In our new burger joint (soon to open now !) the building owners fitted a
new bathroom as part of the conversion process. In there, is a
floor-standing unvented multipoint hot water unit. Not very big. Probably no
more than 20 lt. The cold feed to it comes via a really convoluted route,
looping round past the feed to the cold tap on the hand basin. Initially, I
couldn't figure why anyone would run a pipe from the rising main, all the
way round to the basin, then all the way back to the water heater, which is
right by the rising main. I thought perhaps it was some 'Eastern European'
way of doing it ... :-)

I then obtained a similar unvented water heater for our prep area, which is
rather too far away from the heater in the bog, to sensibly run a pipe back
to. When I came to study the installation instructions, there is a section
about expansion, and it says that the unit uses the incoming cold supply
pipe as its expansion means, so that some 4 m of uninterupted 15mm pipe run,
must be allowed to accommodate this. So I guess this is the answer as to why
the 'loop' is in the pipework in the bog, to provide whatever length is
required to satisfy the needs of that one.

Fair enough. So I've done a similar loopback now on the one that I am
fitting. The thing is, I can't for the life of me figure how this works. How
is any expansion being taken care of, in 4 m of copper pipe, filled with
cold water under mains pressure ? The pipe isn't going to expand, and you
can't compress water, so what's going on ? There is a safety over-pressure
valve supplied with the heater, which has to go on the cold feed close to
the unit. This is preset at 6 bar, so the manufacturers are clearly not
expecting the pressure to rise to anything near that, except under fault
conditions. Anyone know the hydraulic principles involved here ? I'm sure
it's something simple and obvious, but it's evading me at the moment ... !

Arfa