Hello everyone,
to be direct:
We are currently planning the heating system for our new build. It will definitely include an air-source heat pump with an internal heat exchanger.
The question now is whether to install a water-bearing stove (wood stove with water jacket) to support the heating system.
The big advantage: during times when the heat pump operates inefficiently (evenings in the cold season), the stove can provide support. Other support systems tend to operate effectively only when they are not directly needed (photovoltaics → sunny and usually warmer, solar water heating → summer / sunny winter days).
Our advantage is that we own our own woodland.
What surprises me is that most of the threads I have found so far (here and elsewhere) have a rather negative view of the system (many are also somewhat outdated).
This seems to be based on several points:
* the hot water storage tank must be larger → the heat pump heats a lot of “unnecessary” water
* does not work as expected
* higher investment costs do not pay off
* and a notable comment saying “it’s simply a bad combination”
However, with my theoretical layperson knowledge, I think:
There are water storage tanks of (just to throw a number out there) 500 liters (130 gallons), where the stove feeds water at the bottom and heats the full 500 liters, while the heat pump only heats the top 150 liters (40 gallons). Therefore, theoretically, the extra effort for the heat pump should not exist.
The point about “does not work” can only be explained by control technology in my case—meaning somewhere a small error was made during installation (for example, even when the stove heats the storage, the heat pump is still running simultaneously).
The higher investment costs can be easily calculated. My estimate would be: +2,500 euros for additional installation (pumps, larger tank, piping, installation, etc.) and 1,500 euros more for the stove. This is just an estimate.
And I absolutely cannot understand the point about the “bad combination.” The heat pump gets relieved during its least efficient operating times.
What I’m also thinking: I’m unsure whether a normal stove with 3-4 kW (10,000-13,600 BTU), if placed in the living room, might be too large and cause overheating. It would therefore be good to divert any “excess” heat into the heating system. Unless, of course, you leave doors open and the stove heats the whole house, assuming that works thermally (the stove is located behind a small 90-degree corner).
Has anyone here already had experience with this system? I’m a bit stuck because I personally think it’s a good solution, but the many negative threads have made me hesitate about whether it’s worthwhile.
Best regards
to be direct:
We are currently planning the heating system for our new build. It will definitely include an air-source heat pump with an internal heat exchanger.
The question now is whether to install a water-bearing stove (wood stove with water jacket) to support the heating system.
The big advantage: during times when the heat pump operates inefficiently (evenings in the cold season), the stove can provide support. Other support systems tend to operate effectively only when they are not directly needed (photovoltaics → sunny and usually warmer, solar water heating → summer / sunny winter days).
Our advantage is that we own our own woodland.
What surprises me is that most of the threads I have found so far (here and elsewhere) have a rather negative view of the system (many are also somewhat outdated).
This seems to be based on several points:
* the hot water storage tank must be larger → the heat pump heats a lot of “unnecessary” water
* does not work as expected
* higher investment costs do not pay off
* and a notable comment saying “it’s simply a bad combination”
However, with my theoretical layperson knowledge, I think:
There are water storage tanks of (just to throw a number out there) 500 liters (130 gallons), where the stove feeds water at the bottom and heats the full 500 liters, while the heat pump only heats the top 150 liters (40 gallons). Therefore, theoretically, the extra effort for the heat pump should not exist.
The point about “does not work” can only be explained by control technology in my case—meaning somewhere a small error was made during installation (for example, even when the stove heats the storage, the heat pump is still running simultaneously).
The higher investment costs can be easily calculated. My estimate would be: +2,500 euros for additional installation (pumps, larger tank, piping, installation, etc.) and 1,500 euros more for the stove. This is just an estimate.
And I absolutely cannot understand the point about the “bad combination.” The heat pump gets relieved during its least efficient operating times.
What I’m also thinking: I’m unsure whether a normal stove with 3-4 kW (10,000-13,600 BTU), if placed in the living room, might be too large and cause overheating. It would therefore be good to divert any “excess” heat into the heating system. Unless, of course, you leave doors open and the stove heats the whole house, assuming that works thermally (the stove is located behind a small 90-degree corner).
Has anyone here already had experience with this system? I’m a bit stuck because I personally think it’s a good solution, but the many negative threads have made me hesitate about whether it’s worthwhile.
