gas supply to new hob

[TMC]

Hi all

Will be getting a gas man in to do the gas work as although I am not being paid for the work it is not my house

There is already a 22mm pipe running most of the way to the location as it feeds the combi boiler tI have not looks at the boiler spec but in is a small 3 bed semi with 6 rads

would the gas man be able to take a 15 mm feed off of that and extend it to the 4or 5 burner hob or would a new feed be required from the meter and if so would that be 15 or 22 mm?

Regards

[A.Lee]

I think you may be looking at either a new pipe, or a separate feed to the hob, especially if it is 5 ring.

I'm doing the same, and under advice from my 'man', he wants a 28mm pipe for at least the first 3 metres, going down to 22mm for the last 2 metres to cooker and boiler. He says GasSafe are really keen on this recently, whereas you used to be able to get away with a 22mm pipe all the way. Alan.

[John Rumm]

Easy to work out correctly if you have the input figures. You need to know:

The maximum gas rate of the boiler in m^3/h (you can work back from kw input if you have that - approx 10.6 kw per cubic metre per hour).

The maximum gas rate for the hob.

The length of pipe in total, where you count a metre of pipe as a metre, a swept bend as 0.3m and an elbow as 0.5m.

You have a maximum pressure drop to work with of 1 mbar, from which you can use the following figures for 22mm tube:

Pipe Maximum gas rate (m^3/hr) length

3m 8.7 6m 5.8 9m 4.6 12m 3.9 15m 3.4 20m 2.9 25m 2.5 30m 2.3

So, for example say the pipe run to the boiler was 6m, and then there was another 3 to the hob, and the boiler can draw 2m^3 and the hob 1 m^3

You check the total load of 3m^3 on the 9m section - see its less that the 4.6 maximum discharge rate, and know it would be ok.

You can work out the marginal drop on sections of the pipe by looking for the figure that matches you load - say 20m will give you 3m^3 using the maximum drop of 1 mbar, so in this case actual length / 20m = pressure drop for that section.

[Dean Heighington]

Wow! Where do you get all this information :)

[Andrew Gabriel]

The wider pipe can go anywhere along a single pipe run - it doesn't have to be at the beginning. Actually, best place from low pressure drop PoV is where you have the most pipe bends.

[TMC]

Hi John

Thanks for this will get the figures and see what is needed

[John Rumm]

Yes sorry, was rather remiss not quote the source. The Copper Development Association publish a document on pipe sizing. Alas I could not find a ready version to link to at the time that would open in my ancient version of agrobat, so lifted the pertinent figures from my local copy. I will put the whole table up on the wiki at some point.

[Dean Heighington]

And there's me thinking you penned it all from memory :)

[John Rumm]

Well the bit about 1 bar pressure limit and the effective length of bends and elbows I did. The table I had to check (although I remembered that 20m was the limit on 3m^3 since that was what the previous boiler I installed could suck!) ;-)