Is this for practical or aesthetic reasons? If practical (i.e. washing hands
without being scalded) you could consider an external thermostatic mixing
valve. These are frequently used in care homes to limit the water
temperature (preset to 40C or less) but are also great for ensuring that the
basin hot tap is a usable hand washing facility without dropping the
temperature on the bath taps (I like these hot for top ups) and kitchen tap
(hot good for dishes). Not as cheap as a new mixer tap, but cheaper than
changing the basin for a monobloc capable one.
Before you go ahead, check whether the hot and cold water supplies are
at the same pressure. Both low pressure (from loft cistern supply) is
fine, and presumably both high (from a combi boiler?) is also OK, but
mixing low-pressure hot and high-pressure cold is more problematic.
I actually did this myself in the last place, which involved
installing check valves in both suplies and a pressure reducing valve
in the cold. That worked reasonably well except that the flow through
the pressure reducing valve tended to slow down through time and need
readjusting. (possibly due to scale buildup.)
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