1/2" valve: use 1/2" or 3/4" aquapex?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by kobryn, Nov 30, 2019.

  1. kobryn

    kobryn New Member

    Joined:
    Jun 26, 2019
    Location:
    Seattle
    I'm installing a Grohe SmartControl 1/2" valve with two 2.5 GPM shower heads that can potentially run simultaneously. I've gone thru all 1/2" vs 3/4" pex threads on this forum but I'm still not sure which one should I use.

    It's a second floor bathroom with 3/4" trunk copper line that I'm going to convert to 3/4" PEX and run 1/2" PEX branches to the fixtures. I'm using ProPex/Uponor expansion fittings. Water pressure on the first floor is measured at 80 PSI. Shower branch line is approximately 7 feet long.

    Looks like 3/4" for the shower is a safe bet, but it's a bit inconvenient to run and it looks like Uponor doesn't manufacture a 3/4" PEX to 1/2" male NPT adapter. Other than transitioning back to 1/2" copper using this fitting and soldering on male NPT adapter is there any other way to connect 3/4" PEX to 1/2" female NPT? Or do you think 1/2" PEX should be sufficient in my case?

    Any help is much appreciated!
     
  2. wwhitney

    wwhitney Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    FWIW, per Uponor's pressure drop tables, each foot of 1/2" PEX is equivalent to 6.4 ft of 3/4" PEX in terms of pressure drop.

    But for 5.0 gpm of 80F water (to average hot and cold temps for simplicity, although maybe I should have used 100F), 1/2" PEX drops only 0.71 ft of head per foot of pipe. So your pressure drop with 1/2" PEX would be 5 ft of head for 7 ft of pipe, or about 2.2 psi. For 3/4" PEX, it would be about 0.33 psi.

    If you do prefer 3/4" PEX, Apollo makes an F1960 3/4" expansion pex to 1/2" MIP adapter:

    https://www.homedepot.com/p/Apollo-...-1-2-in-MNPT-Male-Adapter-EPXMA3412/302741319

    Cheers, Wayne
     
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  4. kobryn

    kobryn New Member

    Joined:
    Jun 26, 2019
    Location:
    Seattle
    Thanks Wayne! Looks like pressure should be an issue, but what about flow restriction? Would 1/2" PEX be able to supply 5 GPM to shower heads? According to Uponor manual 1/2" pipe should be able to flow 2.8 GPM. But I'm not sure how to calculate total GPM for hot and cold supply lines.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Retired Systems engineer for defense industry.
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    Look at it this way...in the winter, you'd be trying to use mostly hot water since the cold could be near freezing (I've measured mine at 33-degrees during a cold spell). So, let's take a WAG at 80% hot, 20% cold. That would give you about 103-degrees, if your hot was 120 and your cold was 33. Say you needed 5g, so 5*.8=4g of hot when a 1/2" line can provide 2.8...not going to cut it! The line might provide more, but your dynamic pressure drop starts to go way up. Now, you will still get that temp water out, but the flow rate, and thus the performance of the showerheads will be diminished. Note, some 1/2" valves only put out around 5gpm. If the valve can't output at least what the showerheads want, that will decrease the velocity of the water coming out of the heads. Think a hose with no nozzle on it...all of the water goes out unrestricted, it doesn't get sped up. You need more capacity or at least the stated capacity to force that Bernoulli effect from a restriction (nozzle) to speed up the water out of the nozzles of the showerhead.
     
  6. wwhitney

    wwhitney Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    A couple points:

    That figure of 2.8 gpm maximum flow through a 1/2" pex line is based on limiting the velocity to 5 ft/s. Uponor says the maximum allowable velocity is 12 ft/s, but recommends a velocity cap of 10 ft/s for cold water and 8 ft/s for hot water.

    My previous response was based on a flow rate of 5.0 gpm in a single 1/2" pex line, but of course there are both hot and cold supplies. So say the controlling case is 4.0 gpm (a velocity of 7.24 ft/s) at 120F. Now the pressure loss is 0.43 ft of head per ft of pipe, or 3 ft of head for 7 feet of pipe, or 1.3 psi of pressure drop.

    So 1/2" pex will be fine, but use 3/4" if it makes you feel better.

    Cheers, Wayne
     
  7. wwhitney

    wwhitney Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Flow and pressure are related, not independent. Flow through any component creates a pressure loss, which is increasing with increasing flow. You're concerned about this short 7 foot section of pipe, so the only question is how much pressure loss will it cause at the design level flow rate. Which is what I calculated.

    Cheers, Wayne
     
  8. kobryn

    kobryn New Member

    Joined:
    Jun 26, 2019
    Location:
    Seattle
    Thanks folks! I'll run 3/4" just to be on a safe side - I kinda have to get it right on a first try. Will install it using Apollo fittings that Wayne suggested.
     
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