Posting a message to the forum will remove the above advertisement
  1. Checkout our Plumbers Insurance area - heavily discounted Public Liability Insurance and Van Insurance specifically designed for plumbers.

Discuss Two opinions. Who is right? in the Gas Safe Register Forum - Public Forum area at

  1. Mbrains

    Mbrains New Member

    I have been given two quotes for a boiler replacement both of which require a 28mm pipe gas supply. The meter/mains supply is at the opposite end of the house to the boiler. A 22mm pipe runs into the outer wall and ends up at the boiler, route unknown. At some point it feeds a gas fire and a hob before hitting the boiler on a 15mm pipe.

    Gas safe engineer 1 thinks it goes through concrete floors at ground level and engineer 2 thinks that it runs under the first floor floorboards across the house. Engineer 1 wants run a fresh 28mm pipe right from the meter to the new boiler (partly on the outside of the house) which i want to avoid as it will look ugly. Engineer 2 wants to tap into the 22mm pipe as it enters the house through the wall at first floor level (approx 3m run from the meter).

    My questions are:
    1. Is tapping a 22mm pipe and running 28mm of it 3m downpipe ok to do. Will the boiler run ok?
    2. Is there a way to actually work out where the existing 22mm pipe runs?
    3. Is it even possible to run a 28mm fresh pipe through the cavity wall vertically up to first floor (and then send it under floorboards across thd house)?
  2. Stanios

    Stanios Plumber GSR

    Upsizing from 22mm halfway down the run won't do anything in fact because something silly like that was even suggested I wouldn't consider taking up that guy's quote.
    Imagine you had a straw for a garden hose then in order to improve its pitiful watering ability you decided to strap a standard hose on the end of the straw. Would that work? Same with gas.
    Can't really answer other questions without seeing the job, I would advise though to speak to neighbours see if they know anything about how the gas was run (if you are part of same developement ofc)
    • Dislike Dislike x 1
  3. steadyon

    steadyon Active Member

    The straw / hose analogy ignores the fact that water is not (to all intents and purposes) compressible, whereas gas is.

    If the pressure drop (with all appliances running at maximum gas rate) at the point where the 22mm becomes 28mm leaves sufficient residual pressure drop to give the permissible meter / appliance drop at boiler, gas and hob then it could be done.

    However, without knowing the run, or measuring the pressure drops at different points, no one can tell for certain if it would work.

    Depending on the age of the property, buried pipes may well be low carbon steel, which have different internal measurement and gas carrying capacities from copper.
    • Like Like x 1
  4. king of pipes

    king of pipes Trusted Plumber GSR

    I would go with enginner 1 i know the pipework is not the best looking thing to have on the outside off your property but its needed if your upgrading , could it be run outside underground in track pipe ? Just a thought , as above tapping onto 22 mm will gain you very little if anything at all , you would be better running as much 28mmm from the meter to where it disappears but i would recommend it is all replaced and sized accordingly cheers kop
  5. Darren Jackson

    Darren Jackson Plumber GSR

    1 - It might work but is best to start with 28 at the meter.
    2 - Only way would be to physically inspect & follow the pipe run to find where it goes.
    3 - NO you cannot run a gas pipe inside the cavity Vertically or Horizontally. It has to pass through a cavity via the shortest distance ie straight through it and it must be suitably sleeved and sealed. (only exception to this rule is if it is runs through the cavity in a sealed ducting).
    • Like Like x 2
    • Agree Agree x 1
  6. GH77

    GH77 GSR Top Contributor!!

    In my experience you should start with the largest diameter of Pipework at the meter and only decrease as you work away from the meter to the appliances, engineer 1 sounds more knowledgeable, if you do want to run externally and avoid unsightly Pipework then you could have someone competent mole in a PE pipe below ground, would need a grp sleeve where it entered and exited the ground and would be a lot more expensive than surface running copper, depends on where your priorities lie with regards to aesthetics vs cost.
  7. townfanjon

    townfanjon Trusted Plumber GSR

    Apart from the obvious gas flow rate required , I am pretty sure you can increase pipework where you want as it increases the actual volume of the gas .
    Personally I have never done it but I have read it somewhere
    • Agree Agree x 1
  8. galaxy_plumbing

    galaxy_plumbing Plumber GSR

    we had to do section of piperwork in 35mm as gas supply wasnt adequate for new combi, prior to this change it was undergased. just 2 metres of 35mm solved the problem. I know BG sometimes install larger section of pipe as some accumulator when they cant change whole supply but they go 64mm or so. Prefer option 1 however if possible.
  9. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    I wouldnt never tee into 22mm to run a new 28mm gas run. I see this all the time and when you do a gas rate and tests on the boiler 90 percent of the time the boiler is under gassed. For this reason alone I would also use engineer number 1 and not number 2, but I would go back to engineer 1 and talk through the gas pipe again. He has suggested an external pipe run as it is much easier for both parties. There may be another option but it would obviously cost you more to do.
    Hope this helps.
  10. bright spark

    bright spark Plumber GSR

    King of pipes might be onto something g burying d pipe.
  11. Ric2013

    Ric2013 Plumber

    This seems to have been shot down. However, what I'm wondering is whether you would have said the same if the installer had suggested running the majority of the gas run in 28mm, and then reducing to 22mm for 3 metres closest to the boiler? Would it make any difference, or would it amount to the same thing?
    I suppose the difference is that the hob and fire are sharing the 22mm's capacity?
  12. kris

    kris Trusted Plumber GSR

    Defo better to have 28mm back to meter as that's the right way to do it , but even having some 22mm then increasing to inch and have inch most the way to the boiler will work because your increasing the volume of gas being held in the pipework , we only need to increase pipe sizes because gas is low pressure , it's not a case of it will work or it won't (well it is but ) it's a case of being within pressure drop, the boiler connection is 22mm ,
    • Like Like x 1
  13. GH77

    GH77 GSR Top Contributor!!

    You could try uprating 28mm later in the carcass, but the acid test will be at commissioning when you have the Gas installation at full load and check the inlet working at the boiler, if it's below the minimum the manufacturer allows, what is the gsr going to record on the benchmark? . there's a chance it might be okay, but also a chance it might not, the only certainty is that if you do it belt and braces and carry out a correct pipe sizing exercise and install that then you will have a successful commissioning ( based on the working pressure being correct at the meter )
    • Like Like x 1
    • Agree Agree x 1
  14. king of pipes

    king of pipes Trusted Plumber GSR

    Its the same thing ric 22mm will do a fire and hob providing the gas run is a not to long but you need to up size the pipework close as physically possible to the meter and before any appliances it would work i have done it when i have had more than the allowed pressure drop on a system but i would not stick 2 lengths of 28mm in the middle of the 22mm carcass cheers kop
    • Like Like x 1
    • Agree Agree x 1
    Last edited: Jul 23, 2017