PEX Manifold Design

[LesP]

I am planning to make a custom PEX manifold and would like to confirm that the line sizes and layout are correct.

The house was built in the mid-60's and has a 3/4" supply from the city water meter. I am thinking a hybrid layout would work best with four main trunks as follows:

K/L: kitchen & laundry & hose bibb on main floor
B3: full bath (shower, lav, WC) & wet bar & hose bibb in basement
B2: guest bath (shower, lav, WC) on main floor
B1: master bath (soaking tub, lav, WC) on main floor (back to back with guest bath)
Exp: future expansion for utility sink in basement &/or a future hottub



Do the manifolds need to be 1" to maximize flow or will 3/4" be sufficient?
Will it work to run 3/4" trunk lines and 1/2" branch lines to each fixture?
Should the B2 hot trunk line be 1/2" to speed up the arrival of the hot water to the shower and lav?
Should the B1 hot branch line be 3/4" to speed up filling the soaking tub?
Are diagnostic ports needed on both manifolds or just the cold?

Thanks, as always, for your help with this!

 

[BruceL]

So I'm not an expert or pro, but I would be concerned about this approach. But it might help to understand what problem you're trying to solve.

Presumably your house has a 3/4" trunk for both hot and cold spanning the whole house. Looking only at the hot lines of your plan, I think you need a 3/4" line from the manifold to all those rooms (except maybe B2). So you won't get faster hot water to any of them than you would with a conventional trunk. If you can split up the draws (i.e. run separate 1/2 or even 3/8 to the individual fixtures in the kitchen and laundry) then you would see a benefit, but if you wind up running 3/4" PEX to each room, you will tend to get hot water slower than with a conventional trunk, not faster.

Additionally, 3/4 PEX isn't that easy to work with. Having just run a couple lines into my new master bath, fishing the lines in between existing plumbing and HVAC, getting access through a few holes in the basement ceiling, it was difficult to get installed. I would much rather pull a couple 1/2" PEX lines than one 3/4". If I had to do it again, I would take a different approach, possibly using copper into the bathroom.

(and going back to your plan, you definitely need 3/4" cold supplies. There it isn't an issue with water propagation like the hot, but the difficulty of pulling the pipe is still important.)

So my question would be why look at this sort of "manifold" approach, rather than get a manabloc or other standard manifold and run 1/2" or 3/8 to each fixture/small group?

Bruce

 

[Sponsor]

 

[Terry]

Water pipe sizing

I normally run up to two plumbing fixtures on 1/2". So for a full bath, I would have a 1/2" line for the hot, and 3/4" for the cold.
Running from a manifold, for some you can reduce down to 3/8" if you are running each one on it's own.
Normally a three bath home would have a 3/4" meter and a 1" supply.
The kitchen and laundry can share a 1/2" line.
The soaking tub, if you have 3/4" run to the valve you may want to keep in that in 3/4". That depends on the valve you have. I've seen new ones that come out of the floor with 3/8" lines. Takes forever to fill those tubs.

https://terrylove.com/forums/index.php?threads/water-supply-pipe-sizing.42939/

 

[Jadnashua]

I'd consider putting in a dedicated hot water return line, so if you found it useful, you could add the pump and control later. The ID of a 3/4" pex line is barely larger than a 1/2" copper one. The area of the opening does benefit from the r^2 factor, but it might be an issue.

When using copper, for hot water, the industry recommendation is to limit the flow rate to 5fps, and the available volume at that flow rate obviously varies based on the size of the pipe. With copper, 1/2"~5gpm, 3/4"~8gpm. Most pex allows higher velocities, but note that higher velocities also increases friction, which can result in lower operating pressures (static pressure would be the same with no flow). Your diagram implies three bathrooms, and depending on whether they're all likely to be in use at once, 3/4" pex may be pushing it. If one of those baths was filling a tub, it would slow everyone down. What is the max flow rate recommended for the pex you're considering?

 

[LesP]

Thanks for all the helpful replies -- greatly appreciated!

Bruce, I'm no expert either, but I read an article in Fine Homebuilding (http://www.finehomebuilding.com/2016/07/06/build-pex-manifold) that recommended building a DIY PEX manifold in order to have more flexibility than you get with an off-the-shelf product. From reading the PEX Design Guide (https://www.huduser.gov/portal/publications/pex_design_guide.pdf), it seemed like the remote manifold approach provides the best of both worlds between traditional trunk & branch vs. pure home run systems.

