Cold pipe shuts off if blocked

Discussion in 'Plumbers' Talk' started by PeteOr, Jul 25, 2021.

  1. PeteOr

    PeteOr New Member

    Hi. I recently got our house replumbed (renovation project). The cold supply to the shower upstairs has been capped off for a few months. When I uncap it, the cold water belts out at what seems like a good pressure. But when I block the end of the pipe again, ie with my thumb or the cap, it completely shuts off. When I run the sink tap, it starts up again. But the problem means I can’t connect a shower because the cold feed just won’t operate. No problems with the hot.

    Anyone got any ideas? Sounds like an airlock problem maybe?? but I don’t have a clue how to fix that. Also the pipe is sticking out of an expensive tiled wall …
     
  2. AnotherTopJob

    AnotherTopJob Screwfix Select

    How long did you run the water from the affected pipe? It probably just has air in the system and needs running for a while.
     
    just pumps likes this.
  3. PeteOr

    PeteOr New Member

    Thanks. Could that be right? When blocked it stops very suddenly, no spluttering or anything. I’ll have a go and let water out all the upstairs taps.
     
  4. just pumps

    just pumps Screwfix Select

    Yes that could be right.
    Only need to let flow from the affected pipe as ATJ said above
     
  5. exbg

    exbg Active Member

    Is this pipe of the mains or tank fed? If off the mains, it is difficult to airlock. If tank fed it should be a dedicated supply from the tank, in which case an airlock should not be shiftable by opening another tap.
     
  6. PeteOr

    PeteOr New Member

    It’s off the mains, no tank. I’ve let it run for about 15 minutes and then blocked the protruding pipe with my thumb. It just gives up. Also I can restart the flow again by opening the bath tap.

    I’m absolutely stumped.
     
  7. exbg

    exbg Active Member

    1. If you go to your kitchen sink, turn the tap on hard , can you stop the flow with a thumb. Normally a good cold pressure will not allow it. If you cannot stop it with a thumb, how does that pressure compare against the bathroom?
     
  8. Jimbo

    Jimbo Screwfix Select

    Do you mean that if the flow is stopped (with your thumb) the flow doesn't resume when thumb is removed?
     
  9. PeteOr

    PeteOr New Member

    That’s exactly right. The pressure is good and it’s difficult to stop it with my thumb. But once I do, it stops and doesn’t resume. And it only resumes when I open any other tap in the bathroom ie sink or bath. Pressure is good elsewhere in the house.
     
  10. Jimbo

    Jimbo Screwfix Select

    Did you consider calling GhostBusters already?
     
    PeteOr likes this.
  11. Jimbo

    Jimbo Screwfix Select

    jimbobby likes this.
  12. Jimbo

    Jimbo Screwfix Select

    My working theory is that a PRV is fitted backwards. Therefore the water flows when an upstream outlet is opened, since the static pressure at the inlet (which should be the outlet) falls below the spring pressure. The water then continues to run as the pipe is open. But once flow is slowed enough for the supply pressure to rise, the spring forces the valve closed. When the pipe is again opened, the high pressure on the now static supply keeps the valve closed.

    Could be rubbish obviously.
     
  13. PeteOr

    PeteOr New Member

    Thanks Jimbo. It’s a good theory and makes sense. Unfortunately the wall might have to get stripped to test it out.
     
  14. dcox

    dcox Screwfix Select

    It's not coming through a pump, is it?
     
  15. PeteOr

    PeteOr New Member

    I’ve now taken up the floor and there is indeed a pump. It was unplugged but still all rigged up to the hot pipes. It was there when fed from the tank in the loft but now the hot water comes from a combi boiler. So if hot water still flows out through the pump why would this affect the cold?

    I also found a switch ball valve in line with the cold. Could this be the culprit?
     
  16. spirits are real 2016

    spirits are real 2016 Screwfix Select

    Has the switch ball valve got a arrow on it in the direction of flow of water..
     
  17. PeteOr

    PeteOr New Member

    Not that I can see. I’ve tried to upload a picture but it won’t let me. It looks exactly like this minus the arrow, and there’s ‘25’ embossed on it
    https://www.bossvalves.com/product/brass-swing-check-valve-art-16/?lang=en

    UPDATE yes there is an arrow. Unfortunately for me it looks like it’s going in the right direction. I’ll try to remove the thing though.
     
  18. Jimbo

    Jimbo Screwfix Select

    It sounds like there is a lot that can be removed now if it is combi fed. It probably doesn’t even need a check valve.
     

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