Heat pump LWZ 403 Sol high supply + return in solar thermal. Underfloor heating is malfunctioning...

Thank you for your reply!
I’m grateful for any help/idea :)

What do you mean by the compressor runs 1 sec.? You mean every time the integral was reset it twitched? Seen live?

Actually, I started to deal with FHEM after I noticed these short compressor starts and wanted to know what the heat pump is doing there... So yes, witnessed live multiple times :(
Compressor twitches briefly (turns on, starts and honestly after about a second, actually less, it turns off again and then stays off for a while until this happens again).

Can you exclude a hydraulic short circuit? How about ERR and ÜSV?

How can I best exclude a hydraulic short circuit? ERR is deactivated in almost all rooms (except one) and flow seems to be there at least according to the underfloor heating distributors, since the floors also get measurably warm. Even if I close all rooms with ERR (that’s how it was with the previous owners), the hallways and bathrooms are always heated – that alone should allow flow.
However, I have also considered whether something is not 100% right with the heating pump, since it audibly squeals and maybe sometimes doesn’t move the water away fast enough? (Short circuit?!)
What can I check/do with the ÜSV (overflow valve?!)?

Does the compressor always run normally long with the solar pump? Or is that just coincidence now?
What values do you have set for I and P?

The compressor runtime was actually rather coincidence I think (maybe it was colder AT, too little heat from the collector and therefore the compressor kicked in too)
What do you mean by I and P values?

Are you actually running 100% return flow controlled?

Currently with default 30/70 setting.

Thanks for the finger-pointing forum tip, I’ll have a look there as well!

Okay, I just saw that you showed up in the HTD, very nice.
And with Bignose, an owner of an older LWZ (forget my question about the P portion - you do have an ON/OFF compressor) also bit right away, who has dealt intensively with his heating system, has the basic knowledge and explains it well too, perfect! :)

To me, that sounds like some kind of error, you really can’t generate a half-second Vd cycle with settings, I would say.
If it’s due to any safety shutdowns, there should actually be an error every time.

Somehow it sounds to me as if the Vd suddenly loses some release parameter, or it only queries it at startup. But that’s just speculation.
If I had to guess, I’d say some measurement is defective/intermittent and sometimes works properly.

Well, since you are now in far more competent hands, I’ll just read along “over there” with interest and look forward to the solution.
And I hope you won’t need a Stiebel Eltron service call - they supposedly have absolutely ridiculous wait times (at least where we are).