3/4" PEX enough flow on DHW?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Rossn, Oct 28, 2020.

  1. Rossn

    Rossn Member

    Joined:
    Dec 12, 2017
    Location:
    Denver, CO
    As part of my remodel, I am replacing the recalled water distribution tubing, as everything is open.

    I am looking to add DHW recirculation - something like the Comfort PM Auto. I will install a dedicated return line.

    Does 3/4" PEX (Uponor/Pex-A) have sufficient enough capacity to flow for the below fixtures, or do I need to look at 1" PEX or 3/4" Copper? Of course, the larger I go, the more heat loss there is, and the longer it will take for DHW to reach the fixture if the recirculator isn't operational.

    Serves:
    Kitchen - Primary sink, prep sink, dishwasher
    Half Bath - used most frequently
    Master Bath - Tub, Shower with 2 heads (or 1 head and rain shower), 2 sinks
    Lower Bath - Tub with shower head (potentially rain shower), 2 sinks
    Laundry - Laundry Sink, Washer (could potentially run the washer separately instead of off the main line)
     
    Last edited: Oct 28, 2020
  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Are you talking about sizing the dedicated return line, or are you talking about sizing between the water heater and the fixtures? If the latter, what topology are you using--manifold, trunk and branch, etc?

    Cheers, Wayne
     
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  4. Rossn

    Rossn Member

    Joined:
    Dec 12, 2017
    Location:
    Denver, CO
    The current plumbing is Kitek, so while it is open I will replace it, and I'm referring to the trunk line size feeding the fixtures. I understand the return can be downsized, though I'm not sure there is much saved in doing so.

    Since it is open, I am considering various topology, though if running a recirculator, then I am thinking that drives me to trunk and branch. To be honest, the only areas that have issues getting warm water to them are the shower and one sink in the master. They are maybe 35-40 feet from the side arm DHW tank to the valve. Everything else is 15-23 feet. I feel like that distance would keep me from using a manifold near the DHW tank (there isn't another good location).
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Based on that, how about a manifold with one line to each room, and a recirculator for just the master bath? In which case an under sink recirculator might be a convenient options.

    With a manifold, the kitchen and laundry could be 1/2" branches, and the half bath could be a 3/8" branch, combined with another branch if nearby to one, or just run as a 1/2" branch if it's too much trouble to deal with 3/8".

    The two full baths, if they were just one showerhead and a lavatory, I would say also 1/2" branch. But if you have are going to have multiple showerheads, or want to fill the tub quickly (and have the water heater capacity to do that), then you'll need to look at the flow rates and sizing more closely, 1/2" may be too small.

    To answer your original question, I think in a trunk and branch system, for the trunk carrying all the loads, 3/4" PEX would definitely be on the small size. It depends on the length of that segment, you can downsize the trunk as you have fewer downstream branches. For all the fixtures listed, if you calculated the flow and look at the pressure drop per foot with 3/4" PEX, I expect it would be on the high side. Which means it would be OK if it's just 4' before you start branching, but would be problematic if it were 20'. That's just off the top of my head, you can run the numbers for an exact answer.

    Cheers, Wayne
     
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