1/2" supply and GPM questions

brian3b · Feb 20, 2009

I have a 1/2" supply to master bath. I tested and filled a 5 gallon bucket in 25 seconds - which puts me at 12gpm flow rate.

If my thermostatic valve (http://www.danze.com/products/pdfs/D151000_ss.pdf) can sustain 19gpm at 80 psi (I have 85 psi), would it be reasonable to assume that I can run 4-5 shower heads (2.5gpm each)? The reason I ask is b/c on another forum EVERYONE kept telling me, "1/2 inch won't work, you need 3/4 inch" and "you might be able to run 2 heads at once, but that's it."

I could never get an explanation other than "it just wont' work, you need to install 3/4 inch"

I'm farely analytical, and the numbers appear to add up correctly... Am I missing something? can someone please explain if I'm approaching this correctly and what the best method to determine how many showerheads I can use simultaneously? Thanks!

Brian

Jadnashua · Feb 20, 2009

Is that new valve 3/4"? My guess is that it is. You may get a little more volume using that valve on 1/2" supply lines than a 1/2" valve, but you might be dissapointed if your supply stays 1/2".

Sponsor · 2022-01-29T12:20:58-0800

hj · Feb 21, 2009

shower

You may be confusing volume with pressure. The 1/2" line might supply the volume you need, but the water would probably just fall out of the heads instead of spraying, since there would be little residual pressure to create the spray.

Jimbo · Feb 21, 2009

1/2" pipe will reasonably supply in the range of 7 gpm. Draw more than that, and velocity is out of spec ( bad for pipes) and pressure loss per foot of pipe increases.

I hear new showers have a GPS device built in, and if you try to draw 18GPM for a shower the AlGore police will be at your house in no time!

brian3b · Feb 21, 2009

jadnashua said:
Is that new valve 3/4"? My guess is that it is. You may get a little more volume using that valve on 1/2" supply lines than a 1/2" valve, but you might be dissapointed if your supply stays 1/2".

NO sir, the new valve is 1/2 inch. and my supply is 1/2 inch.

hj said:
You may be confusing volume with pressure. The 1/2" line might supply the volume you need, but the water would probably just fall out of the heads instead of spraying, since there would be little residual pressure to create the spray.

I'm not sure if I'm confusing... I calculated 12gpm based on the rate in which I filled up the 5 gallon bucket....

jimbo said:
1/2" pipe will reasonably supply in the range of 7 gpm. Draw more than that, and velocity is out of spec ( bad for pipes) and pressure loss per foot of pipe increases.

I hear new showers have a GPS device built in, and if you try to draw 18GPM for a shower the AlGore police will be at your house in no time!

So what do you recommend? How can I determine what the maximum amount of shower heads I can run? Thanks a lot!

hj · Feb 21, 2009

shower

No, they have an internal flow meter and if exceeded, they shut down for 7 days before resetting so the shower can be used again.

brian3b · Feb 21, 2009

hj said:
No, they have an internal flow meter and if exceeded, they shut down for 7 days before resetting so the shower can be used again.

No what? Who shuts it down? ???

I'm trying to determine how many shower heads I can run simultaneously... do you recommend something? Thank you!

Jimbo · Feb 21, 2009

Anything you do will work. Will it work well....only if installed per manufacturer's specs. We have been fooling around with you, but the straight answer is that 5 to 7 gpm is it for 1/2". Doesn't mean that at midnight you turn into a pumpkin, but if you are trying to flow 18 gpm, there will be side effects like low pressure and premature erosion of piping.

This is now strictly personal, but I think we are past the point where anyone should be considering multiple head mega gpm showers. But it is your choice, and in many places you will pay accordingly for the water.

brian3b · Feb 21, 2009

jimbo said:
Anything you do will work. Will it work well....only if installed per manufacturer's specs. We have been fooling around with you, but the straight answer is that 5 to 7 gpm is it for 1/2". Doesn't mean that at midnight you turn into a pumpkin, but if you are trying to flow 18 gpm, there will be side effects like low pressure and premature erosion of piping.

This is now strictly personal, but I think we are past the point where anyone should be considering multiple head mega gpm showers. But it is your choice, and in many places you will pay accordingly for the water.

Thank you for the answer... is that 5-7gpm based on experience, code, or what? Is there not a specific way to make a determination? I've been on two forums now and people are shunning me b/c I'm asking for a method to make this determination. It's like I'm breaking some unwritten plumbing rule. Obviously I'm a newbie, and I give all you experienced guys all the credit in the world b/c I know you know more than me.

