Body sprayers on 1/2" PEX

[Brad McClelland]

Thank you in advance for any help! I currently am remodeling my master bathroom shower and am wanting to make sure i'm not making a mistake. I have a older house built in '69 in central NC that is currently plumbed for the most part in CPVC (ick...). I've been working on upgrading and changing it out but have only got a few fixtures changed over so far. Everything is being converted to 1/2" PEX with crimp rings, would love to go the expansion fittings but think with the manifold should be OK with the pressure loss and lack of fittings. To start with I installed a Viega manabloc 30 output system that is only running the master and a few other things so far as I'm changing things over. It's input is 3/4" on cold and hot currently. In the master bathroom I am installing 2 shower heads each dedicated obviously off of the manifold, off of one diverter it will run a shower head and a handheld that will switch between the two. Off of the other diverter it will run a shower head and body sprays on the opposite side of the shower head to give a spa experience. I am currently planning on installing 3 Moen A501 body sprayers and want to make sure I have sufficient water pressure. The sprayers say that they take max 1.75 gpm and from what I've seen a 1/2" Moen can only handle about 5-6 gpms, so I'm thinking no more than 3 body sprayers and definitely not 6 as I would like, thoughts...?? And yes I know I'll drain my water heater quickly lol, gas tankless here we come.

 

[Jadnashua]

The ID of PEX is smaller than that of copper, so 3 sprays may see some degradation using pex. There's static pressure (size of pipe doesn't matter) and dynamic pressure where the size of the pipe, length, and flow rate will determine how much can flow through it without degradation. To work at optimum, you need MORE source than what the outlets desire. That way, there's always some back pressure, and it will work great. The more the outlets can flow versus the inlets, the lower the outlets will be as experienced by lower flow and perceived pressure. In reality, the pressure is the same, but the velocity will decrease. That restriction enables the Bernoulli effect, where the fluid speeds up to pass through the restriction. When there's no longer a restriction, it doesn't speed up as you'd notice when there isn't enough flow to max things out - the water doesn't need to speed up to pass through, it just flows like your hose without a nozzle on it.

There's a PEX design handbook you can download for free that will give you more information on what works or not.

 

[Sponsor]

 

[Brad McClelland]

The ID of PEX is smaller than that of copper, so 3 sprays may see some degradation using pex. There's static pressure (size of pipe doesn't matter) and dynamic pressure where the size of the pipe, length, and flow rate will determine how much can flow through it without degradation. To work at optimum, you need MORE source than what the outlets desire. That way, there's always some back pressure, and it will work great. The more the outlets can flow versus the inlets, the lower the outlets will be as experienced by lower flow and perceived pressure. In reality, the pressure is the same, but the velocity will decrease. That restriction enables the Bernoulli effect, where the fluid speeds up to pass through the restriction. When there's no longer a restriction, it doesn't speed up as you'd notice when there isn't enough flow to max things out - the water doesn't need to speed up to pass through, it just flows like your hose without a nozzle on it.

There's a PEX design handbook you can download for free that will give you more information on what works or not.

Much agreed, the concept of supply being greater than demand is a crucial part of analyzing so many aspects, but this is coming from a request of personal experience. 1.75 gpm max x 4 like I would like to do is 7 gpm, this is a bit much to expect from a standard line with a bunch of branching. Theres a lot of expression of geting 6 gpm from a standard 1/2" pex line, but none have said they had a direct line from a manifold. This to me decreases friction and presents a whole different analysis to the situation. I have read direct lines from 1/2" pex can deliver up to 14 gpm, but perfect results are not what we experience in the field most of the time. To me manifolds change the whole equation but to one I don't have much experience with. I wouldn't try this with a standard "branch" home water system, but find that the pressure does seem better with the manifold from what little experience I have. Thanks!

 

[Jadnashua]

The PEX industry handbook does allow a higher flow velocity on pex than that for copper. That can help offset the smaller diameter of the pipe. But, the faster it goes, the more dynamic pressure losses there will be. That handbook (you can find it with a Google search) will tell you what to expect, and give you best practices. 7gpm out of a 1/2" valve is expecting a lot, too!