Exchange oil heating for heat pump with underfloor heating and wood heating

That's simple: The classic air-to-air heat pumps ("air conditioners") are almost always oversized. There's nothing you can do about it because they simply don't go below 2 kW. And even in an unrenovated old building, at most a huge living room requires 2 kW of heating or cooling capacity. Regarding heating and cooling load: theoretically, they are the same. But only if delta T were equal. And in Germany, in winter there's about a 30° difference between outside and inside, and in summer 15°. So only for cooling could you dimension smaller. But that doesn't exist.

By the way, your assumption is correct: multiple single units (singlesplit) are cheaper, more efficient, and more redundant than a multisplit device. The only disadvantage is the higher number of outdoor units.

If you want to heat with them, you should make sure that the devices really reach down to -15 or -20°. There are often special "arctic" versions from the manufacturers. Also, floor-standing units are quieter when heating because they release warm air at the floor and don't have to blow it downwards first. However, they do have comfort disadvantages when cooling.

So, the plumber just said that the old underfloor heating (OG) cannot be connected directly to the heat pump, as there is a risk of damaging the pump. If I understood correctly, it would have to be connected via a heat exchanger. I don't find that optimal, because that also means a certain temperature difference is lost along the way, but since I don't want to risk damaging the heat pump, I would bite the bullet... I can live with a lower flow temperature upstairs anyway, because other heating options and solar yields are also available.

The question that concerns me more is: what does that mean for the rest of the system? Would it still be possible to connect the new underfloor heating (UG) and the heat exchanger directly to the heat pump without a buffer tank (or at most a small one on the return flow) and thus operate at least that part efficiently? Or are there hydraulic or other reasons against it (I have no idea how such a heat exchanger would affect the overall system)?

What exactly did he name as the reason? It is quite possible that there could be problems without further measures. However, in my lay opinion, this would lead to problems with efficiency and possibly the minimal flow rate. I also know the option of installing microbubble separators and a sludge separator. Specifically in the return flow, so that the heat pump is protected. Whether that is sufficient, I cannot judge and would probably clarify this in a forum specialized in building services engineering.

What exactly did he state as the reason? It is quite possible that there could be problems without further measures. However, in my layman's opinion, this would lead to issues with efficiency and possibly the minimal flow rate. I also know of the possibility to install microbubble separators and a sludge separator. Specifically in the return flow, so that the heat pump is protected. Whether that is sufficient I cannot judge and would probably clarify this in a forum specialized in building services engineering.

It couldn't have been said better. The explanation by the heating guy is unsatisfactory because a reason is missing.