In message , Adrian
writes

I have something similar but not Ariston. I suspect these are imports
from a country with rather different rules on plumbing installation.

Do you mean that the Ariston is from a country with different plumbing
regs ?

Mine came from Screwfix!

IME anything with more than one language on the instructions is likely
to be an import.

I spoke to one UK manufacturer (forgotten who now) in an attempt to
establish why it was necessary for the feed to be from the cold supply
and failed to get a clear explanation. The obvious issue is the risk of
a local *shut off* valve leading to over pressure. ISTM a thermostat
failure is more likely, where putting hot water/steam back into the
mains supply system ought to be less attractive than into a vented hot
water cistern. Oh well! None of this is relevant to your borehole set
up.

regards

--
Tim Lamb