Water supply for body spray/shower head combo

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jasondavid

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Hi, great forum and site by the way. I'm taking on my first major plumbing effort by installing a new shower with 4 1GPM body sprays, rain shower head, and wand. The rough valve is a combination volume/temp + 6 way diverter from Delta with only 2 fixtures running at max (ex: 4 body sprays + shower head). I used an existing 1/2 cold and 1/2 hot from a soaking tub that we demolished. I suspected that the supply would not be ideal, but wanted to confirm first before deciding to upgrade to 3/4 supply. After testing the installed fixtures on the 1/2 supply I noticed a notable drop in pressure when two fixtures run at once.

Here's what I'm thinking about doing and would appreciate any guidance. In the attic I have a 3/4 cold supply going to a tankless and 3/4 hot coming out. I was considering tapping into those with 3/4 tee and running 3/4 hot and cold to the shower (about 25 feet away). Once in the wall with the valve, I would reduce these down to 1/2 hoping to not have to upgrade the delta valve. Any feedback is appreciated including if there are better ways to solve the problem (aside from hiring a plumber because that would prove my wife right and I can't afford any more of those scenarios - she already has too many to count ;-)
 

Dana

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Are you sure it's not the tankless rather than the plumbing that is limiting the flow? The heat exchangers in a tankless are pretty resisitive to high flow (much more so than half-inch copper), and the burner modulation range isn't infinite- many will automatically restrict flow to maintain output temperature when at the burner's limits. What tankless are you running?

Half inch copper will deliver well over 4 gpm, even though it can suffer erosion at the ells & tees if it sees well over 3 gpm a lot of the time. But unless it's a really long and convoluted path it's not likely to be what's limiting your deluxe shower flow. It's worth increasing the hot supply to 3/4" but the flow through the cold side won't be anywhere near the pipe erosion zone.

With the tankless set to 115F and the incoming water at 50F it takes 5 gpm of hot and 1 gpm of cold to hit the showerhead at a decently hot 104F at a 6 gpm total flow. If you increase the temp at the tankless it'll decrease the flow through the tankless a bit delivering the same amount of heat, but not a whole lot until you're in serious scald risk territory.

At the same 50F incoming/115F tankless 6gpm flow you installed a drainwater heat recovery heat exchanger rated at 55% or higher ( tested @2.5gpm under the NRCAN protocol) it reduces the flow through the tankless to 4 gpm, and increases the cold side flow to 2 gpm. This is not a cheap fix (figure about a grand as a DIY) and needs at least 5' of vertical drainpipe downstream of the shower to get the performance needed to run multi-sidespray gusher shower off a tankless at wintertime incoming water temps. There are good/better/best versions, some introduce their own flow limitations. With more information I can come up with the short list of what works.

Drainwater%20heat%20recovery%20-%20equal%20flow%20installation%20diagram%20-%202%20-%20RenewAbility.jpg


A drainwater heat exchanger is equivalent to boosting the burner output, but this "burner" doesn't use any fuel- it just takes the heat going down the drain and putting it back into the incoming water stream. It may or may not ever pay for itself in fuel savings, but it's a lot cheaper than installing a second tankless to get the increased hot water performance.

So, go ahead and increase your plumbing size, and be sure to insulate the 3/4" stuff with at least R3 pipe insulation. When that doesn't fix the problem it's time to consider other measures.
 

Dana

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BTW: 6 gpm= 3000 lbs/hr. At a 55 F rise (50F in, 105F at the showerhead) that takes a burner output of 55F x 3000lbs/hr= 165,000 BTU/hr, which is about the max output of non-condensing 199KBTU/hr tankless, or a 180KBTU/hr condensing tankless. During the coldest months your incoming water in Virginia Beach can be as low as 40-45F, for a 60-65F rise, which will completely max out even a 199K condensing tankless @ 6gpm.
 

jasondavid

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Dana,
Thanks for your generosity and thought in your reply. I'm very appreciative of your help.

I am using a Rinnai R94LSI. No, I don't know that it's not the tankless, but I did some rough experimentation with the whole system once installed.

1) I ran another shower in the house full heat at the same time as running the new shower head and body sprays and noticed no notable change in spray vs just the new shower and body sprays alone.

2) I ran all hot and compared to all cold in new shower/body sprays and noticed no difference between either

Your post did make me think about a setting I made on the tankless some years ago. I like the option to have really hot water and to dial it down in some bathrooms (kids for example), so I set the tankless to 130F. I'm going to test this again with it now at 120 and again at 115.

Finally, great point about not needing to run 3/4 from the cold line considering we'll mix in more hot in a shower than cold volume.

Also, the system appears to be mostly PEX. the line out and into the tankless is all 3/4 PEX up to the last few feet and that PEX seems to continue (though reduced) up to the fixtures.
 

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The Rinnai R94LSI puts out about 180-185,000 BTU/hr @ max fire.

At the same firing rate and shower temperature lowering the temperature of the tankless increases the rate of hot water flow. There may be a noticable drop in max flow rate due to the restriction of the heat exchanger and plumbing when you do that, but it probably won't be dead-obvious. You'd have to measure it to be sure.

At high rates of flow the pressure drop across the tankless can interact with the anti-scald valves in shower mixers throttling back flow.

