Direct Supply PEX Diameter to Shower and Tub

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Dave Osborne

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I have read numerous posts on these forums about the subject but questions remain for my situation.

I am connected to the city water utility with a 3/4" meter, there is a 3/4" PVC pipe entering the house from the meter and a 1" copper manifold that provides home-run service to each fixture via PEX-B of various sizes.

Question 1: is 1/2" PEX large enough to supply hot and cold water to my shower mixing valve from the manifold?
  • The shower valve can simultaneously supply two shower heads (a rain shower and hand shower). The rain shower head has a 2.5 GPM (gallons per minute) flow and the hand-shower is 1.5 GPM.
  • Depending on the temperature mix (balance of hot/cold, per the user's desired temperature), let's say it's 75% hot and 25% cold; that would demand 3 GPM from the hot supply and 1 GPM from the cold supply. Or, maybe the user is a lunatic and wants 100% hot or a complete psycho and wants 100% cold, then hot or cold would need to provide the full 4 GPM. For 1/2" PEX, this would require a sustained water velocity of 7.24 FPS (feet per second). Is this possible? Within reason for PEX to transport water at this velocity? Within reason for the water from the city to provide ample pressure (PSI, pounds/square inch) to force water through tubes at this velocity?
  • There are different flavors of rough mixing valve available, I can get a valve with 1/2" PEX crimp ports or a valve with 1/2" - 14 NPT OD 5/8" ID For 1/2" Nom. Copper Tubing. I can use the copper port version to connect 3/4" PEX adapters and run 3/4" PEX hot and cold lines directly from the manifold to the shower valve.
Question 2: My other bathroom has a bathtub with a single shower head. Surely, 1/2" PEX would be sufficient to supply the shower head (it's another eco-friendly 2.5 GPM fixture) but is it sufficient to fill the tub in a reasonable amount of time? The tub has a capacity of 68 gallons, so at a rate of 6 GPM, I'd be staring at the tub filling for over 10 minutes. Is 6 GPM even plausible with a valve with 1/2" copper ports?
  • This tub/shower valve has all [1/2" - 14 NPT OD 5/8" ID For 1/2" Nom. Copper Tubing] ports. So the pipe running from the valve to the tub faucet is 1/2" copper.
  • Again, user-preference of the bather becomes a factor. If the bather is nuts (or an elite athlete) and wants to take an ice bath, then the cold supply line will be solely responsible for providing the GPM to fill the tub. Likewise for someone who wants to take a near scalding-hot bath, the hot side is providing everything.
  • My plan is to run 3/4" PEX from the manifold directly to this tub/shower valve, with 3/4" PEX to 1/2" Copper adapters directly preceding entry to the valve. Tub fill performance is still limited by the 1/2" copper tub faucet line (so, yes 3/4" PEX is overkill perhaps, but necessary to overcome the limits of 1/2" PEX's inner diameter of 0.475 inches for tub filling purposes). Is this a rational plan? Would 1/2" PEX be sufficient and I just wait a little bit longer for the tub to fill up? What is a reasonable expectation for water velocity in FPM through 1/2" PEX with average residential PSI from city utility?
Thank you for any and all advice or insights!
 

wwhitney

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1) I'll say yes, 1/2" PEX is probably fine for a 4 GPM shower. I've seen 8 fps as an acceptable design velocity for PEX hot water systems.

It will depend on the length of run from the manifold to the shower valve. Per http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php 4 gpm through 1/2" PEX will drop 20.7 psi/100 ft. Not a problem at 10 ft.

Say you're at 50 ft, that's a 10 psi drop. Probably also OK, but you can come up with scenarios where you'd want to upsize to 3/4" PEX to reduce the pressure drop to 2 psi. E.g. if your city water pressure is only 50 psi, and upstream of the manifold already drops 10 psi at 4 GPM (that would be a lot), and your shower valve specifies a minimum input pressure of 35 psi. Or if you look at the shower valve's flow vs pressure curve and decide you really prefer the flow numbers at 38 psi over the flow numbers at 30 psi.

