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Community and Q&A

Is it reasonable to use 3/8″ PEX supply lines to all fixtures?

Gerald Blycker | Posted in Mechanicals on

I am planning the plumbing system for my Net-Zero Energy home that is being built to Passive House standards. I will be using PEX tubing with a manifold system using home runs. I designed the house so that the bathrooms and laundry room are centrally located, making the supply runs pretty short. I want to minimize the amount of water and energy being used and low-flow fixtures provide plenty of water for my needs (2.2 gpm at sinks and 2.5 gpm for showers).

Most people seem to be using 1/2″ PEX for supply lines, but that seems to be more water than is necessary for most fixtures. A 3/8″ PEX line would get hot water to the fixture twice as fast and waste half the amount of water in the process. Considering that the fixtures will be low-flow and the braided line leading to sinks is pretty small, would 3/8″ PEX provide enough water? I can’t see it being an issue for the toilets, sinks, dishwasher or clothes washer. But I’m concerned about the shower and tub. Will I be able to get 2.5 gpm through 20 feet of 3/8″ PEX at about 40 psi? I am on a well, so I can increase the pressure of the system if needed.

I don’t believe that a tub’s faucet has any restrictors, so what flow rate can I expect there? I am fine with waiting a few extra minutes for the occasional soak in the tub.

Has anyone tried using 3/8″ PEX supply lines and what has your experience been? I’ve seen 3/8″ manifolds for sale on line, so I don’t think I’m crazy in considering this.

Thanks for any feedback or input.

-Gerald

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Gerald,
    There are two aspects to your question:
    (1) What is reasonable?
    (2) What does the code require?

    Concerning what is reasonable: the national expert on this issue, Gary Klein, suggests that 3/8-inch tubing is only reasonable for a single fixture rated at 1.5 gallon per minute or less. Fixtures with greater flow rates need 1/2-inch or larger tubing.

    To learn more about this issue, see Gary's article, Inefficient Hot Water Piping Layouts Waste Hot Water.

    No matter what you think is reasonable, your desires may be trumped by your local plumbing code. Before proceeding with your plan, check with your local code official to learn what local regulations require.

  2. User avatar
    John Semmelhack | | #2

    There are pressure drop tables in the PEX Design Guide available here - http://plasticpipe.org/publications/pex_handbook.html. See page 96 for pressure drop table for straight pipe. Page 95 for representative pressure drop for typical fittings.

  3. Mike MacFarland | | #3

    Martin- I believe Gary was referring to a code maximum flow rate for piping, not specifically PEX tubing. My understanding is that copper is limited by code to 5 FPS velocity, while PEX is allowed twice that at 10 FPS. So a 3/8 copper line is currently code compliant at 1.5 GPM, while the same nominal PEX is allowed to deliver 3.0 GPM at exactly 10 FPS.

    Gerald, we have done this successfully on multiple projects with up to 2.5 GPM fixtures with very happy occupants who don't have to wait or waste unnecessarily. You might consider going 1/2" on the tub. Pay very, very close attention to the volume of water between your water heater outlet and the start of your PEX. Note also that code mandates a minimum of 18" of separation between the outlet and any PEX piping- something you might discuss with your code official to see if you could get an exception to in the event you are using a water heater without a hot exhaust system near the outlets. I'd have this discussion so that the manifold can mount right at the outlet.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Mike,
    Thanks for sharing your experience.

  5. Daniel Ernst | | #5

    Gerald,

    In a current project we are using a combination of 1/2" and 3/8" PEX tubing---supplied through a Manabloc manifold to create a parallel system. Using this type of system has certain advantages and disadvantages, so it all depends on design and fixture type.

