"Christian McArdle" wrote in message
. net...
If I wanted to use the combi side of a boiler for running 2 showers (and
maybe the kitchen tap) only, would 15mm be big enough or would it
restrict
flow?

15mm is marginal. However, you might get away with it if the pressure and
maximum flow rate is high, but you'd have to be lucky and there could well
be some interaction with the kitchen tap (i.e. scalding/freezing of shower
occupant when kitchen tap is used).

It would be much better to run a new 22mm pipe from the combi input back
to
the stopcock. Run 15mm from the combi to the hot taps, to reduce lag and
energy wastage.

Also restrict the sink taps to reduce influence on the showers.