So knowing your static pressure from the city would help guide your decision. If you can determine the residual pressure at your hot water manifold with a 4 gpm flow rate, that would be even better. [That would mean having a pressure gauge just before the manifold, flowing 4 gpm through the manifold, and see what the pressure reading drops to. Relative to the static pressure, the difference is the pressure loss attributable to everything between the city water main and the manifold from 4 gpm of flow.]

2) So this is a tradeoff between filling the tub quickly and reducing water waste and wait time every time you take a shower. With 3/4" PEX, the volume of stale hot water in the pipe is about double that of 1/2" PEX of the same length. If you're mostly going to be showering, then I would go with 1/2" PEX. If you're mostly going to be bathing, go with 3/4" PEX.

As to the flow rate you'd get with 1/2" PEX hot only to a tub, if your 7.24 fps computation for 4 gpm is correct (I didn't check), then you wouldn't really want to go much above 4.5 gpm, to stay near the 8 fps guidance.

Cheers, Wayne
 

Dave Osborne

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Thank you for the replies, they are very much appreciated.

Wayne, you hit on all the points I was wondering about. In my search for documentation of what the city's static pressure might be, I discovered news articles about pressure being an issue for people in the area/county, due to a massive boom of development over the past year. In any case though, I will give the utility a call to inquire.

I should have mentioned that this is new construction and the meter is not currently connected to the manifold, it is connected to a temporary hose spigot. To pass the rough-in inspection I have to pump the manifold and PEX pipes full of pressurized air, so it's kind of a bummer that I have to commit to an installed supply/distribution design based purely on theory. In any case though, it is encouraging to hear that 8 fps has been considered an acceptable design velocity and I do get the feeling that 1/2" PEX is the way to go for the master shower. The shower is about 11 feet away from the manifold and water heater

I already built the manifold, it is predominantly 1/2" valves with a few 3/4" valves on both the hot and cold sections. So, I have some freedom to rearrange the connections. My plan now is to abort the use of 3/4" supply lines to the master shower and instead use the 1/2" valves that would serve the kitchen sink and dishwasher. I would then use the now-free hot and cold 3/4" valves (and 3/4" PEX) to make mini-trunks to the kitchen sink base cabinet, where they would split into 1/2" hot and cold branches for the sink and dishwasher. This way I don't need to rip apart and rebuild my manifold and I can run two PEX pipes to the kitchen sink base instead of four. Plus, that creates two free 1/2" valves (one hot, one cold) that I can use for something else.


I am thinking this through in print here for posterity's sake:
That "something else", could be the tub/shower valve in the second bathroom. I do agree that the volume of stale hot water wasted in the run of 3/4" PEX is not ideal. The bathtub valve is about 14 feet away from the manifold, really more like like 23 feet due to the circuitous route that the PEX takes to get there, so a bit less than half a gallon (0.4209) of water between the manifold and the valve. In theory, it takes something like 6.3 seconds to clear the stale hot water in the tube at a flow rate of 4 GPM (3.52 fps velocity) or 3.15 seconds at 8 GPM (7.05 fps velocity). Not terrible as far as wait time **for tub filling**, but wasteful nonetheless.

The shower is another story. The shower head limits flow to something like 2.5 GPM, which would take 10.1016 seconds to purge. Usually the water comes out of the tub faucet first before the diverter is engaged, but still that takes awhile.

If I use 1/2" PEX for the tub instead, that is roughly 0.2116 gallon of stale hot water between manifold and tub valve, basically half the volume of 3/4" PEX and (maybe) half the time to clear it (but higher velocity than 3/4" is required). If I can clear the 0.2116 gallon out of the 1/2" PEX at a velocity of, say, 8 fps maximum, that moves 4 GPM so it would take 3.174 seconds. The shower head is a choke point, so in shower mode at 2.5 GPM it would take 5.0784 seconds to purge the stale hot water.

