Mixing Valve question

[Plumber69]

Mixing valve has 3/4 inlets and a 1" outlet. I was told it could handle 8 showers. Does this sound right? How could 3/4 be enough. I don't understand why the inlet is smaller then the outlet

 

[Reach4]

Because there are two inlets and one outlet, I suspect. If the cold and hot water are contributing nearly equally, the two 3/4 lines will be a good match for the inch output.

 

[Sponsor]

 

[Jadnashua]

For hot water, the copper institute's max flow recommendation on hot water is 5fps water velocity. On a 3/4" pipe, that equates to about 8gpm. Cold can safely flow faster, so if you combined the two, depending on the desired outlet temp and the inlet temp, you'd have about a max of 20gpm, which is barely at the needed flow for 8 2.5gpm showerheads...and, that assumes quite hot inlet so you can maximize the cold contribution to the flow. Yes, the pipes can flow more than that, but that goes against the copper institute's guidelines. If your inlet hot water is limited to the recommended 120-degrees and flow velocity...eight showers will end up quite cool if you want to have them all running at the same time, especially in the winter when the cold water is, naturally, colder than in the summertime. Excessive water velocity can do several things: create noises and literally, start to erode the pipe (hot water is worse on this, thus the lower maximum velocity recommendation). When you don't have excess capacity, you don't get the same Bernoulli effect (speed up via the restriction) in the shower head, and it starts to act like a rain shower head, and more like dribbles out. IMHO, you're on the edge with that valve and 8 showerheads. Depends on the effective length of the run (each change of direction adds to the effective length) after it how well it will work.

 

[Plumber69]

For hot water, the copper institute's max flow recommendation on hot water is 5fps water velocity. On a 3/4" pipe, that equates to about 8gpm. Cold can safely flow faster, so if you combined the two, depending on the desired outlet temp and the inlet temp, you'd have about a max of 20gpm, which is barely at the needed flow for 8 2.5gpm showerheads...and, that assumes quite hot inlet so you can maximize the cold contribution to the flow. Yes, the pipes can flow more than that, but that goes against the copper institute's guidelines. If your inlet hot water is limited to the recommended 120-degrees and flow velocity...eight showers will end up quite cool if you want to have them all running at the same time, especially in the winter when the cold water is, naturally, colder than in the summertime. Excessive water velocity can do several things: create noises and literally, start to erode the pipe (hot water is worse on this, thus the lower maximum velocity recommendation). When you don't have excess capacity, you don't get the same Bernoulli effect (speed up via the restriction) in the shower head, and it starts to act like a rain shower head, and more like dribbles out. IMHO, you're on the edge with that valve and 8 showerheads. Depends on the effective length of the run (each change of direction adds to the effective length) after it how well it will work.

Right now there's 4 shower rooms with 4 showers in each one. Each room has a 3/4 copper cold and 3/4 hot line. But I only have 2 mixing valves cause I was told one could handle 8 showers. Would joining the 2 hots and 2 colds before it went in the mixing valve help

 

[Plumber69]

Right now there's 4 shower rooms with 4 showers in each one. Each room has a 3/4 copper cold and 3/4 hot line. But I only have 2 mixing valves cause I was told one could handle 8 showers. Would joining the 2 hots and 2 colds before it went in the mixing valve help

Keep in mind the outlet on the mix valve is 1 inch. But inlets are 3/4

 

[Jadnashua]

Because of the square factor in calculating the area of the pipe opening, each 3/4" pipe's relative area is 0.14, while the one-inch outlet comes to 0.25, so 0.14*2=0.28 verses the outlet pipe's factor of 0.25. So, the combination of the hot and cold are close to being able to be supported by the 1" outlet...maybe with some acceleration, but that will be slowed down after a short distance via friction.

I'm not an expert on this, but it seems that, unless your desired outlet temp is fairly moderate, getting good flow and pressure out of 8 heads, IF they're not low-flow ones (a 'standard' showerhead can't use more than 2.5gpm, and most try to achieve close to that unless labeled as ECO), will be marginal. The pipes CAN flow more than the recommendations, but it also increases the friction, which will drop the pressure some. Somebody with more practical experience, verses reading the books and manuals, may be able to help you further. IMHO, it will be close, and no, combining the inlets probably won't make a difference going through that one valve.

 

[Plumber69]

Because of the square factor in calculating the area of the pipe opening, each 3/4" pipe's relative area is 0.14, while the one-inch outlet comes to 0.25, so 0.14*2=0.28 verses the outlet pipe's factor of 0.25. So, the combination of the hot and cold are close to being able to be supported by the 1" outlet...maybe with some acceleration, but that will be slowed down after a short distance via friction.

I'm not an expert on this, but it seems that, unless your desired outlet temp is fairly moderate, getting good flow and pressure out of 8 heads, IF they're not low-flow ones (a 'standard' showerhead can't use more than 2.5gpm, and most try to achieve close to that unless labeled as ECO), will be marginal. The pipes CAN flow more than the recommendations, but it also increases the friction, which will drop the pressure some. Somebody with more practical experience, verses reading the books and manuals, may be able to help you further. IMHO, it will be close, and no, combining the inlets probably won't make a difference going through that one valve.

Thanks for help. I ended up hooking all shower heads to the 8 connections. I ended up with even good pressure.