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Thread: Cold usage pulls hot water lines?

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    DIY Junior Member obsoperator's Avatar
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    Default Cold usage pulls hot water lines?

    I live in a large 1955 ranch house with a 50' insulated 1/2" copper horizontal run from the water heater to the kitchen. The 3/4" copper cold water runs 50' from the service valve to the water heater. When I run a lawn sprinkler from a hose, it seems like even a recently used hot water faucet nearer the heater runs cold even longer than it might "first thing in the morning".

    The newer Kenmore gas hot water heater seems to have silver check valve nipples. There is a fancy Watts MMV tempering valve (which nominally has check valves) in the greenhouse room. The single-lever faucets in the bathrooms have new cartridges-because the city water is very hard, they were limed-up. The kitchen has a cheaper Delta single-lever with a nylon ball.

    Am I right in imagining that the high cold water flow is pulling hot water out of the hot lines? Is it safe to put a flapper check valve into the 3/4" cold feed before the water heater? Thanks.

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    Moderator & Master Plumber hj's Avatar
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    Default water

    Am I right in imagining that the high cold water flow is pulling hot water out of the hot lines? Is it safe to put a flapper check valve into the 3/4" cold feed before the water heater? Thanks.

    Can''t happen. To do so would require having a suction in the hot lines and a vent to allow air to enter as the water is sucked out. Neither of which can happen in a water system in the USA. You either have an inaccurate perception or something else is occuring.

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    DIY Junior Member obsoperator's Avatar
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    Default Cold water flow into hot pipes

    Thanks for your comment hj. I was trying to raise the possiblity (?) that the check valves are limed up in the high-end Watts tempering valve, and that cold water usage is pulling hot water backwards through the water heater. That hot water would be replaced by cold water through the Watts or a faulty faucet cartridge (?), not by air. But I'm guessing.

    Although I've found a post here suggesting that a washing machine can cross-connect, it's hard to belive a new front-loader would do this. But I'm trying turning off the washer valves.

    The reason I don't think it's a misperception is that the "hot" water found later at the faucet actually seems colder than overnight standing cold water found "in the morning". And it takes longer to get hot than in the morning.

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    Plumber jimbo's Avatar
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    You possibly have a crossover between hot and cold....probably inside that tempering valve.

    A hot/cold crossover, in the absence of a pump, will usually cause cold water to migrate into the hot line, due to a slight DP in favor of cold.

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    Moderator & Master Plumber hj's Avatar
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    Default flow

    You just said that the cold water pressure is low causing the hot water to migrate out of the hot system, and now you want the cold water pressure to be higher so it can replace the hot water in the piping. You can't have it both ways, and your theory is invalid.

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    Plumber patrick88's Avatar
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    Quote Originally Posted by obsoperator View Post
    Thanks for your comment hj. I was trying to raise the possiblity (?) that the check valves are limed up in the high-end Watts tempering valve, and that cold water usage is pulling hot water backwards through the water heater. That hot water would be replaced by cold water through the Watts or a faulty faucet cartridge (?), not by air. But I'm guessing.

    Although I've found a post here suggesting that a washing machine can cross-connect, it's hard to belive a new front-loader would do this. But I'm trying turning off the washer valves.

    The reason I don't think it's a misperception is that the "hot" water found later at the faucet actually seems colder than overnight standing cold water found "in the morning". And it takes longer to get hot than in the morning.
    What makes the watts tempering valve high end? The mixing valve could be the cause of your problems. The hot water should not be backing out the tank.
    You should turn off your washer valves after each use any ways.
    I'm just starting to work with an old friend of mine to bring solar electric and hot water systems, wind turbines, Flex Fuel Boilers, batteries, hydroponic gardening, books, pellet grills and more. Also the parts for DIY installation.

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    DIY Junior Member obsoperator's Avatar
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    Quote Originally Posted by patrick88 View Post
    What makes the watts tempering valve high end?
    Well, +$150 price alone, if not the adjustable thermostat with a plastic knob. And the diagram indicates that it has check valves built into it. I really meant "high-end" in comparison to the old-style, steam-whistle shaped Watts tempering valve I bought for my toilet tanks years ago. I decided not to install it because it was (...just an adjustable orifice) too direct a connection between hot and cold lines for my taste.

    I did the work today, and confirmed that the Watts MMV's integral check valves are not functioning. When I drained my new hot line work, I found that cold water flowed through the Watts and back to the hot line also feeding the Watts. Why did I drain my new work? Here's why:

    I want to suggest that anyone who uses Made In China flapper check valves from Home Depot work on their side drain plugs. Before you turn the water back on, remove and wrap the drain plugs with Teflon tape, and use enough to create a Teflon washer effect under the tiny hex head of the plug. Also, have the right small socket to tighten the plug without rounding over the soft brass. (The valve includes a good rigid Teflon washer on the large, top access cap-that's not what I'm referring to here.)

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    Plumber patrick88's Avatar
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    Quote Originally Posted by obsoperator View Post
    Well, +$150 price alone, if not the adjustable thermostat with a plastic knob. And the diagram indicates that it has check valves built into it. I really meant "high-end" in comparison to the old-style, steam-whistle shaped Watts tempering valve I bought for my toilet tanks years ago. I decided not to install it because it was (...just an adjustable orifice) too direct a connection between hot and cold lines for my taste.
    Ok cool. I thought you were talking about the cheap ones.

    Quote Originally Posted by obsoperator View Post
    I did the work today, and confirmed that the Watts MMV's integral check valves are not functioning. When I drained my new hot line work, I found that cold water flowed through the Watts and back to the hot line also feeding the Watts. Why did I drain my new work? Here's why:
    Ya I had a feeling it was that causing the back flow. To bad it was the better style that had a problem.

    Quote Originally Posted by obsoperator View Post
    I want to suggest that anyone who uses Made In China flapper check valves from Home Depot work on their side drain plugs. Before you turn the water back on, remove and wrap the drain plugs with Teflon tape, and use enough to create a Teflon washer effect under the tiny hex head of the plug. Also, have the right small socket to tighten the plug without rounding over the soft brass. (The valve includes a good rigid Teflon washer on the large, top access cap-that's not what I'm referring to here.)
    I tend not to get anything from the HD. At least you know how to make things work. Thanks for the info.
    I'm just starting to work with an old friend of mine to bring solar electric and hot water systems, wind turbines, Flex Fuel Boilers, batteries, hydroponic gardening, books, pellet grills and more. Also the parts for DIY installation.

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