Which storage should be used with an air-to-water heat pump?

Our heating engineer recommends a 200l buffer tank for the 300l domestic hot water tank with the reasoning that the heat pump then runs better and cycles less.

Yes, it IS more difficult to plan and set up a heat pump without a buffer tank so that it runs reliably even without one. Not every heating engineer can do that, and that's why he wants to sell his buffer. "I don't know anything else and we've always done it like this."

It's obvious - if the system runs unreliably, he has a warranty case on his hands. If the system runs inefficiently, he doesn't care, because you pay for the electricity.

We had to install a return buffer tank - people got so upset on the pink forum.

We had to install a return buffer tank - the pink forum was really upset about that .

That makes me curious, why was it necessary? So what is the technical reason?

Specification in the system diagram from Vaillant - without which no guarantee.

The technical reason is that there must be a buffer when all valves in the house are closed and the heat pump then pumps in the heat.

I also found it pointless, my heating technician did too, but Vaillant wanted it. But it doesn't really bother because it is not a classic upstream buffer. Just material costs (the little guy costs almost 400 euros).

So we also received the warranty - but it's possible that it didn't apply to us since we don't have any thermostats....

We also don’t have any thermostats, and whether you really get anything in case of a warranty, you’ll see. We have asked explicitly several times...