    We are shooting for the WaterSense standard. We installed 3/8" for the majority of the fixtures:
    * Lavatories - < 1.5 GPM
    * Kitchen Sink < 1.8 GPM
    * Toilets - 1.28 GPF
    * Shower - 1.75 GPM

    We installed 1/2" PEX for the following:
    * Shower / Tub Combo - Variable GPM
    * Washing Machine - Variable GPM
    * Outdoor Faucets - Variable GPM

    The water supply is public; our pressure reducing valve brings the supply down to 60 psi. Your performance with a standard well pump and pressure tank may not be comparable.

    Another consideration is supply line length. The pressure drop on 3/8" PEX tubing is considerable. compared to 1/2" and 3/4" diameter tubing / pipe. See the tables that John references.

    Depending on your well and well pump you can either specify a higher operational pressure (40/60 psi switch on your pressure tank) or use a constant pressure controller (typically 50 psi +/- 5 psi).

    You can also reduce your fixture flow rates by using WaterSense labeled fixtures. That will drop your kitchen sink and shower flow rates closer to the 1.5 GPM suggested limit, lower your risk of poor performance.

    Here's one example of a great low-flow showerhead:

    http://www.amazon.com/Niagara-N2515CH-Fixed-Mount-Showerhead-Chrome/dp/B0089CGG9K/ref=pd_sim_hi_5/176-7430389-8139822?ie=UTF8&refRID=0E7JADBJ9FXJK6RAA44F

  6. Benjamin Reeve | | #6

    This is being answered by a person (me) who also only very rarely uses #14 wire. I have had some experience with pex, both for water supply and in radiant heating, and my question with respect to using 3/8 pex would be "Really - why would you?" When the supply pressure drops to 20 psi (...because the water from the Town is grungy as can be and you have installed a filter bank, because the main in your street is done for and won't be replaced for months) you are gong to ... put in a booster pump and ... use energy for that instead. When you decide to add on to your house, and (home runs notwithstanding, you want to 'borrow' a little water from that 3/8ths line, the bit of water then dribbling at that new sink will be ... just fine - no doubt.
    Mine is a dissident voice here, I know, but this discussion has the feel of 'too much skinning the numbers' and not enough 'build it so it is going to want to work out fine' for my experience with buildings of many and various kinds, and water supply from vegetable farm irrigation to apartment buildings to laboratories. Right, save energy, save water, save materials, all good. Enjoy your low-flow showerhead. Great. But squeezing the capacity of what you are putting in the walls (without any real materials or economic benefit) just ends up being constricting and cheezy and a built-in dependence upon 'one infrastructural corner' of things-as-they-happen-to-be, but might not always be.

  7. Gerald Blycker | | #7

    Daniel,

    My plumbing inspector said that I could use 3/8 PEX for everything except for a bathtub, washing machine and hose bibs. I am also using a Manabloc manifold with 1/2" ports and reducing down to 3/8" for hot water lines going to the lavs, shower, kitchen sink and dishwasher. I will be using 1/2" for all of the cold lines.

    Could there be issues with a faucet mixing water from a 3/8" hot and a 1/2" cold line?

    Both hot and cold will go through the 3/8" braided line before being mixed by the low flow faucet. All of my fixtures are between 10-25 feet from the manifold. If the only issue is I need to move the faucet handle up and to the left instead of straight up to get warm water, I'm fine with that.

    Thanks for everyone's feedback.

  8. User avatar
    Jon R | | #8

    I use less than 1 GPM for showers and always have high water pressure. So the faster warmup of 3/8" makes sense for me - thanks for the idea.

    Generically, I agree with doing the calculations for the specific length, pressure and flow rate.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Sounds like very good advice amongst the posted responces and I concur with all of them.

    That being said, I have a shower that only puts out 4 gpm and will someday hopefully turn it into a 20 gpm waterfall. I feel for those in water shortage areas.

    aj

  10. Nik Fiorito | | #10

    Not a technical answer, but I outfitted our previous home with a Manabloc manifold with both 3/8" and 1/2" outlets, and for the most part I regretted putting 3/8ths everywhere but the toilets. 1/2" provided much higher pressure and seemed to get warm water to the taps much more quickly, and the 3/8ths to the second-story shower was not enough pressure-wise.
    Again, nothing technical, and maybe not the greenest choice? But I will be doing only 1/2" pex in my new place; it also allows for purchasing one size of tubing and fittings (bigger packs and lengths save money).

