It sounds like (no pun intended) that the 8m requirement kills any idea of a heat pump.
But, if that can be solved, then using a "minisplit" system would avoid all the problems related to heating up water. Plus it would give you cooling in the summer.
If the actual "8m" distance is a function of noise output, perhaps a minisplit system would enable you to choose a lower powered heatpump to be in spec. Of course, these kinds of noise laws vary greatly but minisplits are used in very densely populated areas.
It may bring its own problems, such as the need to devote some internal wall space to the head unit and to bore a small hole through to the exterior.