That said, I wasn't planning to use remote manifolds, but simply branch out to each fixture at the end of the "home run" to each fixture grouping. Is that acceptable?

Terry, thanks for the links to the very helpful water pipe sizing information. If I did my math right, I have 32.5 fixture units and less than 80' of developed pipe distance. Could you clarify these terms in the table for me?

Meter and street service (ends at the meter): is that the pipe between the city water main and the meter? I'm guessing it is already 3/4" so I don't think there is any issue there.

Building supply and branches (begins at the meter): The table indicates that I should have 1" pipe but I currently have 3/4" galvanized to the house and the main trunks and only 1/2" branches to the fixtures. I would prefer not to dig up the line between the meter and the house until there is a problem. So that leaves the sizing of the new PEX I am putting in: should the hot and cold manifold be 1" with 3/4" runs to the fixture groupings and then 1/2" to the individual fixture? Except for the soaking tub, where I would like to have a 3/4" valve if I can find one (any recommendation?)

Jim, the main bathrooms are very close to the hot manifold, so I'm hoping a hot return line won't be needed. I'll be using Uponor AquaPex which has a max hot water flow rate of 7 gpm for 3/4" pipe and 13.8 gpm for 1" pipe. (Complete details here: http://www.uponorengineering.com/~/...sizing_is_p736_0314.ashx?version=062720161015). But if the tub valve is only 1/2" or 3/4" won't that be the limiting factor?

Thanks again for everyone's input. I'm not an engineer or a plumber so I appreciate the help!

 

[Reach4]

There are also things that look like this:

They can have 2, 3, 4, 6,7,8 side ports. This example is a "flow through" type, but they have them where one end is closed.

This particular one is Wirsbo, for their PEX A connections.

 

[Jadnashua]

A typical 1/2" tub/shower valve will flow about 6gpm, give or take a bit. A code shower head is limited to 2.5gpm, so even 1/2" pex would be okay for the shower, but could slow a tub fill. The highest flow rate I've found on a 3/4" tub/shower valve is about 17gpm, but I think you'd only achieve that if you had copper feeding it, not 3/4" pex (1", probably could, reduced immediately at the valve).

Pex has some insulating properties verses copper, but if you have 80' of pipe, at say 5fps (7gpm would give about that velocity given the ID), you're talking about 14-15 seconds to flush out the room temperature water before you get hot each time. That's at max flow. Using say a vanity, where the faucet is flow restricted to a max of 2.5gpm, if it was all hot, you're talking about 3-minutes to flush all of the room temp water out! Most modern WH have a heat trap on them, so you don't get much conductive or convective flow that you might get with copper.

One study I read years ago after considering the energy cost for pumping, heating, wasted water, and sewerage costs, found that recirculation saved both money and resources. Your results may differ. It's a pleasure having hot water available almost instantly. In my small townhouse, prior to installing a recirculation system, it could take 2-minutes for it to arrive. Mine's on a timer, and if I get up earlier than when it turns on for the day, it reminds me of why I put it in!

 

[LesP]

Thanks for the picture Reach4! I have looked at those and honestly don't understand the benefit of remote manifolds...but I'm always happy to learn.

My plan is to put in a main hot and cold manifold with shut-offs at the start of each line, since I will be doing this work in phases. The PEX to the kitchen & laundry is already in, but I'll be doing the bathrooms one by one. I figure the central valved manifolds will allow me to keep the water on in parts of the house while working in other parts.

At the end of each major line to a fixture group, I was planning to install tees and elbows as needed to serve individual fixtures. How do the mini remote manifolds differ?

 

[Reach4]

How do the mini remote manifolds differ?

Fewer connections to make.

How does what you propose work better than classic branching?

 

[LesP]

Fewer connections to make.

How does what you propose work better than classic branching?

I don't think my plan necessarily works any better, but I'm hoping it allows me to do the work in stages without having to shut off water to the whole house. Ideally I can build the custom manifold out to the valves and then add lines as I tackle each fixture group.

Fewer connections is good, but isn't there a tradeoff of more pipe?

 

[Reach4]

I don't think my plan necessarily works any better, but I'm hoping it allows me to do the work in stages without having to shut off water to the whole house. Ideally I can build the custom manifold out to the valves and then add lines as I tackle each fixture group.

Fewer connections is good, but isn't there a tradeoff of more pipe?

Maybe. One advantage is that if you run 1/2 to the tub or lav most of the distance, the water will get there quicker.

I have to be slower than you. I blocked off the end of some PEX temporarily with Sharkbite end stops, such as U514LF, until I got a piece of PEX hooked to the fixture.

