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3/4" PEX enough flow on DHW?

rossn Member Posts: 76
edited October 2020 in Domestic Hot Water
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.

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)


  • hot_rod
    hot_rod Member Posts: 22,251
    The way water piping is sized is by F.U fixture units. However these tables were developed long before low flow fixtures were mandated.
    The biggest unknown in your group is the rain shower head. That load alone couldn use a 3/4" pex line.

    A home run system was designed specifically for jobs like yours, a dedicated line to each fixture, no recirc required. A 3/4 could run to a large shower load as wait time isn't as critical.

    Or, no harm in running a 1" insulated supply to that load, the recirc is just overcoming head loss and heat loss in the loop, the feed size is just to cover the draw of the fixtures.

    Why would you have a recirc that is not operational? Control it by an aquastat and the loop is always ready. Or time it out at night.
    1" wall insulation on the entire loop, assuming it is inside the heated building envelope, should limit that heat loss to the equivalent of running an 80- 100W bulb.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ChrisJ
    ChrisJ Member Posts: 15,767
    It's been a few months, but I plumbed my bathrooms using Uponor pex A and from what I recall, I tried to avoid more than two fixtures pulling water at the same time from a 3/4. I think I used the number 6 GPM as a max for 3/4.

    In my case there are more than 2 fixtures per run, but it had to do with what's likely to be used at the same time etc.

    Many run an entire house on a single run of 3/4 pex btw.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,327
    Hello @rossn , What do you imagine will be the highest use in gallons per minute? Looking at the Uponor chart: https://uponorglobal.azureedge.net/-/media/uponor-global/resources/manuals/us/pdam.pdf?rev=e7c5e0ae767946f197cca154288baf1c I see that at 10 feet per second, you get about 11 gpm. Ten fps can give a pretty high pressure drop, but research has shown that about 99% of the time we don't use more than 3 gpm in homes, so as long as you have pressure balanced showers, you're probably good.
    About recirculation, I'd go with a demand controlled pump. They are FAR more energy and water efficient than any other control system. Using this sort, go with a 3/4" recirc line so it can move water quickly.

    Yours, Larry
  • rossn
    rossn Member Posts: 76
    Fixtures will be watersense, or reasonably close (master primary shower head might be 2.5 GPM. I wouldn't expect to use more than 3 shower heads across the 2 baths at a time, which I'd assume is around 2+1.6+1.6=5.2 GPM on the hot water side. Maybe throw in a lav for another 1 GPM. So, we're looking at peak usage around 6 GPM DHW.

    Since my original post, I have decided to go the propress route, so we'd be looking at 3/4" copper, and it seems like the practical maximum there is 6-7.5 GPM. So I guess a 3/4" DHW trunk for the mentioned area is reasonable.

    Why would the recirc not be working... maybe if I determine it is wasting too much energy. I am a bit concerned about that. For the other end of the house, where I have 50+' of tubing to reach one bath and 90+' for the other bath, I am planning on a D'mand recirc pump which is manually triggered. For the closer end of the house, with furthest fixture at around 35', I am thinking of using a Grundfos Comfort PM Auto.