"Steve" wrote in message
...

Have the cold line to the
showers off the 22mm line to the combi, just before the combi. Then if a
pressure drop around the combi there will be equal pressure to the
showers hot & cold.

Interesting idea. However there will be some extra resistance to flow for
the water passing through the combi's heat exchanger.. Also, in my case,
there will be a much longer path for the water going through the combi
compared to the shower's cold supply.. If the combi and shower cold
supplies are both 22m, then the cold would end up at a higher pressure
than the hot. The answer is a thermostatic shower!

Get the situation. 22mm from stoptap to combi. Tee at stoptap and all cold
off this in 15mm. A restrictor on this line, or flow regulator. So you
don't want the shower's cold off this cold line, as if a dishwasher kicks in
the shower's cold will be robbed and DHW will get too hot, cause a temporary
hot blip at the shower which may scald.

The DHW will have a higher resistance than the cold as it runs through the
combi. However, some have less resistance than others, and such great issue.
Do not have 22mm cold to the shower, only 15mm. When there is a pressure
fluctuation around the combi both the DHW and shower cold will be affected
and rise up and down pretty well at teh same rate. The mixer must be "combi
compatible", with a metal thermostatic strip, not a wax operated cartridge.
Best if it has an integral pressure balancing valve, as a combi gives a
reasonable stable temperature output.