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Old 02-05-2013, 01:14 PM   #1
jkinder
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Default two 1/2" lines feeding one 3/4" supply line?

Hello,

I am in the process of installing a new Manabloc pex manifold. I am doing home runs on each appliance in the kitchen, 2 bathrooms, utility room , and mud room, which are all centrally located near the utility room where the manifold is being installed.

The problem comes with a master bath that has a 3/4" supply going to it. All of the piping is not accessible (buried below sub floor from above, hidden above spray foam from below). Furthermore, it is a decent run to the master bath, 30' or so. So supplying hot water on a shower home run and then getting out of the shower and supplying hot water to the sink on a home run is a lot of waste, sending it over in one supply is preferable.

With all that being said my question is can I use 2 of the ports on the manifold to feed into one of the 3/4" supply lines for that bath? The math says that two 1/2" lines (.39 sq in) is just a tad smaller in volume than one 3/4" line (.44 sq in).

My concern is not the volume, rather it is the circuit looping back on itself. Maybe it's a non-issue due to equal pressure, or maybe there is some other possible problem with this setup that I don't know about. I wanted to get advice before I did it.

Thanks,
Josh



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Old 02-05-2013, 09:09 PM   #2
Beni
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Well if the old piping is copper then the difference is .17" sq area for pex (actual inside dia) to .50" sq area for copper (type M). So for the same pressure drop you would need three Pex lines. Using multiple lines is fine BTW.

Pex of a given size has a smaller inside diameter due to a thicker wall of the tubing. While the PEX folks will say 1/2" pex is equal to 1/2" copper, that's not quite true. Pex uses insert fittings. So a 1/2" pex tee for example will have about a 3/8" waterway at the tee. Compared to copper or steel where the fitting waterway is the same diameter as for the pipe. So PEX automatically has a built in flow restriction. Thats the main reason the PEX folks use a manifold system as PEX does not really scale up 1 to 1 if piped as a conventional system. Using the manifold system overcomes that limitation...

So to equal a 3/4" copper pipe use at least three 1/2" PEX lines...



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Old 02-06-2013, 12:47 PM   #3
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Makes sense. Thanks for confirming that multiple lines will suffice.

You are correct about the I.D. I hadn't thought of that. Although I get some of that pressure back due to no 90 degree elbows that are inherent using copper.

Thanks again!

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Old 02-06-2013, 02:08 PM   #4
speedbump
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Maybe another thing to consider is the friction loss of three 1/2" pipes as opposed to one 3/4" line.

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Old 02-06-2013, 11:15 PM   #5
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Actually the pressure drop is not that bad. Your only flowing 1/3 of the total flow and weather 3 lines or 300 the pressure drop for a given flow rate will be about the same as for a single pipe. The flows add but the pressure loss or friction loss per foot is the same. That is, it does not get added across parallel runs, only flow rate does.

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Old 02-07-2013, 12:15 AM   #6
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Quote:
Originally Posted by speedbump
Maybe another thing to consider is the friction loss of three 1/2" pipes as opposed to one 3/4" line.
Agreed. Maybe not a huge difference but something to keep in mind. Depends, if all I had was 1/2 I'd probably just run that instead of buying 3/4.


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