  11. Richard McGrath | | #11

    In a parallel manifold distribution system 3/8" tubing is allowed as long as you have code minimum pressure at the fixture . That being said 3/8" is not that much less expensive than 1/2" . Here is the tricky part , 3/8" tubing at similar pressure has exponentially higher turbulence which has the uncanny ability to have greater heat loss in the run to the fixture . Is it cheaper first cost , maybe , will your hot water lines have higher losses between A & B , YES ? If saving energy is your goal , use the 1/2" .
    We have used 3/8" tubing many times because it was spec'd . Did the end user like flow rates , not always . Did we spend more or less first cost , negligible .

  12. Charlie Sullivan | | #12

    Richard (#11), if you have any insulation on the pipe whatsoever, that will overwhelm the difference due to the amount of turbulence. And then the smaller tube just gives you savings on hot water because of its smaller volume.

    Mike (#3), I would think that a manifold mounted right at the outlet of that water heater would lead to high standby losses--a manifold is hard to insulate well, and I would want to have a well insulated heat trap plus at least 6" more of well insulated line past the heat trap to minimize standby losses.

    (If anyone has good ideas about how to easily insulate a manifold well, I'd be interested)

  13. Eric Habegger | | #13

    I think it makes sense to use homerun 3/8" pex on bathroom lavs. They don't push much water and I wait forever for hot on my current large diameter lav lines that are connected to the shower and bathtub hot. As far as toilets go, they use only cold and will stop by themselves when the float hits it mark so no water saving there whichever you use. I would use separate lines for cold to the shower and the toilet so when you flush the toilet there isn't as much decrease in cold water to the shower.

  14. Richard McGrath | | #14

    Charlie , Turbulence is turbulence and Reynolds numbers don't lie . Heat lost through the tube wall is heat lost . The fact that it is now contained and cooling within the insulation does not negate that the minute it passes through it is lost because it cannot re enter . Hot goes to cold .
    Remember also that faucets have minimum pressures at which they will deliver the stated gpm at the faucet and 40 psi - something could be an issue .
    smaller tubing size means nothing towards savings on hot water . 1.5 gpm is 1.5 gpm regardless of tubing size .

  15. Malcolm Taylor | | #15

    Richard wrote:
    "Heat lost through the tube wall is heat lost . The fact that it is now contained and cooling within the insulation does not negate that the minute it passes through it is lost because it cannot re enter ."

    This runs against everything I thought I understood about insulation.

  16. Richard McGrath | | #16

    Does heat leave a building or does cold enter it Malcolm ? Did insulation at some point that I am not aware of begin to possess a property whereas it can stop dead all flow to the colder ? Maybe that colder energy which has reached the insulation can reverse and return to where it originated , No wait that doesn't work . Could the building cavity which this water line passes through be at thermal equilibrium with fluid inside the pipe ? I suppose it is possible , but not in the houses discussed here . Maybe there is some R60 pipe insulation just marketed . Assuming a 50* Delta T between the fluid being moved and the interior of the building in winter are you saying 2" of fibreglass would be sufficient to stave off a 100* Delta .
    Pardon me Malcolm , but could you explain ?

  17. User avatar GBA Editor
    Martin Holladay | | #17

    Richard,
    You said that turbulence causes higher heat loss. Charlie said that if you insulate the pipes, that addresses your worries about heat loss. Your incomprehensible answer, and I quote: "Heat lost through the tube wall is heat lost. The fact that it is now contained and cooling within the insulation does not negate that the minute it passes through it is lost because it cannot re enter."

    That's a nutty theory. Let's see -- the air in my house is at 72 degrees F. Oh, no! The heat has been transferred to my drywall! Now the heat is gone forever, because my drywall has warmed up to 72 degrees! Oh, I forgot. I have insulation on the exterior side of my drywall.

