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the threads I have found so far (here and elsewhere) have a rather negative view of the system (many are also somewhat outdated).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
You need to coordinate three trades that you often won’t find combined under one contract:
- Stove setter
- Heating engineer who doesn’t just follow standard templates
- Control technology specialist
If you don’t have a fireplace, you need a domestic hot water storage tank (not a buffer tank) of sufficient size for the heat pump. Ideally, this includes a fresh water station.
The heat pump system should NOT require a buffer tank.
The fireplace is then connected via return temperature boost and a switching valve to a buffer tank used exclusively for the fireplace. You will need an appropriate control system for this.
You also need a proper hydraulic balancing with correctly sized heating pipework. Individual room controllers are usually unnecessary and counterproductive in most rooms.
A combination buffer tank has the exact disadvantage that you generate heating water during domestic hot water mode (lower efficiency) because the hydraulics of both circuits are linked through the buffer tank.
Standard heating system manufacturers’ setups usually provide a combination buffer tank and individual room controllers but no return temperature boost.
I don’t want to discourage you, but you can see there are many interfaces, a lot of additional technology, and above all, trades outside their usual specialties that need to work together.
PS: I have all of this myself, but only because I enjoy it and specifically handled the tuning and control technology personally.
- Stove setter
- Heating engineer who doesn’t just follow standard templates
- Control technology specialist
If you don’t have a fireplace, you need a domestic hot water storage tank (not a buffer tank) of sufficient size for the heat pump. Ideally, this includes a fresh water station.
The heat pump system should NOT require a buffer tank.
The fireplace is then connected via return temperature boost and a switching valve to a buffer tank used exclusively for the fireplace. You will need an appropriate control system for this.
You also need a proper hydraulic balancing with correctly sized heating pipework. Individual room controllers are usually unnecessary and counterproductive in most rooms.
A combination buffer tank has the exact disadvantage that you generate heating water during domestic hot water mode (lower efficiency) because the hydraulics of both circuits are linked through the buffer tank.
Standard heating system manufacturers’ setups usually provide a combination buffer tank and individual room controllers but no return temperature boost.
I don’t want to discourage you, but you can see there are many interfaces, a lot of additional technology, and above all, trades outside their usual specialties that need to work together.
PS: I have all of this myself, but only because I enjoy it and specifically handled the tuning and control technology personally.
In general, I think the combination is great. Just as you said, at the time when the air source heat pump operates most inefficiently, the fireplace provides support. That’s exactly how we planned it as well. If you also have your own forest or can get wood cheaply, and you have the time and interest for chopping wood, even better.
We had few issues with the "interfaces." We clearly defined that the stove installer assembles the stove, the heating technician connects it and provides the buffer tank, etc. There were no discussions at all, except about who installs the differential pressure monitor. However, there are stove installers who want to supply the buffer tank as well.
We haven’t put anything into operation yet, so I can’t say anything specific about the combination of the air source heat pump and the fireplace. However, I have several family members who use a water-bearing fireplace.
If you want a masonry fireplace, in my experience the additional cost is somewhat higher than the mentioned 4,000€ plus the buffer tank surcharge.
I don’t know your heat demand or conditions. You need to find a suitable stove that transfers as much heat as possible into the water. The larger your stove, the more worthwhile it is. You will have costs for the connection, a larger buffer tank, etc., regardless of whether your fireplace delivers 3 or 10 kW into the water. I would definitely recommend using a larger buffer tank. I consider 500 liters (130 gallons) too small.
We had few issues with the "interfaces." We clearly defined that the stove installer assembles the stove, the heating technician connects it and provides the buffer tank, etc. There were no discussions at all, except about who installs the differential pressure monitor. However, there are stove installers who want to supply the buffer tank as well.
We haven’t put anything into operation yet, so I can’t say anything specific about the combination of the air source heat pump and the fireplace. However, I have several family members who use a water-bearing fireplace.
If you want a masonry fireplace, in my experience the additional cost is somewhat higher than the mentioned 4,000€ plus the buffer tank surcharge.
I don’t know your heat demand or conditions. You need to find a suitable stove that transfers as much heat as possible into the water. The larger your stove, the more worthwhile it is. You will have costs for the connection, a larger buffer tank, etc., regardless of whether your fireplace delivers 3 or 10 kW into the water. I would definitely recommend using a larger buffer tank. I consider 500 liters (130 gallons) too small.
If you want to heat (also) with wood and even have your own forest, why not just go for a wood gasification boiler? The chimney is already there anyway, and you’ll need a large buffer tank as well. You avoid the dirt inside the house and save yourself the entire heat pump system. Yes, the downside is you have to light the stove approximately every 2 days (depending on the house’s insulation) to keep it warm. That’s just the trade-off. If you’re away for a longer period and can’t do that, you can use an electric heater to bridge those days and maintain the temperature with electricity alone.
That would be my preferred heating system. And with your own forest, the operating costs for the few cubic meters of wood you need per year are practically zero…
Best regards
Specki
That would be my preferred heating system. And with your own forest, the operating costs for the few cubic meters of wood you need per year are practically zero…
Best regards
Specki
We are currently building as well and had considered this. But the problem is: what do you do with the air-side output of the stove? For example, the Olsberg Tolima has 10 kW, of which 70% is supposed to be water-side. According to forum discussions, it’s more like 40/60. That leaves 4 kW in the room where the stove is installed. If I want to charge my 1,000-liter (264-gallon) storage tank from 40 to 70 degrees Celsius (104 to 158 degrees Fahrenheit), the stove needs to burn for 5 hours straight. That means 20 kW of heat is released into the stove room. This only works in an uninsulated old building.
We are installing the smallest stove we can find. Even though it still has 4–5–6 kW air-side output, I don’t *have* to burn it for 5 hours straight for the system to pay off eventually.
We are installing the smallest stove we can find. Even though it still has 4–5–6 kW air-side output, I don’t *have* to burn it for 5 hours straight for the system to pay off eventually.
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