Delta Multi-choice Valve low pressure on colder temperatures

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by DIY_Noob, Jul 5, 2020.

  1. DIY_Noob

    DIY_Noob New Member

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    Sep 6, 2019
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    Hi. Odd one (to me at least) but I’m working installing a delta tub and shower. The valve is a multi choice temperature only type and I’m seeing low tub spout pressure and then when I move temperature down, shower head leaks. Anybody seen this before? Something with the cartridge?

    ive already messed with the scald guard to no avail except when I make it cooler the shower just runs more.

    any suggestions are greatly appreciated!
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Retired Systems engineer for defense industry.
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    First off, what kind of pipe are you using to your spout? The instructions are pretty specific about that...you CANNOT use PEX or CPVC to the tub spout as that will result in a restriction, and that can force some water up to the showerhead. That piping must be copper.

    Then, consider that as you change to all cold, you're using one supply. When warm, you're tapping off of two pipes, so the available volume will be greater. People often equate pressure when they really are being affected by volume. The smaller the ID of the pipe, the more friction there is, so that could be an issue, too, but normally, the pressure is not the issue. When the water is not flowing, regardless of the pipe size, the pressure will be the same, but you can get lots more volume out of a fire hose than a soda straw! The faster you try to get water through a small diameter pipe, the more friction there is, and the lower the outlet pressure (dynamic pressure) there will be. Volume is a function of not only the ID of the pipe, but the number of fittings, changes of direction, the velocity, and distance it has to travel through it. Smaller pipe, it tries to flow faster, resulting in more friction, and less volume AND pressure.
     
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

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    Check that the O rings are intact at the base of the cartridge. Check that the screen inside the valve body is positioned correctly if any. Did you solder the valve body. Debri may have lodge in the valve body passageways and what Jum mentions about the tub spout pipe, it must be copper.


    Screen Shot 2020-07-05 at 10.08.57 PM.jpg Screen Shot 2020-07-05 at 10.10.00 PM.jpg
     
  5. DIY_Noob

    DIY_Noob New Member

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    Thank you! The drop from the valve to the nipple is 1/2” galvanized as is the spout. There is a coupling and an elbow and that’s it. Initially it was PEX and though it has improved (only water leaks on cold vs all the time) it’s still less than ideal.

    do you guys see an issue with the galvanized? The instructions the way I read it really just cautioned against PEX specifically.

    Also thank you for the explanation on friction. That makes a lot of sense. I was wondering why just a narrower ID would lead to less pressure even if the water is moving faster.

    thanks again
     
  6. LLigetfa

    LLigetfa DIYer, not in the trades

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    If the rough-in was installed upside down, that can cause water to come out the shower head when using the tub filler. Did you verify which side us up?
     
  7. DIY_Noob

    DIY_Noob New Member

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    Thank you for the response. I’m going to check that today to see.
     
  8. WorthFlorida

    WorthFlorida The wife is still training me.

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    Retired
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    [QUOTE="DIY_Noob, post: 634919, member: 91368".......Also thank you for the explanation on friction. That makes a lot of sense. I was wondering why just a narrower ID would lead to less pressure even if the water is moving faster.[/QUOTE]

    A tub spot with a lift lever works like that 5th grade science experiment. Fill a glass with water to the rim and place a thin card board on it. Turn the glass upside down. The card board doesn’t fall and the atmosphere pressure holds it tight. The faster the water moves it’s less pressure and it’s the atmospheric pressure that holds it in the shower mode.
     
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
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    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    IMHO, galvanized piping has absolutely NO place in potable water systems. It WILL start to rust, the question is how soon, and how bad. It's a real pain when that rust starts to stain your tub, and even worse, when the rust starts to impede the flow as it rusts out from the inside out. Potable water always has some entrapped air which includes oxygen, so it WILL rust. The pipe's end and threads are often cut after the main pipe is galvanized, exposing the steel underneath...not good!
     
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