Kohler Thermostatic Valve - 1/2" or 3/4"?

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Hesch

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I am currently remodeling our master bathroom with plans to install a multi-fixture shower setup. I will also be taking the opportunity to re-pipe most of the house since we still have a good amount of old galvanized hanging around. So, I have quite a bit of freedom to install whatever supply lines are required/recommended for this new shower setup. Edit: I should add that I will also be installing a Manabloc with 1/2" PEX runs direct to most fixtures, including the master bathroom lav and toilet.

My plan is for 3 fixtures (2.5 GPM standard head, 2.5 GPM hand shower, 2.5 GPM rain head) fed by a thermostatic valve and 3 separate volume controls. I envision we will use 1 or 2 fixtures about 95% of the time, while rarely running all 3 at once. However, since I am pretty much starting from scratch with my home's plumbing, I would rather do it right and have a system that can still deliver optimal flow and pressure from all 3 fixtures. For context, I measured 10 GPM and 70 PSI at the hose bibb 6' past the meter. I plan to tee off of a 1" copper main line approx. 60' from the meter for the shower supply lines.

If you were plumbing in this new shower, how would you do it? What size pipe and valve would you use?

Option 1: 3/4" copper or 1" PEX supply > Kohler 1/2" MasterShower valve > 3/4" PEX > 1/2" MasterShower volume controls > 1/2" PEX > drop ears

Kohler claims 10.9 GPM @ 45 PSI, which exceeds the 7.5 GPM I need, but I have seen that valve's flow rate called into question on this forum before.

Option 2: 3/4" copper or 1" PEX supply > Kohler 3/4" MasterShower valve > 3/4" PEX > 1/2" MasterShower volume controls > 1/2" PEX > drop ears

The 3/4" valve is rated for 17.2 GPM @ 45 PSI. This would eliminate any concerns about flow rate, but it is $130 more.

Option 3: Your Recommendation?

I know this topic has been beaten to death, but I searched and searched and still feel like I could not find a clear answer/recommendation. Any advice is appreciated.
 
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jadnashua

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It's highly unlikely that you'd get 7.5gpm out of a 1/2" valve, especially if fed with 1/2" pex, and maybe not with 3/4" (but it would be much closer).

1/2" pex is fine for one head, marginal for two, and likely pathetic for three, especially in the winter when you'll be using much less cold. A thermostatically controlled valve helps, as, generally, they can flow more than a simpler pressure balanced one.

It also will make some difference whether you're using expansion pex fittings and tubing, or choose to use pex with crimp rings, as expansion fittings have less restrictions through them. Also, if using pex, don't worry that much about how it looks in the wall. Making it look really neat with right-angle fittings versus just looping it around and eliminating the extra fittings can really help reduce the dynamic pressure drop. FWIW, the bigger issue is volume available, but pressure loss affects volume, too. For a head to accelerate the water, you need MORE water available than it can flow, so matching the available capacity is closer to just a hose without a nozzle on it...it's the restriction in the nozzle that causes the flow to accelerate, which is what you want and that takes available volume (minimizing pressure loss helps, too!).
 

Tuttles Revenge

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spend the money and install the bigger valves. No reason to go cheap on the stuff behind the walls that you can't ever replace.
 

Hesch

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It's highly unlikely that you'd get 7.5gpm out of a 1/2" valve, especially if fed with 1/2" pex, and maybe not with 3/4" (but it would be much closer).

1/2" pex is fine for one head, marginal for two, and likely pathetic for three, especially in the winter when you'll be using much less cold. A thermostatically controlled valve helps, as, generally, they can flow more than a simpler pressure balanced one.

It also will make some difference whether you're using expansion pex fittings and tubing, or choose to use pex with crimp rings, as expansion fittings have less restrictions through them. Also, if using pex, don't worry that much about how it looks in the wall. Making it look really neat with right-angle fittings versus just looping it around and eliminating the extra fittings can really help reduce the dynamic pressure drop. FWIW, the bigger issue is volume available, but pressure loss affects volume, too. For a head to accelerate the water, you need MORE water available than it can flow, so matching the available capacity is closer to just a hose without a nozzle on it...it's the restriction in the nozzle that causes the flow to accelerate, which is what you want and that takes available volume (minimizing pressure loss helps, too!).

