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New Pex in Residential Addition


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I have new Pex plumbing installed in an addition onto my house. The water pressure and flow are fine in the existing house. The existing house was re-plumbed with Pex last year. My new addition is not complete and framing is open. A new bathroom is rough plumbed. Two new exterior hose bibs are connected and flow water at about half the amount as the other existing house hose bib. I am concerned how the new bathroom flow/pressure will be when finished. I need to solve this before insulating the new addition. Ideas on where to start?

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Everything is 1/2"? That's too small. You need at least 3/4" laterals to distribute through the house, and the last fixture in any run can then be reduced to 1/2". If it's all 1/2", you're going to have substantially reduced flow.

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Shows what I know.....

I'm thinking conventional plumbing practice with copper pipe. That's what you get living in a city where the local plumbers union effectively kept PEX completely out of the equation. Completely. I don't think in PEX language yet. Folks here don't even know what it is.

Maybe there's already a manifold and home runs, in which case, what I said is wrong.

If it's plumbed conventionally, with primary laterals and risers to fixtures, you want/need the larger pipe. If it's home runs to a manifold, you don't necessarily need the larger pipe.

Which is yours? Manifold and home runs or conventional laterals and risers?

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We have had two clients that used Pex in their home to save money (we specified copper). The fittings fail at the angles and both houses had to be gutted, remediated for mold and new water piping was installed. They both collected on product liability claims.

Some plumbers install the pex like copper. There is a fitting at every turn with a clamp. This can be a nightmare if not installed perfectly and with the proper hardware.

Listen to Bill about home runs for the pipes. The less fittings, the less of a chance of problems.

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I see lots of PEX as replacement plumbing. Every instance is 1/2" tubing with fittings following the path of the original galvanized.

The only manifold distribution I've seen was made of enough galvanized fittings to open a hardware and plumbed with vinyl beverage tubing.

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Pretty much all homes in my area built after 2006 will have PEX. Manifolds are seldom used simply because it adds a few dollars to the project. The restrictions come at the fittings with PEX, the 1/2" pipe slides over the the fitting which has a smaller diameter in order to allow the pipe to slide onto the fitting. You also loose flow rate for every root of rise, so second floor homes have a pretty good pressure/flow reduction. Most of the better plumbers and builders in my area are now upsizing PEX installs to compensate for the decreased ID of the pipe fittings.

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I see PEX every day and much of it is 1/2" to the fixtures. But do not see a lot of manifolds. Yes most runs are made with as few elbows as possible.

Something to check out - sometimes a piece of plastic will get lodged in the pipe somewhere and reduce the flow. You could have a plumber flush the lines to your addition.

If that's not it, you should upsize to 3/4".

What Scott just said a minute ago. [:)]

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Thank you for all the information! The existing house and the addition to it are conventionally plumbed with Pex and no manifolds. There does seem to be quite a few 90 degree connections in the new work where maybe some could have been avoided, but who knows. The question about low water flow came from the two new hose bibs that are connected and functioning. There are three new bathroom fixtures in the addition yet to be connected with unknown flow that I am also concerned about since once covered the fix could be very expensive. The rough plumbing was recently passed by city inspection. Is anyone aware that the 1/2" Pex is not allowed by the UPC in my situation? Is the 3/4" a recommendation, or a code requirement?

Thanks again for the education!

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My house is plumbed almost entirely with 1/2" PEX and flow is fine. There is 3/4" from the pressure tank to the water heater, with 1/2" home runs to each fixture from there. There are no fittings anywhere in the 1/2"... it's basically run more like Romex than water pipe.

If a guy is going to run it like copper, with a bunch of fittings, it has to be 3/4". Take a look at the I.D. of a fitting and you'll see that it's quite a bit smaller than nominal. Put in enough of those and it really reduces flow. This might not be an issue at a lav faucet, but at the tub and at hose bibbs it will be noticeable.

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