Why no buffer? [...]
Are there any disadvantages besides the heat loss (which stays in the house anyway) that I am overlooking?
Yes, there are disadvantages. These have also been proven in practice by the Fraunhofer study. Search for it. It is even
boldly highlighted there.
Essentially, a buffer causes a higher flow temperature at the heat pump. That can make a difference of 5K or more. For every 1K, you have to reckon with about a 2.5% efficiency loss. That means at 5K, the efficiency would be 12.5% lower.
Also, why do you need a buffer?
The screed is already a much better buffer. In it, you can easily absorb the photovoltaic surpluses that occur rather rarely during the heating season by increasing the flow temperature by 1-2K. You won’t really notice that on the room temperature, if at all.
Otherwise, regarding efficient operation of the heat pump, a forum specialized in building services engineering would be a better place to get information. Because I would suspect that instead of a buffer, an outdoor reset valve is installed with you, which also lowers efficiency. Moreover, there are other measures to increase efficiency. If you optimize the operating mode of your heat pump, you will get more out of it than through the buffer.
Oh yes, independent of efficiency. Since a buffer costs money, the storage costs in a buffer would be higher than the costs for feeding in and buying back photovoltaic electricity. That is also one of the reasons why the specific heat generation costs of a solar thermal system are so high.
Regards Nika