Gas Pipe Sizing

[megawatt]

Hi all,

Whilst doing some electrical work recently with all the floorboards up and lovely neat pipework visible I couldn't help noticing the gas pipe to the boiler and, as I had just been reading an article about gas pipe sizing, it left me a little puzzled.

Main pipe was 22mm and measured (roughly by eye) around 15m's or so from the meter to the boiler and, due to the complicated structure, had 9 soldered elbows in the length. 15mm pipe tee'd off about 2m from the meter to feed hob and gas fire point.

Boiler being fed was a Worcester Heatslave 3.5 (25kw) which, according to the book, requires 2.8 m3/h of gas at max slog and the house also had a 5 burner hob (about 14kw if all used at once) and point for a gas fire (currently disconnected) ... No ovens though as these were electric.

The owner has had the installation certified when the hob was fitted, and all the pressure drops were okay as was the supply pressure to the appliances and the installation works fine.

Question is: Doing the sums based on the nice article at http://www.cda.org.uk/megab2/build/pub124/sec3.htm#4.3 Copper Tube Sizing how can there be sufficient volume of gas for it all to work?

Just curious if any experts can clarify

 

[Gasman1015]

There should be enough volume for the boiler (22mm pipe,20 metres max will give allowance of 2.9 m3/h) however with an allowance of say 1.0 m3/H for the hob if would be insufficient only if the hob was being used at max at the same time.

Charts are really for designing from scratch as in most cases we have unknown lengths.

The installer may not have been running both appliances at max, while testing for drop.

In the majority of new or replacement systems installations, I do allow for a new supply direct from the meter, taking into account existing appliances.

I have been to many house's though that have two boilers running from one gas meter.

Also manufacturers agents and corgi inspectors never seem to take into account other appliances when checking pressure at the appliance under test.

Bit like having diversity, like you sparks.

 

[Paul Barker]

It';s very common here for me to loose quoptes because of all the top floor flats in very large seaside town building supplied by 3/4" gas pipe. Even a 19kw regular boiler requires 28mm and if there is much else up there or a combi you vertually can't even do the job.

No doubt cowboys just hang a Biasi off the end of this 3/4". Bish bash 300 notes for a days half hearted work and on to mug the next ignorant fool who chooses installer by price..

 

[ChrisR]

how can there be sufficient volume of gas for it all to work?

Boilers turn themselves down in normal operation, so would run on far less than the intended nominal 20 mbar. Usually it means you just never get max output. Bit some current boilers won't even fire if the inout pressure is less than say 18mbar. Typical of burners in condensers.

Corgi are winding themselves up to letting 35mm be used on domestic, so they can charge us extra in as many ways as possible.
In the meantime we can use 2 or more x 28mm in parallel if we want, otherwise we can't put big domestic boilers far from the meter.

 

[Gasman1015]

Cowboys come assorted guises, had a customer with an injured boiler that had been fitted by BG and was connected to 10 metres of 15mm pipe with 10 bends( and that was the kitchen section alone).

 

[megawatt]

So presumably, if my grasp of this is correct, if the boiler and the hob were both running flat out and the volume was not quite up to scratch everything gets slightly less gas than normal and therefore less kw ... Not harmful just not getting the best out of the equipment?

One other question (which will probably highlight my lack of knowledge in this area ) ... If the meter is set to output say 25mb and the pipework is on the edge of tolerance, why can't the regulator be tweaked to up the pressure from the meter to say 30mb?

Thanks all for your insight.

 

[Gasman1015]

Regulators are set and sealed to give 21 mbar.

I've no doubt some will and have adjusted the regulator beyond its setting, however as well as testing for supply pressure at the boiler we also test at the meter whilst the boiler is operating to confirm regulator.

It may well be that the regulator is incorrect and the installer had sized the pipe work correctly.

 

[meldrew's_mate]

If the meter is set to output say 25mb and the pipework is on the edge of tolerance, why can't the regulator be tweaked to up the pressure from the meter to say 30mb?

Wash your keyboard out with soap and water Cooker is designed to work on 20mB, boiler will have an internal regulator so it COULD tolerate higher inlet pressures at low gas rates, however, if you turned up the main regulator to a dynamic setting of (as you suggest) 25 or 30 mB when the boiler is off the poor cooker burner flame would be trying to lift off (function of gas velocity and flame propagation) and producing heaps of CO (which is not good)

Most boilers will work at 18mB and as the calcs for pipe size are based on 1mB drop, then a 2 or even 3mB drop would still allow the boiler to reach full rate.

Doesn't mean the installer won't deserve a good boll**king if he does't design it for 1mB though.

 

[megawatt]

Meldrew's Mate wrote:

Wash your keyboard out with soap and water

I did this as you advised and now none of the keys work?

ChrisR wrote:

Corgi are winding themselves up to letting 35mm be used on domestic

This may be great for gas volume but not for the structure of the house. Our 22mm gas pipe runs through every joist in the master bedroom which is 18' long and the joists span the 14' width ... 36/37mm notches (to allow clearance) ... Ouch!

 

[esra_ptrap]

We'll just get all UK homes with sewer sized gas pipework in time for the gas supplies to run out - Fossil fuel is dead long live dilithium crystals and warp drive