Feeding both outputs from American Standard Flash Mixing Valve into Diverter Valve Supply

[Clay033]

Hello All, I am a homeowner desperately trying to get some feedback on what I am led to believe is a good plumbing configuration. I am currently preparing to plumb the hot/cold water supply for a deep, jetted tub. The tub has a water heater but obviously, the function of the heater is to maintain the heated water as heated from the hot water tank or tankless system. Based on the need to maintain the tub water temperature, I want to get the tub filled as fast as I can. I also want to get the most volume out of the shower head and handheld. I am setting up the American Standard Flash mixing valve (RU101SS) with the R433S three-way diverter valve. When looking at the pair of outlets (Tub Spout w/o diverter and Shower Head) provided in the mixing valve, the hole diameters allowing water to feed out from the mixing valve are each smaller than the supply hole feeding into the supply for the diverter valve. Based on the small holes in the mixing valve, I am inclined to join both of the water outputs from the mixing valve and feed the joined outputs into the supply for the diverter valve. My thoughts are that I will maximize the supply into the diverter vale and hence that will provide the optimum scenario for output from the tub spout, handheld and shower head. I was curious if anyone has plumbed the noted configuration and was it beneficial, or has there been flow problems resulting in the diverter valve that I am not thinking about? Thank you!

 

[Breplum]

I know what you mean, the flow out of those cheap valves with marginal openings is going to be poor.
Actually, most shower valves like that have very small waterways through the cartridge.
I am pretty sure no plumber would take the tub and shower outlets and run them in to a diverter valve because the flow from the hot is still limited to the single hot pathway through the cartridge
To get fast fill on a large tub we use 3/4" fast fill tub fillers, period.

 

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[John Gayewski]

When making a post like this its helpful to provide picture or links. It's hard to expect someone to research the products you're taking about


But generally, the reason to use larger pipe is to minimize further friction loss. You will lose when you go through the valve, but after that loss you don't want to loose further.

 

[Jadnashua]

You do get a pressure drop with higher flow on a smaller pipe, but you'll get higher flow more easily with a larger valve assuming your pipes to it are at least that size, too.

The Copper Institute's maximum flow recommendations are NGT 5fps for hot water in a copper pipe. For 1/2" lines, that's only 4gpm. A 3/4" line at the same velocity is 8gpm, so you can see a larger pipe can provide more volume at the same pressure within the design guidelines.

For restrictions that are short, the fluid flow tries to increase velocity through the restriction, so depending on how long it is, it doesn't decrease the flow as much as if the whole line was that smaller diameter. So, while the openings in the valve may be relatively small compared to the inlet, the end effect isn't as bad as it might seem since the path is short.

FWIW, most 1/2" valves are rated for approximately 6gpm to account for the mixing of hot and cold, while the hot and cold without restrictions could normally supply more than that if just openly dumping it through.