"Mike Hibbert" wrote in message
...
"Roger Mills" wrote in message
...

I don't profess to be an expert on combis - but surely, if you connect
the
output of the combi to a cold supply at mains pressure, you won't have
any
flow through the combi - because the input and output will be at the
same
pressure.



Not sure I understand your first point, what I thought about doing would
be
a "Y" shaped connection with the hot (from combi) and the mains cold
coming
in at the top bits of the Y and then the shower being supplied from the
the
down bit. The water should draw from both supplies?


Er, no! As I understand it, the only thing that makes water flow through the
combi is the pressure difference between input and output. In other words,
the input is at mains pressure and the output is at a much lower pressure -
equal to the back pressure generated by the flow going through the pipework
and open tap(s).

However, if you connect the combi output to a source of mains pressure
through a Y-piece, you immediately raise the output to mains pressure - thus
removing the differential. Don't forget that pressure is homogeneous - and
acts equally in all directions - so there's no inherent reason why flow
can't come down one branch of the Y and go back up the other branch rather
than along the down pipe.

You might still get *some* flow through the combi, because there will be a
small amount of back pressure in the cold feed to the Y-piece, but the vast
majority of the water is going to take the line of least resistance - which
is the direct feed to the Y-piece, by-passing the combi. So you will be
little better off than if you simply took the cold mains straight to the
electric shower.

Roger