Why is no one using 3/8" pex?

I am building a new house and wish to put in a home run system using a manabloc manifold. I am planning on using 3/8" pex to all fixtures except tub/showers and sill cocks, which will be 1/2". I do not find local supply houses stocking 3/8" and several plumbers say 3/8" is to small. The following company, http://www.mvsupply.biz/index.htm is a large supplier of pex and pex related supplies. According to them 3/8" is the way to go. Why is there this difference of opinions? Is this so new that the majority of plumbers are unwilling to change? I see the home run system in some cities but I don't think they are using anything smaller than 1/2". The advantage of the 3/8" is the speed at which the hot water gets to the fixture. The fixtures served by the 3/8" use less than 2 1/2 gpm, so what's wrong with this? Is Mountain View Supply wrong?

If you have a system that can handle 20 psi/drop per 100 feet at 1.7 gpm I guess it will work. It would be a bit tough to balance the hot and cold sides I suppose...having one at say 75% hot water for the shower is going to create a nasty differential in the supplied pressure to the shower valve. 3/8" PEX is going to be higher percentage pressure loss than the shower valve or showerhead.

What sort of home supply pressure do you intend on having? Is this multistory? The upper level could be really challenged because there will be perhaps 5-10 psi elevation loss, a longer run equalling maybe 50 feet, and then the shower valve and finally the showerhead. If you have 50 psig supply pressure at a PRV the showerhead would likely see about 20-30 psig...and that's not likely to make for a great shower.

The wall of pex tubing is thicker than copper, and at 3/8", your max flow at acceptable velocity is just a bit too small. Now, if you had a manifold in the bathroom, you might get away with it, but not home runs from the entry point. Most valves are designed for 1/2 or 3/4" inputs. the internal passages are often smaller, but while pex has less friction losses than rigid, 3/8" is probably just too small. Might work for a sink, but I wouldn't want it for a shower or tub filler. Wouldn't work that great for your washing machine either. You'd curse it on a hose bib when watering the garden, too.

Pex

Our supply houses stock 3/8" o.d. PEX, and many plumbers do use it for the runs from the manifold to the fixtures. Long runs can develop excessive pressure drop leading to less than desirable flows from the faucets. You have to understand that 3/8" o.d. PEX, (I assume that is what you are referring to since 3/8" i.d. PEX and copper is a "bastard" size and no one has tubing OR fittings in that size), is less than a 1/4" i.d. which borders on inadequate for many uses, other than from the stop at the wall to the faucet.

comfun1 said:

I am building a new house and wish to put in a home run system using a manabloc manifold. I am planning on using 3/8" pex to all fixtures except tub/showers and sill cocks, which will be 1/2". I do not find local supply houses stocking 3/8" and several plumbers say 3/8" is to small.

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Why on earth would you do this? Honestly what's the point?

Run 1/2" min.....

Check code min. requirements

I believe some (maybe all) of the plumbing codes only allow 1/2" as the minimum size connection to a fixture. Might want to check that first.

pex sizing

Lots of supply houses and home centers stock 3/8" pex. With the Manabloc you will have to get a different fitting to connect 3/8 to the ports. My experience is this: 3/8 is good for lavatory faucets where flow doesn't need to be high and you get hot water faster due to the smaller volume in the pipe. For short runs it's ok for showers, kitchen sinks, etc. For sillcocks, bathtubs, washers, and such, where you want some flow, I'd use 1/2."

what's the advantage? It costs the same as 1/2" and is a PITA to transition to fixtures with. BTW the manablock system is one giant piece of leaking poorly made crap. Get copper manifolds with ball valves shutoffs.

I agree, copper manifold with 1/2" pex X 1/2" sweat ball valve. For clean and professional looking, that's what I think looks best. And it makes for the least about of soldering and fittings too, but that's not too big a deal.

I would not use a Manablock. My experience with one is that the valve actuating piece twists off and the wrench breaks.
Vanguard sent me new valve cores but the design defect remains.
It's also pretty easy to cross-thread the inlet ports if you aren't careful.