Best regards
@fragg: That’s right. All "sheet metal" stoves (even the higher-quality ones) release a relatively large amount of heat into the air because they only have an attached heat exchanger and relatively little thermal mass. For this reason, I chose a fully water-jacketed boiler, which was installed and built into the masonry by the stove maker. That means I have 700-800kg (1,540-1,760 lbs) of thermal mass. According to the datasheet, the boiler provides 10 kW to the water and 4 kW to the air, but it can easily be operated at about 4-5 kW water side without getting dirty. Then, heat is mainly released through the glass. Sitting on the sofa 4 m (13 ft) away it does get a bit warmer, but the room doesn’t overheat. And there’s nothing like the cozy warmth you feel the next morning, when you come down the stairs and the stove is still radiating just a little residual heat.
Of course, if I completely fill it three times and burn it with the air intake fully open, despite having a 60 m² (645 ft²) installation room, it feels like a sauna.
From a calculation standpoint, the investment only becomes profitable after 20 years if you have free wood and fire it every day. So it’s more of a hobby than a financially calculated choice.
Of course, if I completely fill it three times and burn it with the air intake fully open, despite having a 60 m² (645 ft²) installation room, it feels like a sauna.
From a calculation standpoint, the investment only becomes profitable after 20 years if you have free wood and fire it every day. So it’s more of a hobby than a financially calculated choice.
K
kkk27272923 Jan 2018 10:26fragg schrieb:
... it's rather a 40/60 split. So that leaves 4 kW in the installation room. If I want to heat my 1000-liter (264-gallon) storage tank from 40 to 70 degrees Celsius (104 to 158°F), the stove needs to run for 5 hours. You calculated 7 kW on the water side but didn’t take into account that the heating system continues to run during firing (ongoing heating load).
To heat 1000 liters (264 gallons) from 40 to 70 degrees Celsius (104 to 158°F) you need approximately 35 kWh (as you correctly calculated).
That would take about 6 hours at 6 kW (without considering the ongoing heating load).
The stove’s full efficiency is probably only reached under lab conditions and does not start from the first minute of ignition.
For 35 kWh, you need about 9 kg (20 lb) of beech wood (15% moisture, losses not considered). How often would you need to reload?
Furthermore, it’s questionable whether the buffer is really at 40 degrees Celsius (104°F) when heating starts. The return temperatures of the underfloor heating are usually around 20–25 degrees Celsius (68–77°F).
Personally, I am not a fan of water-based fireplaces.
I have a 30 kW wood gasification boiler with a 2000-liter (528-gallon) buffer tank plus a gas boiler as backup.
A wood gasification boiler is definitely the most efficient way to heat with wood. However, after careful consideration, we decided against it for the following reasons:
- We did not want a 2,000-liter (530-gallon) buffer tank in the basement
- We planned two fireplaces in the house and also enjoy watching the fire
- A backup heating system is necessary for us regardless. The additional cost of a wood gasification boiler is not worthwhile if you end up installing more fireplaces in the house anyway.
I also believe that a water-based fireplace is especially worthwhile in combination with an air-source heat pump, because on very cold days, when the heat pump becomes inefficient, the fireplace can support or even cover the entire heating load. In our case, both fireplace locations are quite spacious, and since we also have a controlled ventilation system, the heat distributes well throughout the house.
I have no concerns about overheating the rooms, and as mentioned, I think this setup definitely makes sense, especially if wood is available at a low cost.
- We did not want a 2,000-liter (530-gallon) buffer tank in the basement
- We planned two fireplaces in the house and also enjoy watching the fire
- A backup heating system is necessary for us regardless. The additional cost of a wood gasification boiler is not worthwhile if you end up installing more fireplaces in the house anyway.
I also believe that a water-based fireplace is especially worthwhile in combination with an air-source heat pump, because on very cold days, when the heat pump becomes inefficient, the fireplace can support or even cover the entire heating load. In our case, both fireplace locations are quite spacious, and since we also have a controlled ventilation system, the heat distributes well throughout the house.
I have no concerns about overheating the rooms, and as mentioned, I think this setup definitely makes sense, especially if wood is available at a low cost.
Good morning everyone,
thank you very much for the many replies. Unfortunately, I could only respond today.
The three different trades all come from the same company; they have staff who have done this kind of work occasionally (how experienced and competent they are... I don’t know, especially since the control technology is really not that simple).
