It depends on the distances involved.
Have you calculated that 22mm is enough?
Have a look at
and do the calculations as well as looking at Ed Sirett's FAQ.
Assuming he uses nothing but 90 degree elbows the maximun length of pipe
required to reach the combi
is 9m then adding 6 elbows and 1 for good luck = 9 + 7*1.5 = 19.5 call it
The installation manual says;
2.3 Gas supply
The appliance requires a maximum of 3.17 m3/h of natural gas
(G20).The installation and the connection of the gas supply to
the appliance must be in accordance with BS6891.
The meter or regulator should deliver a dynamic pressure of 20
mbar (G20)at the appliance,which is equivalent to about 19-
19.5 mbar at the gas valve inlet pressure test point.
On Wed, 22 Jun 2005 13:05:13 +0100, John Rumm wrote:
The extra for the hob. Is likely around 0.6m3/hr.
The do the calcs properly you would allow 0.5 mbar drop for the meter to
the branch point and then 0.5mbar for the rest of the run to the combi and
for the short run to the hob.
Chances are that the installation will be just about OK with the hob added.
A full cooker (likely nearer 0.9-1.2 cu. m. /hr) would probably
require the initial section in 28mm.
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
My brother is installing a combi, then a 5 ring hob, then connecting up to a
He is using 22mm pipe mainly, though the run isn't a long one. He had a
with the hob worktop surround and the integrated cooker fouling the 15mm gas
pipe exiting the rear of the hob. He took a piccy of this and showed it to
the man at
Plumbworld, or whatever it's called, and he said that 8mm pipe would be
sold my brother 3m of 8mm pipe and a 15mm - 8mm adapter.
My brother asked my opinion on this, and seeing the website that was posted
the thread had a calculation table on pipe sizing in it, I suggested we give
it a shot.
Well, the website doesn't go down to 8mm or down to 1m lengths of pipe but I
able to figure the power laws involved, and it turns out that a 1.1cubic
will drop 1mBar along 66cm of 8mm pipe. In fact, if you use an elbow that
allowed run to 16cm, and that is with no losses elsewhere!!
So beware, the steep pressure loss with thin pipes such as 8mm means they
effectively unable to supply a hob, certainly of the 1 cubic metre/hour
variety. Go down
to 0.5 cubic metres/hour and suddenly reasonably long lengths of 8mm pipe
> I will be using 22mm from the meter to the new combi.
> My question is, can I branch off this (15mm) to the
> gas cooker socket?
In theory yes. In practice don't. Boilers come on with whoomph, and a
combi is drawing around over half what the meter can supply. Also the cheap
regulators Transco fit are slow acting. All this may rob the cooker. It is
not much to take a cooker pipe back to the meter.
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