    I think that my panic was premature.

  18. Richard McGrath | | #18

    The intended destination for that 120* water was the fixture Martin . Whatever heat did not make it to the fixture is lost , it did not arrive at it's destination . Correct ?

    What was the temperature of the air that left the registers or evaporator ? That heat alreday reached it's destination , the room air and it has done what it was supposed to , given up it's heat to the room and occupants . The sheetrock in your example would be the tube wall in my example . Once it reaches the cold side , what happens ?
    Is avoiding something that is avoidable and undesirable somehow not fashionable because it came from me ? Stop !
    Fluid moving through a pipe has different types of flow . Turbulent , laminar and transitional . In a heating system we want heat transfer so transitional or turbulent is desirable . Laminar and less heat transfer is what we want when we expect as much of that 120* fluid to reach the faucet where it was intended to go . The less heated water that is at the business end to mix with cold or not will cause the user to use more water to get the desired effect .
    What is so difficult to understand ?

    To the Op , I would size the pipe however you want . Just know the facts and don't let first cost be the only deciding factor .

  19. Daniel Young | | #19

    Richard,

    Heat lost IS heat lost. But adding insulation reduces that heat lost. That's it's main purpose. But to address your point on the small diameter pipe.

    The type of heat loss you focused on was only one link in the chain, the convective heat loss from the fluid to the pipe wall. This heat loss is calculated by [q=h*A*dT (heat loss = convective heat transfer coefficient *Area*temperature difference between fluid and the surface)] The velocity of fluid (and turbulence) is much higher. So the convective heat transfer coefficient will go up, the area of the pipe will go down by 25% since the 3/8" pipe is 25% smaller than the 1/2", but the difference in temp?

    If the pipes were both insulated, then the above is basically insignificant. The insulation essentially allows the pipe wall to remain near the fluid temperature in both cases. The conductive heat loss in the insulation becomes the dominant heat loss factor. The insulation is the throttle, not the turbulence in the pipe. In fact, because of the heat transfer mechanics or radial insulation (pipe insulation) the heat loss will be lower on the 3/8" than the 1/2" pipe, if they have the same thickness of pipe insulation.

    If the pipes were bare, the turbulence and fluid velocity might be a factor, but the PEX may still be the throttle.

  20. User avatar GBA Editor
    Martin Holladay | | #20

    Richard,
    I agree that the drywall in my living room is analogous to the PEX in your example. But when the air in my house warms the drywall to room temperature, it is not "heat lost." Similarly, I'm not particularly worried if the PEX tubing is at the same temperature as the warm water flowing through it -- as long as the PEX tubing is insulated.

    Heat will always flow from a warm material to an adjacent cold material. The relevant factor is the rate of heat flow. Insulation slows the rate of heat flow -- and as Daniel Young just explained, it slows the rate so well that it makes turbulence irrelevant.

  21. Richard McGrath | | #21

    Nik said
    " Not a technical answer, but I outfitted our previous home with a Manabloc manifold with both 3/8" and 1/2" outlets, and for the most part I regretted putting 3/8ths everywhere but the toilets. 1/2" provided much higher pressure and seemed to get warm water to the taps much more quickly, and the 3/8ths to the second-story shower was not enough pressure-wise.
    Again, nothing technical, and maybe not the greenest choice? But I will be doing only 1/2" pex in my new place; it also allows for purchasing one size of tubing and fittings (bigger packs and lengths save money)." This is a real world account and opinion .
    Answered by Nik Fiorito
    Posted Apr 21, 2015 12:04 AM ET

    Additionally I add the pertinent pages from the handbook linked to above , see attached .

    When we want heat transfer for that purpose we usually use between 2' - 4' per second flows . Please note the FPS rates for 3/8 and 1/2 on the document . Keep in mind that we should probably be looking at 1.1 and 1.25 GPM as our goals for hot water lines . You can clearly see which size tubing has less pressure loss and also which pipe size has velocities conducive to heat transfer . The pages also explain what Nik experienced .

