Sink Supply Question

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by cls33, Sep 1, 2013.

  1. cls33

    cls33 New Member

    Sep 1, 2013
    Super easy amateur question...

    I had rough plumbing run for a basement bathroom I'm putting in and I finally got around to attempting to install the faucet for the new sink. To tap into the supply lines I had to cut the capped copper pipes coming out of the wall to install the local shutoff valves for the faucet. The bathroom itself has shutoff valves for both the hot and cold supply lines as they come into the room so I shut off the hot water valve and proceeded to cut the capped copper pipe on the hot supply line under the sink... which resulted torrents of water rushing out :) I ended up having to shut off the cold water supply line as it came into the bathroom to stop the flow.

    After a little investigation, it seems that the cold water line is supplying water to *both* the hot and cold supply lines under the sink.

    So my question, is that right? Should the cold water line also feed the hot supply to the sink? Was I wrong in expecting that shutting off just the hot water supply to the entire bathroom would shut off supply to the hot water line under the sink? Or did the plumber mix something up?
  2. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    Before you cut any pipe, you need to drain down the home by opening up faucets upstairs and down. It may take fifteen minutes before you have a complete draindown.
    If you have an expansion tank near the water heater, expect that to add to the gush of water after it has been shutdown.

    You should have a cold line, and a hot line from the heater.
    Two lines.
    Last edited: Sep 1, 2013
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  4. dj2

    dj2 In the Trades

    Aug 13, 2013
    Without seeing what you have and without knowing what your plumber did, the answer is no, the hot water doesn't come out the cold side. You need to get the plumber back there,double check to verify what he did, and make changes, if needed.
    Also ask him what the 'hot' shut off valve controls.
  5. cls33

    cls33 New Member

    Sep 1, 2013
    This wasn't a drainage issue. I have valve's that control just this bathroom that effectively separate it from the rest of the plumbing system. The only pipes that would have to drain are the pipes for just that room. I expected a bit of water, but this was clearly an actively supplied gush that quit almost immediately after I shut of the "cold" water valve that supplies that bathroom. (remember, I cut the HOT water line)

    I just wasn't sure if the cold and hot lines were mixing before they supplied the sink and wanted to check because I guess I'm more comfortable potentially sounding like an idiot on a web forum than I am potentially wasting a plumber's time to drag him back out if he did in fact do it correctly. But it sounds like I was right, the cold water valve shouldn't affect the hot water supply line whatsoever and it's probably just misplumbed somehow... so I'll go back to the plumber.

  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    If you've done the shower rough-in plumbing, but do not have the cartridge in (many Delta valves are like this) or have the plug installed, this will allow the cold water into the hot line in the room.
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