaic power.
Also, the actual values probably refer to your current consumption (in the apartment?). In the house, you will additionally have the mechanical ventilation system, heating circulation pumps, etc. This means consumption will increase even without the heat pump.
To give you a rough estimate of how much your photovoltaic can cover your heat pump’s demand… With my 22 kWp photovoltaic system, I was able to cover the heat demand of a KfW40+ house with 240 m² (2580 ft²) at an actual seasonal performance factor of 4.5 as follows:
Nov: 33%
Dec: 28%
Jan: 28%
Feb: 46%
Mar: 66%
The allocation of photovoltaic yields to the heat pump or Hamburg was done proportionally to the demand. That means, if you correctly assign only the leftover photovoltaic energy to the heat pump, the coverage is significantly worse. It won’t be much different for you, maybe even worse due to the higher heat demand with KfW55. If you implement your hydraulic concept as suggested, it will get even worse. Therefore, the heat pump should definitely be operated without a buffer tank, otherwise, your practical seasonal performance factor will be significantly below 3.7.
GSGaucho schrieb:
Regarding ceiling heating: The upper floor ceiling towards the cold attic is a wooden beam ceiling with some exposed beams. We don’t want to install two systems there, so underfloor heating is fixed. That wasn’t the idea. The ceiling heating should be additional, not instead of underfloor heating. Also, ceiling heating is not feasible in wooden beam ceilings. Additional ceiling heating is really more a question of how much you want to improve the heat pump’s efficiency and whether you want efficient cooling as well. Independently of ceiling heating, I would prefer a solid concrete slab if possible.
GSGaucho schrieb:
A good storage heater or masonry stove costs over 20k. And I know people who have one and actually advise against it because it still overheats, especially when the sun is shining during the day. Our house also has large window areas. Compare the air output power of a storage heater (around 2 kW) with the chosen hydronic stove. But I don’t want to discourage you from choosing a hydronic stove. Everyone is free to enjoy their hobby.
GSGaucho schrieb:
Stiebel advertises this air-to-water heat pump as “can be combined with solar thermal…” I should probably contact them to ask how the hydraulic integration is supposed to work. Or maybe that’s only meant for summer operation. oops: I better say nothing about Stiebel. Just as much: I don’t know anyone who operates an efficient heating system with it. But at least the LWZ 5 CS Premium reaches a good theoretical seasonal performance factor. It would be interesting whether Stiebel is more open than other manufacturers. I would appreciate reports. Since the standard version is not listed in the seasonal performance factor calculator, for BAFA support you should take the premium version, if there is a standard one at all.
Best regards, Nika
GSGaucho schrieb:
Stiebel advertises this air-to-water heat pump as "can be combined with solar thermal." I should probably contact them to find out how the hydraulic integration is supposed to work. Or maybe that only applies to summer operation. Oh dear... I just looked at the documentation for the LWZ 5/8CS Premium... I have to warn you right away. Since the heat is taken from the exhaust air, at low temperatures the air volume flow has to be increased, which causes the humidity in the air to drop to an uncomfortably low level. Due to these ventilation losses, there is a higher heating load, which then also has to be covered by a higher supply temperature, further reducing the efficiency of the heat pump. There is a parallel thread where users are desperate because the air becomes too dry. If you do a bit of research, you will quickly understand why such systems are often criticized by experienced heat pump users as just electric direct heating. The most desperate inquiries from heat pump owners are about exactly these types of systems.
At first, I thought this was a regular indoor air heat pump. But it’s not. So please, please, please don’t put yourself through this and look for an alternative. The calculated annual performance factor for these heat pumps is even more unrealistic than the fuel consumption figures for cars.
Best regards,
Nika
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