Advice - Boiler replacement in renovation project

Discussion in 'Plumbers' Talk' started by Greenspace, Nov 14, 2017.

  1. Greenspace

    Greenspace New Member

    We currently have a gravity fed system and are looking to install (and move to the utility room) a combi boiler to replace the old vented boiler that uses a chimney. So far two plumbers have come around to quote.
    Plumber A has recommended running 22mm pipes back to the original boiler cupboard to connect to the original pipe work.
    Plumber B has said that connecting back to the original cupboard isn't necessary as the pipework will be pressurised. Once moved to the utility room he recommends connecting to the nearest radiator using the 15mm pipes currently in place and the hot/cold from the utility room washing machine pipes.
    Does it matter if you connect to 15mm rather than 22mm when installing a combi like this? House is a small 4 bed.

    Thanks in advance
  2. Mike83

    Mike83 Active Member

    The hot/cold should be ok.
    Plumber B, don’t listen to him. A 4 bed house will have 7+ radiators.
    A new 15mm pipe will struggle with this never mind pipework that’s old and possibly reduced in diameter due to blockages.
    22mm back to the original location is the far better plan.

    Just remember though. All the existing pipework has never been under pressure and could leak when connected to the new boiler. It’s rare but does happen.
    Greenspace likes this.
  3. ramseyman

    ramseyman Active Member

    Agood precaution is to replace all lockshield and TRV valves when moving to a pressurised system
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  4. KIAB

    KIAB Well-Known Member

    Keep the 15mm for drops to radiators, do as much of the main pipe runs in 22mm, will give you a flow rate, so a quieter & more efficent system.
    Greenspace likes this.
  5. Hi Greenspace.

    The combi has two 'outputs' - one to supply the rads (CH) and t'other to supply your hot taps.

    So I wonder if there's some confusion as to what each plumber was referring to?

    For the RADS, then - yes - you need 22mm flow and return pipes to the new boiler. IF the second plumber was suggesting simply connecting the new combi CH flow and return to the nearest rad using 15mm pipe, then he's nuts. It's so wrong that I suspect a misunderstanding.

    I wonder if that's what he actually meant? Perhaps he was actually talking about the DHW flow to your hot taps, in which case this IS best done from a combi boiler in 15mm pipe. This is the 'ideal' size as it is mains-powered, so this pressure can overcome pipe resistance. And also the 15mm pipe will have less cold standing water in it to be shifted before the hot gets to the taps.

    Can you confirm what each plumber actually meant?
  6. dobbie

    dobbie Well-Known Member

    Definately go with plumber A and go back to the original location.
    Plumber would create a couple of dead legs if he just capped the pipes in the airing cupboard.
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  7. Greenspace

    Greenspace New Member

    Thanks for all your advice. We've had a couple more plumbers around and going with Option A. Keep it in original cupboard (running a new flue up to roof) and using current pipes.
  8. Jitender

    Jitender Well-Known Member

    Try to get no more than 3/4 quotes, can get confusing when too many :D

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