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Question regarding pipework size to an expansion vessel

Discussion in 'Plumbing and Central Heating' started by GasmanCam, 14 Nov 2019.

  1. GasmanCam

    GasmanCam

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    HI folks,

    Does anyone know if there are any rules regarding what size pipework should be used to pipe up an expansion vessel that's working with an unvented cylinder.

    I saw one yesterday that was installed with 15mm pipe to the combination valve, and I wasn't sure if this was acceptable or not. (The vessel was also connected into the wrong side of the check valve, which I'm going back to rectify)

    Id appreciate anyones thoughts on this.

    Thanks
     
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  3. muggles

    muggles

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    15mm is fine, the rate of expansion isn't so great that pipework is going to cause a huge resistance. A lot of combi boilers have 3/8" hose with an internal diameter of about 4mm connecting the vessel to the system
     
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  4. GasmanCam

    GasmanCam

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    Thanks for the advise
     
  5. Madrab

    Madrab

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    Don't mean to suggest you're not but if you were qualified to work on unvented cylinders, then you really should know the answer to that.
     
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  6. GasmanCam

    GasmanCam

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    Pipe down Mario, if I wanted to be told off online I'd be on a different website....
     
  7. Madrab

    Madrab

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    Hardly telling you off .... more like pointing out the obvious
     
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  8. dilalio

    dilalio

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    Put it this way... Without "any" room for expansion, the force would "find a way" so, as long as the resistance of the comms tube is less than that of the PRV spring, there is balance (y)
     
  9. GasmanCam

    GasmanCam

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    Thanks Dilalio, what I specifically had in mind was would the smaller diameter pipe hinder the changeover of water within the vessel, which would matter more being secondary water, than if it was primary water.

    Sounds like I was concerned about nothing though...
     
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  11. ReJect

    ReJect

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    Good question! I always have piped them in 22mm copper. The vessels are 3/4” thread on any I install.
    But the flexi hoses that are supplied with some brands are obviously very restrictive.
     
  12. aptsys

    aptsys

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    How much water flow do you think you're going to get to an expansion vessel??
     
  13. ReJect

    ReJect

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    Not a lot and usually very slow heat up, therefore gradual expansion of water anyhow.
    I am not saying anything wrong with using 15mm copper pipe for it. Flexies though in my opinion shouldn’t be supplied with unvented units, but I hate all flexies and see them as a weak point.
    22mm is much stronger pipe which makes it less likely to get damaged when removing expansion vessel.
    On domestic heating systems it is usually 15mm pipe used for to EVs
     
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  14. fixitflav

    fixitflav

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    With (say) 30 kW into the water, I estimate less than 20 litre/h expansion rate. That's only just after the boiler fires, when most of the heat is raising the water temperature. It drops off as the rads etc warm up and put out heat, so net heat into the water falls, down to zero when the whole system is at steady temperature.
     
  15. dilalio

    dilalio

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    Deleted.
     
  16. Gasguru

    Gasguru

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    Water expands just a few % over the full cylinder temperature range, so with a typical 120 litre vessel you're talking no more than a couple of litres.
     
  17. fixitflav

    fixitflav

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    That's right, but it's the flow rate that's needed for the pipe size.
     
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