"geoff" wrote in message
...
In message , Roger Mills
writes
On 13/06/2010 18:25, John wrote:
I am about to have a new Combi fitted (Worcester Bosch 30cdi). The
installer
and I cannot see the existing gas pipe to the current Worcester Highflow
Heatslave so are not sure if it is 22mm or smaller. (obviously we will
see
it when the boiler is removed)

A neighbour in the same type of house found when his Heatslave was
replaced
that the pipe was only 15mm and had to have a new (ugly external) 22
pipe
run installed. This neighbour never had much success with his
Heatslave -
but mine has always performed well and has been inspected every year by
B
Gas.

I told the installer I was concerned that the pipe could be 15mm so he
offered to perform some sort of drop check. He took a reading at the
meter
with a manometer with the gas cooker and boiler running - he then
repeated
the test at the boiler and phoned Worcester Bosch. They confirmed that
the
pressure was acceptable. I think the drop was 2 milli-bar. (from 21 down
to
19)

I realise there is some risk and uncertainly and that I may have to run
a
new 22mm pipe - but what do you think. Does the 'evidence' point to the
hidden pipe being 22mm - or is this irrelevant because the pressure is
okay?

Do you think the installers approach is reasonable?



Is the capacity of the new boiler the same as that of the existing boiler?

If not, the test is invalid because if the new boiler consumes gas at a
faster rate, the pressure drop will be higher - and the pressure at the
boiler may or may not be within acceptable limits.

I was assuming that the fitter had told WB that a 30CDi was going to be
fitted


--
geoff


I presume so. I wasn't listening. However the fitter recently went on one of
their courses and knew who to speak to.

I take your point about the flow rate. I work in an industry that uses
compressed air and understand something of the relationship between mass
flow and pressure. (Which is why we have a 17inch air pipe!)