I could be way off base here, but it seems like the difference in wait-time for hot water arrival would be negligible right? Assuming the water is run out of the tub faucet until hot then diverted to the shower head. 3/4" can move much more water at a given velocity than 1/2", provided the pressure exists to maintain that flow rate.

So, wastefulness and environmental considerations (and water bill) might be the key factor of consequence. 1/2" PEX wastes about one (0.96195264) liter of stale hot water while 3/4" PEX wastes almost two (1.9134493) liters. For tub filling, the initial stale hot water need not go to waste; the drain can be plugged and that not-so-hot water can still contribute to the volume of bath water while the supply gets hotter and hotter. Halfway through the tub fill, the mixing valve can throttle back the amount of hot water balance at bit. The shower is the wrench in the machine and, yes, for adolescents/teens/adults one could assume that many more showers are being taken as opposed to baths.

Verdict: I don't know. I should invent an electronically triggered valve/knob/button that shuts of the fixture at the manifold instead of in the shower/tub itself. But you have given me great food for thought. I can certainly report back on the final decision and its implications as a means of repaying the community with some data for analysis.
 

Reach4

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I could be way off base here, but it seems like the difference in wait-time for hot water arrival would be negligible right? Assuming the water is run out of the tub faucet until hot then diverted to the shower head. 3/4" can move much more water at a given velocity than 1/2", provided the pressure exists to maintain that flow rate.
The gpm for a shower is almost entirely related to the shower head. Velocity will be a function of gpm and cross-sectional area. So the hot will take about twice as long to arrive using 3/4.
 

wwhitney

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In any case though, I will give the utility a call to inquire.
Since it sounds like you already have water supply to the building, the typical place to start would be to put a hose-bib style pressure gauge on a hose bib and just measure your static pressure.

That of course only gives you a data point at the moment, so calling the city to inquire as to their targets and guarantees could be useful. It could be that you have, say, 60 psi now but a bunch of downstream construction is planned and your typical static (in your house) pressure will fall due to everyone else's future concomitant use. So if the city says "well, we have a service guarantee of 45 psi, and will upgrade our water mains as required to maintain that," then you could decide to design around 45 psi static rather than what you measure now.

Cheers, Wayne
 

Dave Osborne

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The gpm for a shower is almost entirely related to the shower head. Velocity will be a function of gpm and cross-sectional area. So the hot will take about twice as long to arrive using 3/4.
Absolutely, I was only using the bathtub/shower in my second bathroom as the basis for comparison. Master bathroom has just a shower but the second bathroom has a shower in the bathtub.

When first turned on, the water spews out of the bathtub faucet at an unrestricted rate (limited only by the valve itself and 1/2" copper pipe, I presume). When toggled, the diverter switch sends the water up to the shower head. So my thinking was to exploit the tub faucet as a means of rapidly purging the stale hot water first, then (once hot) divert the mixture up to the more restrictive shower head.

It's definitely more wasteful but I'm thinking that the bigger PEX and unrestricted tub faucet might negate the added wait time, if that make sense.
 

Dave Osborne

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I also just got the specifics on how much of a bottleneck the valve itself will be. Kohler says 5-6 GPM through the valve, which diminishes some of the value of 3/4" PEX, but to maintain 6 GPM using 1/2" PEX (assuming the mixer is basically set to full hot) would require 10.86 fps velocity through the hot tube (and even higher through the fittings) yikes!

For this reason, I am going to run 3/4" PEX to the tub. Apologies to mother nature, I will have guests fill a watering pail with the stale hot water so that I can water my garden with it or something.
 

wwhitney

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Apologies to mother nature, I will have guests fill a watering pail with the stale hot water so that I can water my garden with it or something.
Or you could put in a greywater system for your tubs and showers. : - )

Cheers, Wayne
 
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