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I have this system installed in my house (fig 1), and I'm having problems with the shower. I want to know whether, if I change the PEX tubes shown in red from 1/2" to 1" as shown in fig 2, I will have more pressure. Will I enjoy the shower?

The head loss due to friction is calculated as $\frac{fL}D \frac{v^2}{2g}$, where

  • $f$ is the friction factor,
  • $L$ is the length of the pipe,
  • $D$ is the pipe diameter,
  • $v$ is the fluid velocity, and
  • $g$ is the gravitational acceleration.

I understand this to mean that when I increase the diameter, the head loss of friction decreases, so I will get more pressure. Is this correct?

Finally, what about the flow rate? How this will change affect it? How does the flow rate depend on these variables?

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  • $\begingroup$ Is this homework or an assignment or similar? $\endgroup$
    – Russell McMahon
    Commented Jul 1, 2015 at 14:27
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    $\begingroup$ It's a real life issue $\endgroup$
    – Loris Doumanian
    Commented Jul 2, 2015 at 14:23
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    $\begingroup$ If that's an open header tank and not pressurised by the mains then the head is 2m + depth of water in tank. Say 3m total. So pressure ~~ 4 or 5 psi. @3jt calculates drop in 1/2" pipe of 0.3psi - if so the MOST improvement you can get is 0.3 psi with a new lossless tube - which is under 10% of head. So on that basis, changing the tube will help, but not stunningly so. NB - don't trust that calculation :-). Is that hot or cold feed? Where is the other feed from (or is this combined?). I'd guesstimate flow rate total to be more like 4 gpm - measure yours with a bucket or calibrated jug .... $\endgroup$
    – Russell McMahon
    Commented Jul 2, 2015 at 18:51
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    $\begingroup$ .. and see how long it takes to fill. Preferred flow rate is a personal preference, and temperature also makes a difference. I like t o shower at 45 degrees C showerhead temperature. My wife prefers 35C which I find too cold. IF this is hot feed, By increasing hot temp you need less hot to flow so get better effective rate. If this is cold feed, by decreasing thot you need more hot and less cold. | ie we need total picture of what is fed how to provide a good answer. Also, measure flow rate you have and also decide what you'd like. Adding an electric pump MAY be easier depending on situation. $\endgroup$
    – Russell McMahon
    Commented Jul 2, 2015 at 18:55

1 Answer 1

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Think about it like this: friction happens at the boundary layer (in the vicinity of where the water touches the pipe). So doubling the diameter will double the circumference (double the friction) but the quadruple the cross sectional area (quadruple the flow).

Will you get a lot more pressure by installing a bigger pipe in that one section? No.

Using an online Hazen-Williams calculator (source) with l = 6 ft, c = 140, q = 2 gal/min and d = 1/2" (conservative, the ID's probably a little higher) gives a pressure drop of about 0.3 psi with the 1/2" pipe.

To put 0.3 psi into perspective, your house water pressure should be somewhere between 40-60 psi. Check your water pressure, and the water pressure at your water heater. Plenty of videos on YouTube show how... note that before you connect the meter you really want to let some water flow, odds are there's gunk in the bottom of the tank that will foul your meter and give you a bad reading.

Most of your friction will be not in the straight run but at the 90 degree turn, although in this case I would guess you've got a bad connection, maybe a leak.

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  • $\begingroup$ Makes sense. I would think that increasing the diameter would have a big effect on the friction in the 90 degree turn, though. I have no idea how to calculate that. $\endgroup$
    – haresfur
    Commented Jul 1, 2015 at 22:38
  • $\begingroup$ 1) why did you selected q=2GPM? did you make some calculations? if yes, how? 2)what is the preferred pressure and flow rate for shower? 3)the "pipes" shown in red are actually PEX tubes, so i don't have any 90 elbows installed. $\endgroup$
    – Loris Doumanian
    Commented Jul 2, 2015 at 14:34
  • $\begingroup$ 1) My shower head has 2 GPM stamped into it, I see it every day :) 2) Pressure is a supply authority/code type thing, although you probably have a pressure regulator somewhere on your property that you can adjust but don't go too high because your pipes aren't built to take it. Flow rate is what your shower head permits, probably a few GPM. 3) No elbows anywhere? Anyway, it sounds to me like you have a leak. $\endgroup$
    – 3jt
    Commented Jul 3, 2015 at 16:42

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