I also gave incorrect information regarding the storage tank. You are right: For a system run solely by a heat pump, there is only the "small" hot water tank for both heating and domestic hot water (or possibly with a fresh water station).
In the system with the fireplace, a larger combined storage tank is used, but here only the heating water circulates. The domestic hot water is heated with a fresh water station.
@Specki: During the planning phase, no one could tell me how much wood I would need. I had said back then that I would not install a wood stove unless someone could at least give me an estimate whether I’d need 2 or 20 rm (solid cubic meters) of wood (just to give an extreme example). But a chimney was installed down to the basement to keep the possibility open to switch to wood in the future.
@Fragg: The air-side output of the stove is the problem for me: If I put a small 3–4 kW stove in the living room, it will probably get too warm. I would prefer, for example, a 5 kW stove in the living room and feed 3 kW of that into the heating system (these numbers are just thrown out as an example). Whether that is enough to warm the storage tank sufficiently, I would need to ask the heating engineer. I am not an expert and would have to rely on their statements.
@kkk and PhiTh: Here we already have two opposing opinions ^^. It’s a difficult decision whether to do it or not. I can understand both sides. After all, this involves additional costs and the control system is not the simplest. So far, no one has been able to explain to me why the system would be bad (except for the complicated control technology). There are more arguments in favor, although these arguments are somewhat vague.
thank you very much for the many replies. Unfortunately, I could only respond today.
The three different trades all come from the same company; they have staff who have done this kind of work occasionally (how experienced and competent they are... I don’t know, especially since the control technology is really not that simple).
I also gave incorrect information regarding the storage tank. You are right: For a system run solely by a heat pump, there is only the "small" hot water tank for both heating and domestic hot water (or possibly with a fresh water station).
In the system with the fireplace, a larger combined storage tank is used, but here only the heating water circulates. The domestic hot water is heated with a fresh water station.
@Specki: During the planning phase, no one could tell me how much wood I would need. I had said back then that I would not install a wood stove unless someone could at least give me an estimate whether I’d need 2 or 20 rm (solid cubic meters) of wood (just to give an extreme example). But a chimney was installed down to the basement to keep the possibility open to switch to wood in the future.
@Fragg: The air-side output of the stove is the problem for me: If I put a small 3–4 kW stove in the living room, it will probably get too warm. I would prefer, for example, a 5 kW stove in the living room and feed 3 kW of that into the heating system (these numbers are just thrown out as an example). Whether that is enough to warm the storage tank sufficiently, I would need to ask the heating engineer. I am not an expert and would have to rely on their statements.
@kkk and PhiTh: Here we already have two opposing opinions ^^. It’s a difficult decision whether to do it or not. I can understand both sides. After all, this involves additional costs and the control system is not the simplest. So far, no one has been able to explain to me why the system would be bad (except for the complicated control technology). There are more arguments in favor, although these arguments are somewhat vague.
Crimson schrieb:
@ Specki: During the planning phase, no one could tell me how much wood I would need. At that time, I said I wouldn’t install a wood stove before anyone could tell me whether I would need 2 or 20 cubic meters of wood (just to take it to the extreme). But a chimney was installed all the way down to the basement to keep the option open to switch to wood heating someday .I find that strange. Surely you have a calculation of the heating demand your house will require. Or it can be calculated quite easily.
What standard did you build to?
For example, KfW 70:
Means heating demand: ≤ 45 kWh/(m²·a)
I don’t think you mentioned your house area, so I’ll assume 150 m² (1600 sq ft). That results in a heating demand of 6750 kWh.
Let’s say the calorific value of the wood is 1700 kWh per cubic meter (mixed beech + spruce).
Then theoretically you would need about 3.9 cubic meters. Then accounting for a 90% efficiency rate and another 90% for other losses, you end up at 5 cubic meters. Adding a bit for hot water and some safety margin, you should be at a maximum of 7 cubic meters per year. That is manageable in terms of work, and if you have your own wood, virtually no cost.
No guarantee I used the exact correct figures, but I think this should be roughly right.
K
kkk27272924 Jan 2018 10:55The question is whether it’s worth the effort for just a few euros in heating costs per month.
In addition, the 7 cubic meters of wood then have to be carried across the apartment to the stove.
In addition, the 7 cubic meters of wood then have to be carried across the apartment to the stove.
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