Install 100L hot water boiler without expansion tank

Hello, I hope I am in the right place.

I have organized a 100L WMB or WWS (hot water boiler / hot water tank) on eBay Kleinanzeigen (whichever is more correct).
It is supposed to be only for the bathroom where the bathtub is, and there is a toilet and a sink. The bathroom with the shower has its own 80L tank which unfortunately is not enough for the bathtub. There is only about a quarter of warm water in the tub. It seems to be slowly giving up.

I have installed a safety group on the 80L WWS because the shower is pressure-resistant.

On the 100L tank, I would like to install a low-pressure fitting so that the expanding water can be released through it.
Unfortunately, to continue operating the bath stove, I would have to redo the roof and rebuild or install a new chimney. That is too expensive and too much for me at the moment. My wife is now pregnant and I want her to be able to run a warm bath if she wants.

Therefore, the bath stove must (for now) be removed. But now I have no room for the expansion vessel and would therefore like to install this WWS as minimally as possible. Is it enough if I

[*]install a check valve from the controlled residential ventilation to the WWB
[*]go from the check valve to the fitting
[*]then lead from the fitting controlled residential ventilation and WWL to the boiler

so carry out a classic low-pressure installation.

Could you approve it like this?

Best regards

You're actually in the wrong forum for that...

I am very sorry, where should it actually go? Should I write a new post?

Best regards

No, I mean the wrong forum altogether... this is about house building and less about DIY sanitary installations.

Well, maybe Joedreck, he knows about gas water sh..... here

You can operate a pressure tank with a pressure cost fitting.
For this, you do not need a safety group, an expansion vessel, or a non-return valve.
The tank only needs to be higher than the outlet fitting.