@G33KatWork I don't know specifically for Stiebel Eltron, but on most heat pumps there is additional hardware needed. When it's active cooling, to reverse the refrigerant circuit, or for passive cooling an additional 3way valve and heat exchanger.

that said, there might be an (unofficial) upgrade path if you get your hands on said hardware components. plus your firmware hax 🙂

@lewurm I know about the valve and I checked a few things. There are open questions about this as one pump which can cool has an additional valve, but another one doesn't. I compared a few internal wiring diagrams that I have. The one which can cool but doesn't have an additional valve seems to use the defrosting 4-way valve to reverse the coolant flow.

But I definitely need more insider info to find out what this additional valve that the one pump has does. If you actually need that valve which I don't have it's infeasible to retrofit. The whole refrigerant needs to be replaced in that case etc.
That's why I looked at this in the past and thought it won't work. But then I found the Tecalor THZ 504 which has the same wiring diagram as my LWZ 5S Plus. And the THZ can cool according to the website (inb4: the website is wrong...)

@lewurm The current theory, after reviewing schematics with a neighbor, is that the additional valve is to reroute warm/cool water to allow for warm water generation using a solar panel while cooling the water that goes through the under floor heating using the compressor.

The one pump which can cool and is wired like ours which can't does not have a solar heat exchanger and pump. The other with the additional valve have. So the theory is that this reroutes water instead of actual coolant. We have all the other valves in the refrigerant loop.

I guess it's time to get familiar with the F2MC-16LX core now...