Let me put it this way... if you go to some guys house and he says "i want a custom shower with multiple showerheads," how do you determine how many he can have? What is your answer to him?

hj · Feb 21, 2009

shower

I'm not sure if I'm confusing... I calculated 12gpm based on the rate in which I filled up the 5 gallon bucket....

Good, and if all you want to do is fill a 5 gallon bucket in the shower it will work. BUT, if you want to have the shower heads spray you with water, that is something else, and that will NOT happen if you use 5 shower heads at once.

brian3b · Feb 22, 2009

hj said:
I'm not sure if I'm confusing... I calculated 12gpm based on the rate in which I filled up the 5 gallon bucket....

Good, and if all you want to do is fill a 5 gallon bucket in the shower it will work. BUT, if you want to have the shower heads spray you with water, that is something else, and that will NOT happen if you use 5 shower heads at once.

Thanks HJ, point taken. I understand now... but I'm still back to my first question... is there a way to determine how many I can use? Or is the standard answer "you can't use that many"? If nothing else, would someone just tell me there isn't a way to determine how many, and that it's just a guess?

NHmaster · Feb 22, 2009

It all depends on the flow rate of the shower heads themselves. If you are using low flow heads you could probably get away with a half dozen or more. If you are using the rainshower heads, one is probably the limit.

brian3b · Feb 22, 2009

nhmaster said:
It all depends on the flow rate of the shower heads themselves. If you are using low flow heads you could probably get away with a half dozen or more. If you are using the rainshower heads, one is probably the limit.

Shower heads are 2.5 gpm.

C NUMB · Feb 22, 2009

This is my opinion but based on the valve you are installing which has 1 volume control, I would say it is for 1 shower head. You may want to contact Danze and ask them.

hj · Feb 22, 2009

flow

The determination is hard to make because the design of the head has a lot to do with it. The smaller the openings and the number of them, the greater the residual pressure. That pressure is what creates the spray, AND allows other heads to be used. If you have a rainfall head with 100 openings, regardless of the size, the water is just going to fall out of it, and there will be little reserve pressure to run any other heads adequately. You can run several needle spray heads, but only a couple of "car wash" ones.

AlwaysInHotWater · Mar 1, 2009

Maybe this will help a bit in determining the water flow available. You can estimate the maximum gpm based on pipe size. This is for bare straight pipe without any valves.

Jimbo · Mar 1, 2009

brian3b said:
Thank you for the answer... is that 5-7gpm based on experience, code, or what? ?

There are charts available on the internet. My answer is based on two factors, pressure loss and velocity. Too much of either will have adverse side effect consequences. Both will vary depending on the total length of pipe, so there is no absolute right answer.

marvinj67 · Sep 12, 2023

I don't understand why these guys can't offer a straight answer. It's very clear what the guy is asking. Don't be a jerk just because the guy is trying to save some cash on a little DIY project. If he has say 5 total heads that are 2.5 gpm each, that's 12.5 gpm total. If it's 1/2" pex pipe, it'll have a 38.1 psi drop at 6 gpm, so he'll need at least a 3/4" supply line with a 3/4" valve to supply 12.5 gpm. At 12 gpm, 3/4" pex will have a 25.4 psi drop, so, if he has a 60 psi system, he'll end up with around 34.6 psi at each fixture head.

Jeff H Young · Sep 12, 2023

marvinj67 said:
I don't understand why these guys can't offer a straight answer. It's very clear what the guy is asking. Don't be a jerk just because the guy is trying to save some cash on a little DIY project. If he has say 5 total heads that are 2.5 gpm each, that's 12.5 gpm total. If it's 1/2" pex pipe, it'll have a 38.1 psi drop at 6 gpm, so he'll need at least a 3/4" supply line with a 3/4" valve to supply 12.5 gpm. At 12 gpm, 3/4" pex will have a 25.4 psi drop, so, if he has a 60 psi system, he'll end up with around 34.6 psi at each fixture head.

I hope he got his Straight answer 14 1/2 years ago if not he is a patient man

1/2" supply and GPM questions

Users who are viewing this thread

New Member

Retired Defense Industry Engineer xxx

Paid Advertisement

Master Plumber

Plumber

New Member

Master Plumber

New Member

Plumber

New Member

Master Plumber

New Member

Master Plumber

New Member

Plumbing Contractor

Master Plumber

New Member

Attachments

Plumber

New Member

In the Trades

Similar threads