"John" wrote in message
news:6w8Rn.42970$4w7.20324@hurricane...
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 cooker on its won pipe back to the meter? His test is not valid.
In one situation like this, I turned on the cooker and took a reading at the
meter dial of what the consumption is in m/h or cu ft/hr. I then from the
boiler gas supply fitted a flexible hose and flame trap at the end. This
went into the garden. Then turned on the cooker and opened the boiler tap
gradually until the combined boiler and cooker reading is enough, and took a
reading. It worked. You have to observe the gas cooker.

In another it was obvious that the boiler pipe was just undersized. I
figured putting in a 28mm gas meter union and a few foot of 28mm pipe at the
meter using a bend (45 degree elbows would have done) would do it - a bit of
a gamble. I did not know how much 22mm or 15mm was under the floor. But by
turning off the cooker and opening up teh bopioer tap, taking a reading,
then look at the charts it is possible to assess how much 22mm is in the
run. It did it anyhow.

Each elbow "increases" the length of a run - each elbow adds "length" to the
pipe run. The 28mm pipe significantly reduced the run length being a larger
bore.