Air heat pump and water-bearing fireplace - experiences

Hello everyone,

just to ask directly:
In our new build, we are currently planning the heating. It will definitely be an air source heat pump with an internal heat exchanger.
The idea now is whether to install a water-bearing fireplace directly to support the heating.
The big advantage: At times when the heating operates inefficiently, it is supported by the fireplace (in the evening during the cold season). All other support systems work effectively when they are not immediately needed (photovoltaics -> sunny and therefore mostly warmer, solar water heating -> summer / sunny days in winter).

Our advantage is also that we have our own forest.

What surprises me is that a large part of the previous threads I have found (here and elsewhere) have a relatively negative attitude towards the system (many are also somewhat older).
Apparently, this comes from several points:
* the hot water storage tank has to 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 nice point with "is just a bad combination"

However, I think with my theoretical layman knowledge:
There are water tanks of (just throwing a number out there) 500 liters, into which the fireplace is fed at the very bottom and heats the entire 500 liters and the heat pump only heats the upper 150 liters. Therefore, the additional effort for the heat pump should theoretically not exist.

The point about "does not work" can only be explained to me by the control technology, that somewhere a small installation error was made (for example: even if the fireplace heats the tank, the heat pump also heats).

The higher investment costs can simply be calculated. My approach would be: +2,500€ for the additional installation (pumps, larger tank, piping, installation, ...) and 1,500€ additional cost for the fireplace. However, this is only an estimate.

And the point "bad combination" I absolutely cannot understand. The heat pump is relieved in its most unfavorable times.

What I also think: I am unsure whether a normal fireplace with 3-4 kW, if it stands in the living room, is already too large and overheats the living room. It would therefore also be nice to dissipate the "excess" heat and feed it into the heating. Unless, of course, you leave the doors open and the fireplace heats the whole house, as long as it works thermally (the fireplace stands behind a small 90° corner).

Has anyone here already had experience with the system? I am a bit stuck because I personally think it is a good system, but because of the many negative threads I am somewhat unsure whether it is worth it.

Best regards

You have to bring together three trades that you very often cannot get from a single source:
- Stove builder
- Heating engineer who does not just follow a standard procedure
- Control technology

Without a chimney, you need a domestic hot water storage tank (not a buffer tank) in sufficient dimensioning for the heat pump. Ideally with a fresh water station.
The heat pump heating system should NOT require a buffer.
The chimney is then connected to a buffer exclusively for the chimney via return flow lifting and a switching valve. Here you need a suitable control system.
Then you also need a real hydraulic balancing with properly dimensioned heating pipework. The individual room controllers in most rooms are not needed and are counterproductive.

A combined buffer has exactly the disadvantage that you produce heating water in domestic hot water mode (poor efficiency) because the hydraulics of both circuits are connected through the buffer.

Standard schemes from heating manufacturers usually provide a combined buffer and individual room controllers and no return flow lifting.

I don't want to scare you, but you see many interfaces, a lot of additional technology and above all non-specialist trades that have to work together.

PS: I have all of this, but only because I enjoy it and specifically did the coordination and control technology myself.

In general, however, the combination is great in my eyes. Just as you said, at the time when the air heat pump operates most inefficiently, it is supported by the fireplace. That is exactly how we planned it. If you also have your own forest or can get wood cheaply and have the time and desire to do wood chopping, perfect.
We have few problems with the "interfaces". We clearly defined it: the stove builder installs the stove, the heating engineer connects it and supplies the buffer tank, etc. There were no discussions at all, except for who installs the underpressure monitor. But there are also stove builders who want to supply the buffer tank.

We have not put anything into operation yet, so I cannot say anything specifically about the combination of air heat pump and fireplace. However, I have some family members who use a water-bearing fireplace.
If you want a masonry fireplace, in my experience the surcharge is somewhat higher than the mentioned €4,000 + surcharge for the buffer tank.

I do not know your heating load and your conditions. You have to find a suitable stove that brings as much heat into the water as possible. The bigger your stove is, the more it pays off. You have costs for the connection, a larger buffer tank, etc., regardless of whether your fireplace delivers 3 or 10 kW into the water. But I would definitely advise you to use a larger buffer tank. I consider 500 liters to be too small.

If (also) heating with wood is intended and you even have your own forest, why not just get a wood gasifier?
The fireplace is already there anyway, and you need a large buffer tank as well.
You save yourself the dirt in the apartment and you save yourself the entire heat pump.
Yes, you have to light the stove about every 2 days (depending on the house’s insulation) to keep it warm. That’s the downside. If you’re away for a longer time and can’t do that, you can bridge those days with an electric heating rod and keep the place at temperature with electricity alone during the few days.

That would be my favorite heating system
And with your own forest, basically with almost zero running costs for the few cubic meters per year you need...

Regards
Specki

Asked the other way around, how big will the house be and roughly what heating load are we talking about? And what targeted hot water demand and which standard [AT]?

We are also currently building and had considered that as well. But the problem is: What do you do with the air-side output of the stove? The Olsberg Tolima, for example, has 10 kW, of which 70% is supposed to be water-side. If you believe the forums, it is more like 40/60. So 4 kW remain in the installation room. If I now want to heat my 1000-liter storage tank from 40 to 70 degrees, the stove has to burn for 5 hours. But then I have pumped 20 kW of heat into my installation room from the stove. That only works in an uninsulated old building.

We are installing the smallest stove we can find. It then also has 4-5-6 kW air-side, BUT I "don't have to" let it burn for 5 hours straight for my setup to eventually pay off.