I'm in the process of fitting a new sink + mono mixer tap (ie. single
hole mixer). (see
http://www.diy.com/diy/jsp/bq/nav/na...141744 746842)

We have mains cold water and gravity hot water (not gravity heated -
just gravity pressure). It was advertised as being suitable for all
water systems (not mains pressure hot).

Upon reading the instructions it states:

"Water supplies shall be at reasonably balanced pressures from a common
source (eg. hot and cold supplies from the same storage or both from a
supply pipe). Where the fitting is supplied from unbalanced supplies
(eg. hot and cold supplies from separate sources) a "Listed" single
check value or some other no less effective backflow prevention device
shall be fitted immediately upstream of both hot and cold water inlets."

As we have unbalanced supplies, it sounds like we need them - but what
is the purpose? To stop mains pressure water ending up in our hot water
pipes, or to stop contamination going back up the cold water pipe from
the hot water?

If it's the former, then I'll fit one on the hot pipe, if it's the
latter, I'll probably not worry too much (based on advice on here in the
past about their benefits).

Oh, and I've also read that check valves reduce the pressure of the
water. We have quite poor hot water pressure, which I'd like to avoid
making any worse if possible.

Thanks

D