On Sun, 13 Jun 2010 18:25:16 +0100, 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?

Possibly not, depending whether the inlet pressure was measured at the
pipework entering the boiler or within the boiler itself (where it would
be lower due to the boiler's inlet connector and pipework). 2mbar is
twice the allowable pressure drop. For an existing installation the excess
pressure drop is allowable as long as the boiler is working OK, but an
installer is not allowed to commission a new installation with excessive
pressure drop. (I'm not saying it never happens though.)

There's also the factor of the relative gas consumptions of the old and
new appliances: if the new requires a greater gas supply then the pressure
drop will be greater.

If the existing appliance's excess pressure drop is caused by something
like a few feet of 15mm pipe at the end of an otherwise 22mm or greater
pipe run from the meter then it may be simple enough to get a correct
supply by continuing the old pipe in 22 all the way to the new appliance.
If the existing appliance has anything like the same consumption as the
replacement then this is likely to be the case. On the other hand if it's
a much lower-rated appliance then replacement of the whole run is likely
to be necessary.

As a general comment when I'm looking at replacing boilers for people then
the size of gas pipework required is one of the first things I look at
when assessing whether I can put in a combi since it can make or break the
installation.

--
John Stumbles -- http://yaph.co.uk

Never believe anyone who claims to be a liar