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Vaillant combi boiler - Hot feed 15mm/22mm?

Discussion in 'Plumbing and Central Heating' started by topman870, 5 Aug 2021.

  1. topman870

    topman870

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    Hello,


    Just purchased a new property and ran the pipes from the kitchen to the bathroom in 22mm for both hot and cold. Now reading online the feed for the hot is 15mm from the boiler. I could convert from 15mm to 22mm but I guess I would run into issue if the boiler can't cope in supplying heat for the diameter. Would you recommend flipping the hot back to 15mm and leaving the cold as is?

    Thanks
     
  2. Agile

    Agile

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    The cold feed to a boiler needs to be sized according to the power output and the cold water supply pressure ( and flow resistance ). In many cases 15mm is adequate.

    15 mm hot pipework is almost always adequate for a combi.

    The disadvantage of using 22 mm is the far larger slug of cold water to be run to waste when a hot tap is turned on.

    Sometimes it can help to run more than one 15mm hot supply from the boiler output in a radial arrangement.
     
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  4. topman870

    topman870

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    Thanks for your reply Agile.

    I shall change the hot feed to 15mm as I don't want to be pushing cold to get hot + its a waste of water...

    Do you think i'm okay to leave the cold as 22mm as i've already ran it now?

    Thanks
     
  5. Agile

    Agile

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    It is fine to leave the 22mm cold supply to the boiler.

    No problems at all and in theory a slightly better flow rate if the cold supply is marginal.

    And the hot pipes should be insulated. Surprising how often that is not done. A huge advantage is the water comes out still warm if it was last used within the last 15 min or so.
     
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  7. topman870

    topman870

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    Nice one!
     
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