3/4 pex capacity

rodr

I would like to know if anyone knows the flow capacity of 3/4 pex at 60 psi.i am trying to size valves for a fast fill tub.
Thanks
Rod

Mike T., Swampeast MO

Tub as in bathing tub?

I believe that the valves are much more the limiting factor than the piping...

I have right at 60# of static domestic water pressure on my 2nd floor where there is a big tub.

¾" cold line home run from 1¼" pressure-balanced manifold loop, new rigid "L" copper.

¾" hot line from 1" recirculation loop, new rigid "L" copper. Conventional tank water heater with 1" connections. Cold inlet to tank off of same pressure-balanced manifold.

1" meter and 1¼" (new rigid "K" copper) entrance piping.

¾" Grohe ceramic valve Roman tub filler with aerator spout. (Grohe is supposed to be a high-flow tub filler.)

From OLD 1922 (only original plumbing left in house) ¾" galvanized hose bib outside I get about 9 gpm.

From tub:

Cold only: 6 gpm

Hot only: 5 gpm

"Typical" bathing temperature: 6.66 gpm

To get a very accurate estimate I believe you'll need this data: 1) pressure drop/flow table from tub filler mfgr 2) length of PEX supply piping to filler 3) length and type of entrance 4) size and pressure drop/flow table of water meter (or well supply) 5) static pressure available at tub filler connection 6) pressure drop through hot water heating device.

For a reasonably easy estimate you still need the flow data from the filler mfgr, available pressure at outlet and meter/well flow/pressure data.

For a rough estimate--as long as all piping AND meter are least ¾" AND ¾" filler: 6-7 gpm @ avg. bathing temp. If meter is 5/8": 5-6 gpm.

rodr

Thanks for your thoughts mike.i have a 1 inch meter with 1.5inch supply.1.25 main line to mechanical room then a 1 inch manifold wih home runs to all fixtures.i was thinking of using 3/4 inch volume control valves with no filler neck since the tub has a waterfall spout built in.The conection to this spout is 3/4.I was hoping to get more than you are seeing.If the valves were full flow what do you think the volume would be?
rod

hr

Gets involved

My calcs show water at 140° a 10 gpm flow would run 11 fps, with a 16 psi drop over 100' in 3/4" pex.

Now you need to determine the maxium velocity acceptable, from the manufacture.

If you have any copper or brass fittings in that line 8 fps seems to be a comfortable flow.

I think the Copper Development charts show 8 fps maxium for copper tube in DHW applications.

Remember increased velocity= increased noise! Especially at the valve, where the restriction (and wear points) are.

hot rod

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DanHolohan

Pocket Ref

has info on discarge through a 3/4" nozzle. It would be 130 GPM at 60 psi. The book goes on to explain that that number is theoretical, and that the actual will be 95% of the above, depending on the shape of the nozzle, bore smoothness, etc.

I think with the PEX, much would depend on the length of the tube, and the pressure drop along the way.

Mike T., Swampeast MO

With the aerator removed and running hot/cold mix at about "normal" bathing temp it flows right at 10 gpm. The fill spout and its connection point on the Grohe Roman tub sets are quite "wide open" and would seem to offer very little in the way of restriction.

With both hot and cold wide open about 13 gpm without the aerator. Your DHW would have to be awfully [dangerously] hot to produce bathing temp water with both valves wide open I believe.

Of course at these velocities, it's making quite a mess

Mike T., Swampeast MO

This arrangement...

...a Grohe ¾" thermostatic mixing valve seems to have no problem delivering 12+ gpm @ about 105° with very little pressure drop (as "measured" by the "feel" of the outlets on the skin). Turn on too many "champagne" sprays and pressure drops noticeably.

Mark Wolff

???

For the customers that may be ok, but I'm not taking a bath in 105* water! Better have a big water heater.

rodr

Thanks for the input guys.i know this is kind of a silly topic.I have a buderus st-300 tank and the tub capacity is 98 gal.After hotrods post i am concerned about the vilocity of the water.I may not be able to send the water to the tub as fast as i would like.The tub is a aquatic century 24.I would like to fill the tub as fast as possible but am still not sure how fast that will be.I need to re-think this after the info you guys have given me.Thats whats so great about the wall.It allows us to toss ideas out there and use eash others minds for great results.
Thanks

Mike T., Swampeast MO

For a shower with multiple sprays...

The center item is the thermostatic mixer. These mix by temperature only and don't provide control of volume as they are always trying to deliver maximum flow. Quite amazing devices as their internal structure give no clue as to how they work [to me at least]. They also instantly stop flowing if either hot or cold supply fail.

The rest of valves are volume control for various sprays/heads in singles and groups with other diverters/volume controls installed further down. Everthing is independently controlled so you have to turn on a lot of outlets to get that much flow...

Hot water heater is big. It's a good quality unit of high efficiency [for a tank water heater at least]. It was literally dropped at some point en route to the first owner and I got it for next to nothing. It only took me a couple hours to repair the sheet metal bottom and put the "guts" back into proper position.

hr

At 10 gpm

you could fill that tub in under 10 minutes! I'd be more worried about walking away and forgeting it and having 10 gpm dumping on the floor

Some Kohler and Jacuzzi brand tubs have level sensors and solnoids to shut down the fixture to prevent this.

I think you could safely flow 10 gpm, remember this is not a constant 24/7 demand fixture. Sounds like a 10 minute burst every day or so. I don't think that would stress the pipe or fittings. Again, the Pex manufacture may have better data on this.

I have this nifty PHCC calculator, and it shows 11.6 gpm in 3/4" copper with a velocity of 7.7 (just the 8 fps max) It is based on the CDA charts. The numbers I spoke of in the other post were from Siggy's Design software, for pex flow rates.

hot rod

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