Air heat pump and water-bearing fireplace - experiences

: 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 heat storage mass. That's why I decided on a completely water-jacketed boiler, which was installed and bricked in by the stove builder. That means I have 700-800kg of heat storage mass. According to the datasheet, the boiler has 10 kW water-side and 4 kW air-side power, but it can comfortably and cleanly be operated at about 4-5 kW on the water side. Then only heat emission through the glass is noticeable. On the sofa 4 meters away it gets a bit warmer, but the room doesn't overheat. And nothing beats the cozy warmth in the morning when you come downstairs and the stove is still radiating just a little residual heat.

Of course, if I fully load it 3 times and burn with the air completely open, I have a sauna despite the 60 sqm installation room.

Mathematically, the investment only pays off after 20 years with free wood and firing every day. So it's more of a hobby than a financially calculated purchase.

...it's rather 40/60. So there are 4 kW left in the installation room. If I now want to charge my 1000 liter storage tank from 40 to 70 degrees, the stove has to burn for 5 hours.

You calculated with 7 kW on the water side and also did not consider that the heating system continues to run during the firing (ongoing heating load). To heat the 1000 liters from 40 to 70 degrees you need (as correctly calculated) about 35 kWh. That would be about 6 hours * 6 kWh (without ongoing heating load). The full efficiency of the stove is probably only reached under laboratory conditions and does not start from minute 1 of ignition. For 35 kWh you need about 9 kg of beech wood (15% dry, losses not considered). How often would you have to reload? Furthermore, it is questionable whether the buffer really already has 40 degrees at the start of heating. The return temperatures of the underfloor heating are around 20-25 degrees. Unfortunately, I don't think much of water-based fireplaces. I have a 30 kW wood gasifier with 2000 l buffer + gas boiler as backup myself.

A wood gasifier is definitely the most efficient way to heat with wood. However, after long consideration, we decided against it for the following reasons:
- We did not want a 2,000L buffer tank in the basement
- We planned 2 fireplaces in the house and also enjoy watching the fire
- A backup heating system is necessary in any case for us as well. The additional cost of the wood gasifier is not worthwhile if you ultimately install more fireplaces in the house anyway.

But I also believe that a water-guided fireplace is worthwhile, especially in combination with an air heat pump, since on cold days when the heat pump operates very inefficiently, the fireplace supports or even covers the entire heating load. In our case, both locations for the fireplaces are quite large and since we also have a controlled residential ventilation system, the heat is distributed throughout the house...
I have no concerns about overheating the rooms and, as I said, I also believe that this setup is definitely worthwhile, especially when you can get wood cheaply...

Good morning everyone,
thank you very much for the many responses. Unfortunately, I was only able to reply today.

The 3 different trades, that all comes from one company, they have the employees who have done this now and then (how experienced and competent they are,... I don't know, after all, it is really not that easy from the control engineering side).
I also misstated something regarding the storage. You are right: For a system that is operated only with a heat pump, there is only the "small" hot water storage tank with heating and domestic water (or possibly with a fresh water station).
For the system with the fireplace, a larger combination storage tank is used, whereby here only the heating water flows through it. The domestic water is heated with a fresh water station.

@ Specki: During the planning phase, no one could tell me how much wood I need. At the time, I said that I would not install a wood stove before anyone could tell me whether I have to use 2 or 20 cubic meters of wood (to go to the extreme). But a chimney was extended down to the basement to have the option to switch to wood at some point.

@ Fragg: The air-side output of the stove is the problem for me: If I place a small 3-4 kW stove in the living room, it will probably get too warm. I would prefer to place a (for example) 5 kW stove in the living room and feed 3 kW from it into the heating system (the numbers are just thrown out there). Whether that is enough to warm the storage tank sufficiently, I would have to ask the heating engineer. I am not enough of an expert for that and would have to rely on the statements.

@ kkk and PhiTh: Here you already have two opposing opinions^^. Difficult matter to decide whether to do it or not. I can understand both sides. After all, it is additional costs and the control is not the easiest. Unfortunately, no one has so far been able to explain to me why the system is bad (except for the complicated control engineering). There are definitely more arguments in favor, even if these arguments are relatively vague.

@ Specki: During the planning phase, no one could tell me how much wood I would need. I had said back then that I wouldn’t install a wood stove before anyone could tell me whether I’d need 2 or 20 rm of wood (to go to the extreme). But a chimney was laid all the way down to the basement to have the option to switch to wood at some point .

I find that strange though. You surely have a calculation of the heating demand your house will require. Or it can be calculated quite easily. According to which standard did you build? For example, Kfw 70: Means heating demand: ≤ 45 kWh/(m²·a) I think you didn’t say anything about your house area, I’ll calculate with 150 sqm now. That results in a heating demand of 6750 kWh. Let’s say the calorific value of the wood is 1700 kWh per Ster (mixed beech + spruce). Then you theoretically need about 3.9 Ster. Then we calculate an efficiency of 90% and other losses of 90% as well, then you are at 5 Ster. Then we say a bit more for hot water, a bit of safety margin, then you should be at a maximum of 7 Ster per year. That is manageable in terms of workload and if you have the wood yourself, basically no costs. No guarantee that I used the correct numbers, but I think that should fit.

The question is whether it is worth the effort for the few euros in heating costs per month.

In addition, the 7 Ster then still have to be carried across the apartment to the stove.