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

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

Asked by Gerald Blycker
Posted Tue, 03/11/2014 - 01:57

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9 Answers

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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.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 03/11/2014 - 05:44

2.
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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.

Answered by John Semmelhack
Posted Tue, 03/11/2014 - 12:47

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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.

Answered by Mike MacFarland
Posted Sat, 03/15/2014 - 14:54

4.
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Mike,
Thanks for sharing your experience.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 03/16/2014 - 04:28

5.
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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/B...

Answered by Daniel Ernst
Posted Sun, 03/16/2014 - 14:24

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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.

Answered by Benjamin Reeve
Posted Sun, 03/16/2014 - 20:33

7.
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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.

Answered by Gerald Blycker
Posted Wed, 03/19/2014 - 01:38

8.
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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.

Answered by Jon R
Posted Fri, 03/21/2014 - 10:42

9.
Helpful? 0

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

Answered by aj builder, Upstate NY Zone 6a
Posted Fri, 03/21/2014 - 17:19

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