I am skeptical too about the claimed gpm. On the other hand, how can they reasonably put those products out on the market if they can't provide the gpm that they are claiming?

I'm planning to use either 3/4" copper or 1" PEX for the supply to the valve, so that should eliminate the concern of having enough flow to supply 3 fixtures. The only question then is the valve. I think I'm going to take Tuttles' advice and get the 3/4" valve for peace of mind.

In that case, do you think I also need 3/4" volume control valves going to each fixture, or can I get away with using to 1/2" volume control valves? And should I run 1/2" or 3/4" PEX from the volume control to the drop ears?
 

Terry

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I got this text message the other day, I guess I can post it onto this thread.

So... plumbing a house here and the client says the shower temperature isn't reliable. Gets too hot, too cold and hard to get it to work right.
It's a K2975-ks-na Kohler thermostatic mixing valve with a separate volume control above it. We piped the valve with 3/4 PEX to the 3/4 IPS ports on the valve.

My first thought was it was the cartridge acting up. But I called Kohler customer service just to confirm. They told me with a tankless water heater installed in the house you have to run 1/2" lines to the valve??

so the valve HAS to have 1/2" plumbed into the 3/4 valve... seriously have to bust out tile now to repipe the runs in 1/2? Has anyone else had this issue?

k-2975.jpg
 
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wwhitney

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They told me with a tankless water heater installed in the house you have to run 1/2" lines to the valve??
That makes zero sense.

But if the client has a tankless, does that tankless have enough capacity to provide all the hot water demanded by the shower? The choice of a shower valve with 3/4" inputs suggest a desire for a high volume with multiple functions. If the demand is, say, 7.5 gpm at 120 degrees, and the tankless can only provide 6.0 gpm at 120 degrees, the problem lies with the tankless, not the shower valve.

So if this is a multifunction shower, maybe ask the client if the temperature is stable when only one function is in use (and no other hot water use from that tankelss), and the instability only occurs with 2 or 3 functions going simultaneously.

Cheers, Wayne
 

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Misread the part about the tankless. Tankless can only put out 5.5gpm so maybe that is their reasoning for the 1/2"... I'd install a buffer tank before ripping out tile.
 

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Misread the part about the tankless. Tankless can only put out 5.5gpm so maybe that is their reasoning for the 1/2"... I'd install a buffer tank before ripping out tile.


We recently installed a U-By Moen shower system capable of 14GPM output. the homeowner wanted to be able to run the shower for 45min. We installed a Rinnai hybrid. 100gal Tank supplied by a tankless with a recirc system.
 

Tuttles Revenge

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I am skeptical too about the claimed gpm. On the other hand, how can they reasonably put those products out on the market if they can't provide the gpm that they are claiming?

I'm planning to use either 3/4" copper or 1" PEX for the supply to the valve, so that should eliminate the concern of having enough flow to supply 3 fixtures. The only question then is the valve. I think I'm going to take Tuttles' advice and get the 3/4" valve for peace of mind.

In that case, do you think I also need 3/4" volume control valves going to each fixture, or can I get away with using to 1/2" volume control valves? And should I run 1/2" or 3/4" PEX from the volume control to the drop ears?

After your mixer you need to calculate how many gpm each control will by supplying. If its just one 2.5gpm head, then the 1/2" valve is fine.. If you had a set of 3 body sprays each putting out 2.5gpm, then I would run a 3/4 valve to that pressure loop to be on the safe side.
 

jadnashua

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When companies rate their valves, it is possible that they're looking at them with both the hot and cold supplies at maximum (i.e., not across the whole temperature range)...that RARELY is possible unless you like a lukewarm shower, and maybe even cold in the winter if your ground water is frigid like mine (measured at 33-degrees after a cold spell). At a more reasonable shower temp, you're using a majority of hot, with some cold to temper it. If you use the Copper Institute's guidelines for maximum velocity, a 1/2" copper pipe for hot is only good for 4gpm (cold is a bit more since it is allowed to flow faster), and on a 3/4" copper pipe, that doubles to 8gpm on the hot side (that max velocity is 5fps). Exceed those values, and the Copper Institute says you can literally erode the pipe from the inside out (that won't happen overnight!), you'll have water flow noises, and the dynamic pressure starts to drop, becoming a potential issue, depending on where you started from.
 
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