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Thread: Cold Water Supply reduced to 3/4" before water heater...

  1. #1
    DIY Senior Member speede541's Avatar
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    Default Cold Water Supply reduced to 3/4" before water heater...

    Hi all,

    I'm making a last minute rerouting to my incoming copper water supply line prior to closing up the wall.

    The plumber who installed our water line ran 1" copper about 60 feet into the back of the house. Along the way, the 1" line hits the basement (ground floor in-law apartment) master bath, branches to the upstairs bath + laundry, branches to the main kitchen + backyard hose bib, and also supplies the basement kitchenette.

    Then, 15 feet before reaching the water heater, it's mysteriously reduced to 3/4" copper. Along that final 15' run, the 3/4' line branches to the basement laundry and a half-bath above, before it finally terminates at the boiler-fired indirect water heater.

    I'm re-routing a portion of this 15-foot run of 3/4" copper around some new duct work, and wondering if I ought to stick with 3/4", or upsize it to 1". I'm sure there's no harm in upsizing it, but it'd be easier not to if there's no real benefit.

    Hot water requirements for the house will be 3 full baths, one half bath two laundries, kitchen + kitchenette, and at least one dishwasher and possibly two.

    Any words of wisdom?

    Water pipe sizing
    Last edited by Terry; 09-01-2011 at 02:46 PM.

  2. #2
    In the Trades Gary Swart's Avatar
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    The 1" for the meter to the house is a good idea, but after reaching the house, 3/4" is plenty large enough. When you branch off of the 3/4 to individual fixtures, 1/2" will be good.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Not sure, but I think most WH inputs are 3/4". WIth a 3/4" line, your max safe water flow is around 18gpm to keep the velocity down to safe levels. If the WH has a 1" input, you can significantly increase the flow available if you increase the size - keep in mind that the area has a squared factor, so it is not linear. But, unless you are going to be using everything simultaneously, you'd probably not notice. this assumes that your output is also available as a larger pipe, which may not be the case. To use that max flow, you'd probably exhaust the indirect if it went on for long even with its quicker recovery over a stand-alone WH.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member speede541's Avatar
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    Thanks for that. But again, it's ultimately my hot water supply I'm concerned with, since the water heater is all the way at the end of the supply line.

    I'm a little familiar with the 3/8" - 1/2" - 3/4" and "groups of three" rule for distribution, but a very likely scenario will be three showers running at the same time plusmaybe a kitchen faucet, dishwasher, or washing machine (or two). If the water heater is supplied by 3/4" supply instead of 1", will pressure remain sufficient under high demand, especially since most of the hot water demand will need to be pushed up to the the two floors above?

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    DIY Senior Member speede541's Avatar
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    @ Jim, Ok, more food for thought (previous reply was to Gary).

    So are you saying it's actually possible to oversupply (overpressurize) a water heater if the line is too large?

    Another concern is the 3/4" line I'm rerouting will have no fewer than six elbows to get around all the ducting. I'm unsure how that affects pressure drop / flow rates.

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    Forum Admin, Expert Plumber Terry's Avatar
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    There is no such thing as too large of pipe when it comes to cold.
    The mains in the street are 8"

    Water pipe sizing

    What you want to do, using the chart below count up the fixture units that the water heater is providing, and the cold fixture units on the way to the water heater minus ten and see if that's overloaded.

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    DIY Senior Member speede541's Avatar
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    Ok, so to clarify, I would count (for instance) each shower twice -- once for the hot side, once for the cold side?

    Does each elbow add to the effective length of the run?

    Do I need to account for vertical rise to the 2nd & 3rd floor?

    And lastly, what's the -10 for?

    Thanks everyone!

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    DIY Senior Member bluebinky's Avatar
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    From a non-pro...

    Ideally, count fixture units.

    If that makes your head hurt and/or you don't mind spending a few more $, run 1" until after the last cold take-off and all the ells.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Each pipe size will be able to supply a maximum volume of water at any specific water pressure before the velocity gets too high to expect longevity of the pipes. If the WH has 1" inputs, then running 1" to it may be worth it. If not, then probably not.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    DIY Senior Member speede541's Avatar
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    Quote Originally Posted by Terry View Post
    There is no such thing as too large of pipe when it comes to cold.
    The mains in the street are 8"

    Water pipe sizing

    What you want to do, using the chart below count up the fixture units that the water heater is providing, and the cold fixture units on the way to the water heater minus ten and see if that's overloaded.
    Adding all my hot water fixtures

    + cold fixtures on just the 3/4" segment
    __________________________________

    = gives me 31.5 Fixture Units


    Subtracting 10 brings me down to 21.5, just a little over the 19 max units recommended for a 3/4" line.

    Is there some fudge factor here? Is this "close enough" I should just stick with the 3/4" copper?

    FWIW, my water heater has 3/4" connections, and I'm leaning towards switching to the 1" line.

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