Combine heat pump and water-bearing fireplace

We decided on a basic stove without water circulation because our stove fitter told us that the water circulation is repeatedly problematic to clean and that he regularly has to make quite a mess at customers’ houses to keep the system properly running. We didn’t investigate the question of cost-effectiveness any further. We will support the hot water tank with a My-Photovoltaic heating rod controlled by photovoltaics – here one can question the cost-effectiveness and calculate exactly when it comes to the cheapest solution. Our main concern is to achieve the highest possible self-consumption and degree of autonomy, so we are willing to accept the second most economical solution. Currently, I am dealing with wind turbines, which are also somewhat questionable economically. So it is always a question of maintenance effort, cost-effectiveness, and attitude towards renewable energy, where people can have different priorities.

As a layperson, something like that makes sense at first, but I would recommend looking at the principle of a modulating heat pump.

I have, please explain to me why that should not make sense in connection with another heating system?!

We decided on a masonry heater without water circulation because our stove builder told us that the water circulation is always problematic to clean and that he regularly has to make a proper mess at customers' homes to keep the system running properly. We did not further investigate the question of economic efficiency. We will support the hot water tank with a My-Photovoltaic heating rod controlled by the photovoltaic system – here one can question the economic efficiency and do precise calculations when it comes to the cheapest solution. For us, it is primarily about achieving the highest possible self-consumption and degree of autonomy, and we are therefore willing to accept the second most economical solution. Currently, I am dealing with wind turbines, which are also somewhat questionable economically. It is always a matter of maintenance effort, economic efficiency, and attitude towards renewable energy, where people can have different priorities.

We also want to heat as independently as possible.

You can do that independently of the rest of the heating system. No matter what you install. You can also do that, just without the water-carrying gimmicks. Because that had its raison d'être in old buildings, etc. Since the Energy Saving Ordinance exists and houses are built according to it, you don't need that nonsense anymore.

why does the water-carrying fireplace only have its raison d'être in old buildings?

As far as I know, you should not supply cold water to the chimney, but preheated water (~60 C). This, in turn, is unfavorable for a heat pump that operates with floor heating at flow temperatures of ~35-40C.
A chimney generates enormous heat. Ergo, the water which is heated to support the heating is actually too hot for the flow temperatures of the heat pump and is mixed with cold water. It all works, but energetically it doesn't make much sense. Moreover, the cozy effect caused by radiant heat suffers somewhat with this technology, as not as much radiant heat is emitted as with a conventional chimney.
Invest rather in a photovoltaic system. It makes much more sense in combination with a heat pump.
The problem with modern houses in connection with a chimney is that they usually emit too much heat. You quickly turn your living room into a sauna. For example, if we have many people visiting (I can hardly remember when that was last the case, thanks to Corona), the temperature in the living room quickly rises by a few degrees.

why does the water-bearing fireplace only have its justification in old buildings?

Because this pre-war technology has little/no use in modern houses, it is energetically nonsensical. The whole thing is a regulatory nightmare and in the end yields almost nothing. No, not everything that existed before and was considered good is sensible nowadays. Because: - it is hardly used (the house simply becomes too warm too quickly) - the effect is only marginal (the water pockets must not be too large and the water must not get too warm) - the investment and operating costs are high because you need a lot of additional hydraulics which also consume electricity during operation.