 

[LesP]

Maybe. One advantage is that if you run 1/2 to the tub or lav most of the distance, the water will get there quicker.

I have to be slower than you. I blocked off the end of some PEX temporarily with Sharkbite end stops, such as U514LF, until I got a piece of PEX hooked to the fixture.

Totally agree with you there on the joys of fast hot water. That's why I'm planning to run 1/2" hot to the guest bathroom (B2) which only has a shower. The master bath (B1) has a soaking tub and I want get as many GPM as the valve can support so I figure 3/4" there.

Do they make valves with 3/4" inlets or is that not needed? Jim, I called Uponor and they told me their pipe is rated for a maximum velocity of 12 ft/sec but they recommend designing to 10 ft/sec for the hot and 8 ft/sec for the cold. Not sure of the implications of all this...

 

[Jadnashua]

Yes, you can buy tub/shower valves with 3/4" inlets, and they do flow more water than a 1/2" valve does...while sometimes hard to find, the specs do show that delta between the two sizes. It's strange they allow higher rates on hot water verses cold. But, check the ID of the tubing you plan, then figure the volume for the lengths involved, and you can figure out how quickly you can get hot water to the destination keeping in mind the actual flow rate of the valve outlet.

 

[LesP]

Thanks, Jim, for the Delta recommendation -- I will check them out.

Here is Uponor's PEX pipe sizing calculator: http://www.uponorpro.com/calculator/

At 80' for the longest fixture run, I got the following results:


So I had it backwards earlier -- it should be a maximum velocity of 12 ft/sec for cold and 8 ft/sec for hot. That makes way more sense since now I understand that the goal is to design the system to keep the water pressure/velocity from potentially tearing up the pipes.

With all that said, I must admit that I am still confused as to which part of my interior plumbing needs to be 1". Should I have 1" pipe from the main shutoff in the basement just to the water heater ( ~5 ft) or all the way through the two manifolds?

I tried to do the Water Supply Fixture Unit (WSFU) calculations, but UPC code doesn't separate cold and hot values like IPC does. Ack...please help, Terry...

 

[Jadnashua]

Just like branches of a tree...the trunk must support all of the branches...so, you need to size each section to handle the needed water for all needs at the trunk, then, as you branch off, the sizes can get smaller. The advantage of a manifold is that one large pipe feeds it, then, you only need much smaller ones to the individual points of use.

 

[Reach4]

it should be a maximum velocity of 12 ft/sec for cold and 8 ft/sec for hot. That makes way more sense since now I understand that the goal is to design the system to keep the water pressure/velocity from potentially tearing up the pipes.

Where do you find those two pieces of info?

 

[LesP]

Where do you find those two pieces of info?

That info came from Uponor's tech rep and is used in their online calculator (see the link in post #14 above).

The entire calculation process is described in this document:
http://www.uponorengineering.com/~/...sizing_is_p736_0314.ashx?version=062720161015

Hope this helps,
Les

 

[Reach4]

That info came from Uponor's tech rep and is used in their online calculator (see the link in post #14 above).

The entire calculation process is described in this document:
http://www.uponorengineering.com/~/...sizing_is_p736_0314.ashx?version=062720161015

Ahh, there it is on page 4:

Domestic Cold-water Piping

• Maximum velocity of 12 ft./sec. through pipe
• Recommended velocity of 10 ft./sec. through pipe​

Domestic Hot-water Piping

• Maximum velocity of 12 ft./sec. through pipe
• Recommended velocity of 8 ft./sec. through pipe
• Maximum operating temperature of 200°F (93.3°C)​

That seems a little different than what you were interpreting, I think.

 

[LesP]

Ahh, there it is on page 4:

​

That seems a little different than what you were interpreting, I think.

That is entirely possible -- I still have a lot to learn! What part am an interpreting incorrectly -- the math, the design purpose, or something else?

I pride myself in not doing hack DIY jobs, so I welcome further education so I can do the job right!

 

[Reach4]

That is entirely possible -- I still have a lot to learn! What part am an interpreting incorrectly -- the math, the design purpose, or something else?

I pride myself in not doing hack DIY jobs, so I welcome further education so I can do the job right!

For one thing, the maximum is shown as 12 for both hot and cold.

If the hot/cold differences were based on protecting the pipe, why would the max number be 12 for both?

I don't know why the "recommended" number differs. I know that cold water is denser. I can't relate that as to why the "recommended" differences exist.

I am not proficient in that. I had never noticed the recommended hot vs cold differences before.