    Insulation is always a good idea unless leaving it out of the equation serves some beneficial purpose . It certainly does lessen the rate of heat transfer by providing resistance to the lower temp on the cold side of it . There will be a boundary layer of fluid in the 1/2 pipe which is another barrier to heat transfer whereas the 3/8 will not have that boundary layer . More contact , more heat loss . Heat transfer will only cease when the insulation is in equilibrium with fluid and pipe wall . No way around it .

    http://en.wikipedia.org/wiki/Convective_heat_transfer

    At the end of the day it is the OPs choice , all we can do is tell someone what we know .

  22. B Haygood | | #22

    I have a very spread-out plumbing system in my ranch style house. I have 3/8" fittings to everything but the hose bibs and bathtub. I don't think the 1/2" there is even necessary. It might take you an extra minute to fill up a tub, and the inside of a garden hose and its fittings are already tiny.

    The three bath home is running off about a 50' run of 3/4" supply PEX, while 1" is recommended. The runs from the manifold (manabloc) to the bathrooms are all over 50'. There is no excess noise, and the pressure is fine at every fitting.

    The only reason to consider using 1/2" for anything is the limited availability of fittings for 3/8". If you just switch to copper for the last bit of the runs, this won't be an issue. However, if you were hoping to be efficient and run PEX all the way up to a faucet, you won't find the faucet adapter in anything other than 1/2" PEX.

    The manufacturers have said nobody is buying enough 3/8" PEX, so they are not making as many 3/8" fittings. I'm sure the only reason this is happening is because of the mythical congecture that 3/8" would be too small.

    For as long a run as I use to each fixture, the smaller internal volume is really appreciated. The shorter bend radii make for nicer installation, too.

  23. sols1dav | | #23

    I retrofitted 1/2 copper with 1/4 pex as an experiment in a home with long runs and wait times for hot water at kitchen sink and lab sinks. Reduction in flow barely noticeable but not objectionable. Many other mothers and housewives have used unknowingly with no comment. Water arrives at kitchen in 11 seconds. I’m planning a new home now and will use 3/8 max.

  24. User avatar GBA Editor
    Martin Holladay | | #24

    I'm a little confused by the "mothers and housewives" comment, but I appreciate your sharing your experience.

  25. Malcolm Taylor | | #25

    There are a number of fixtures that even with high water pressure require more flow than 1/4" or 3/8" can provide, and as Martin pointed out in the very first post, most codes would not allow 1/4" supply for the majority of runs.

    I'm also not sure that the lack of complaints by users is a good measure to judge how well a plumbing system performs.

  26. Zephyr7 | | #26

    1/4" supply lines over any distance would definitely be noticeable. The reverse osmosis system on my well uses a 1/4" line to the fridge dispenser, and even over the approx 10 foot run, you can tell the flow is reduced compared to the kitchen sink that has a 3/8" supply line.

    I agree with Malcolm too. You really need a more objective metric than "lack of complaints" to determine if a system works well. I've seen plenty of scarily bad electrical installations that no one has complained about, but that doesn't mean they aren't time bombs just waiting to start fires.

    Bill

  27. Josh Durston | | #27

    You really need a more objective metric than "lack of complaints" to determine if a system works well.

    So much truth to this, that you can generalize to construction in general.

  28. pierceyslc | | #28

    Gerald, I hope you went ahead with your plan, years ago I see. Change comes slowly, but you were exactly right. With home runs, there is half the cold water to run off to get the hot and less hot water left in the pipe run to go cold when you shut off. My 3/8 runs are vastly better to kitchen and bath faucets and save water and energy. People like you give us a shot at figuring out how to turn around the mess we’ve been making of things. And if we fail in our effort to make things better, we’ll learn so little to lose. Well done. Carry on!

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