With 3/4" pipe you're good for at least 6 gpm before erosion becomes an issue.
 

Dana

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Seem to have posted before refreshing the page...

So the plumbing is all PEX- I was thinking it was copper.

3/4" PEX is good for only about 4.6 gpm before it runs into issues over time, half-inch about 2.3 gpm. It's probably worth bucket-testing the flow rates under a couple of different ways before tackling the job. It's at least conceivable that you may want to go 3/4" on the cold side too. These numbers are all based on a max velocity of 4 feet per second. It's a conservative number, but not ridiculously conservative.
 

Jadnashua

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Pex allows a higher velocity than copper, where the Copper institute calls for a maximum of 5fps, which on 1/2" copper equals 4gpm, and 8gpm on 3/4" copper. The ID is smaller on pex, but they allow a higher velocity. Higher velocities through the pipes also causes more dynamic pressure losses, so it is a benefit to keep the velocity down. Throw in that pex (should be) often is installed with fewer fittings, that can help overcome some of the dynamic pressure losses when comparing it to copper.

FWIW, most 1/2" shower valves are pretty much tapped out at 6gpm, and some can't handle that well. A thermostatically controlled valve of the same size often gets a bit more volume through, but that depends on the design. For full performance with shower heads/body sprays, you really want to be able to supply more than their stated output to ensure they perform at optimum. If they need 6gpm and your valve can only supply 5.8, You'll start to notice a difference in performance.
 

jasondavid

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Thanks @Dana and @jadnashua for the help. Based on your feedback, I did a few tests before making additional changes. Here's what I found so far. I'm still not quite sure on where I need to attack the problem, but listed some examples below of where I might. Any ideas you can provide based on this is appreciated.

-Tested GPM from cold and hot (before the valve) - cold: >6gpm / hot: ~3gpm
-Tested home pressure at hose bib outside: 60psi
-Tested GPM with body sprays and shower running at once: all hot: 2.5gpm / all cold: 2.5gpm
I'm stumped as to why a 6gpm cold supply (right at the valve) would produce only a 2.5gpm out of the fixtures (all fixtures combined total over 6gpm - 4 sprays at 1gpm each and 1 rain shower head at 2.1gpm)
-Reviewed specs for valve and found it should produce 5.8 at 60psi

Until these steps, I considered running 3/4 from the tankless to the shower for the hot supply. I may still do that if I can figure out why a 6gpm cold supply will only produce 2.5gpm from the valve through the fixtures.
 

Dana

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The side sprays and shower heads all have considerable flow resistance, quite a bit more resistance than an opened up 1/2" or 3/4" pipe or hose bib. You may not have sufficient pressure to deliver the flows you like. The fact that you can only get 3gpm all-hot out of the tankless may be an indication of insufficient pressure, since the heat exchanger in the tankless also has a lot of flow resistance to overcome.

Showerheads are all rated at 80 psi. A showerhead rated 2.5 gpm @ 80 psi only delivers 1 gpm when the pressure at the showerhead is 15 psi. This graphic is clipped from a low-flow showerhead vendor's markeing:

Pressure-Flow%20Charts%20-%20USA.png
 

Jadnashua

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Most tub/shower valves have two outlets. The one designed to go to the tub is unrestricted. the one going to the shower head(s) is. If you bought it as a shower only valve, the tub outlet may have been plugged. That one is usually on the bottom. Some valve bodies can be rotated 180-degrees that will put the larger outlet up top making it easier to feed multiple shower heads. Without tapping the larger outlet, the valve itself will likely restrict your flow.

So, that could be an issue if it is installed and tapped with the smaller outlet pointed up and you used it.

Some tankless systems also restrict flow if they can't keep the outlet temperature at your setting when the volume ramps up.
 

jasondavid

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@jadnashua and @Dana all of the help you've provided has been awesome and has really helped me through this. I did some final experimentation to get a true sense of water pressure at the upstairs valve vs. street level and hot vs cold and found that I have about 60 PSI on the backyard hose bib and just around 50 at the upstairs shower. Interestingly, even with a tankless the pressure was 50 for hot and cold. This leaves me with a theory that supply is my problem. So, I ran a 3/4 pex in the attic and have yet to attach it. I think my final hurdle is how I want to connect to the Delta R22000.

Would appreciate any opinions on these options:

1) Connect 3/4 Pex directly to the 1/2 valve - Find a fitting that provides Pex 3/4 x 1/2 threaded female - Lowes and HD don't carry this fitting, but supplyhouse does. I'm wondering if this is because this is less common and not recommended
2) Reduce to 1/2 pex just before the valve - easy to find reducer to do so, but concerned that this just then constrains my supply at the 1/2 pex
3) Run 1/2 copper from valve a few inches up and connect 3/4 pex - gives me more internal diameter, but may be doing it the hard way for no additional value.
 

Jadnashua

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Water pressure will drop at ~0.43#/foot elevation change. So, 10# would be a bit over 20'. Note, that's static pressure. Dynamic pressure (while using water) will be affected by friction caused by the water moving through the piping and especially elbows or other connectors. Minimizing them does help. That's why there are water towers...to store energy so the pump doesn't need to run constantly except to refill the tower. Elevation is potential energy exhibited by the pressure you see.
 
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