Gas Supply to Combi

[Davao]

Hello. 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?

Thanks.

Arthur

[Andy Hall]

It depends on the distances involved.

Have you calculated that 22mm is enough?

Have a look at

do the calculations as well as looking at Ed Sirett's FAQ.

[Ed Sirett]

Indeed so.

[Arthur]

required to reach the combi is 9m then adding 6 elbows and 1 for good luck = 9 + 7*1.5 = 19.5 call it

20m.

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.

Arthur

[John Rumm]

IIRC an elbow should be treated as 0.5m of pipe not 1.5...

So that would give you 12.5m total.

According to the table in the CDA document a run of 22mm pipe 15m long is capable of a discharge rate of 3.4m^3/hour

[Ed Sirett]

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.

[andrewpreece]

My brother is installing a combi, then a 5 ring hob, then connecting up to a gas fire. He is using 22mm pipe mainly, though the run isn't a long one. He had a problem 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 fine, and 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 earlier in 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 was able to figure the power laws involved, and it turns out that a 1.1cubic metre/hour hob will drop 1mBar along 66cm of 8mm pipe. In fact, if you use an elbow that reduces the 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 are 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 are allowed.

Andy.

[Doctor Drivel]

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.

[Ed Sirett]

Yep, The pressure drop is related to the gas flow squared.

As for the sales assistants in Plumbworld which side of the counter would they be on if they had gas fitting qualifications?