Question on estimating shower supply

[Davud]

Hello,

I'm trying to determine whether or not I my home's water supply is equipped to install one of those shower rain heads along with a handheld for a master bath remodel. I have a pretty low PSI but they flow rate is decent, from what I've read.

For reference, here are the specs, along with some info behind them.

Pressure: 35-36 PSI.
This was tested at two spigots and also confirmed by testing from a serviceman from the Water company.

Flow rate: 11.1 GPM
I calculated this by turning off all water faucets in the house, then using a 3/4" drain I installed directly adjacent to the meter at where the city supply directly enters the house, then timing how long it took to fill a five-gallon bucket (see the attached picture where the valve I tested from is circled red, for reference). When I refer to it as a "drain", I mean that in the sense that if I want to drain all the water from my house, I do it there. I'm sure there is an official term for it.

Also for reference, I'm completely remodeling the house, so right now walls are open. So if any of your thoughts or suggestions would be affected by this knowledge, just a heads up - I can make just about any modification you'd recommend.

So as for my question, I've been wracking my brain over trying to figure out whether or not I can install the shower system I want by researching homes with my PSI and flow rate as a way to compare, when I finally figured there is probably a common sense test that I could do instead. While it wouldn't be fool-proof to any degree, it would probably give me the best indication of whether or not my desired shower setup can be done.

I'm planning to simulate a likely "worst case scenario". I want to run my kitchen sink warm at full blast, run the washer on warm, run one of the master bath faucets on warm full blast, and run the dishwasher (the reason I'm specifying warm is that it will utilize the same mixture of hot and cold my shower will generally be). Then immediately perform the five gallon test at the shower on both the hot supply and cold supply.
(note: I understand this is not the "true" worst case scenario, but I figured it's just me and my wife, so the odds of us having any other fixtures running at the same time is highly unlikely, and if we did, then we'll just have to deal with getting dribbled on in the shower ;-) )

If it turns out my flow is, let's just say 5 gpm for both hot and cold for example, would this mean I would be safe to install the shower system if the combined total flow of the rain head and handheld does not exceed 5 gpm?
And on this note, I could also hook up some temp male threads to mount my pressure gauge on to test the pressure, as I know the fixtures also tend to specify PSI requirements.

I know there is a lot that goes into determining this, but I just sort of figured this has to be a relatively decent approach, since it gives the exact PSI and flow of my hot and cold supplies at the exact location where they're to be installed.

Thanks in advance for any advice you guys can give me, I really do appreciate it!

 

[Sylvan]

The velocity @36 PSI through 3/4" tubing would be around 20.87 FPS and GPM of almost 12 GPM ( 11.81 )

Not taking friction losses into account and pressure drop with other fixtures being used

 

[Sponsor]

 

[Davud]

The velocity @36 PSI through 3/4" tubing would be around 20.87 FPS and GPM of almost 12 GPM ( 11.81 )

Not taking friction losses into account and pressure drop with other fixtures being used

Wow, that almost exactly the same as my actual measurement. Good to know there are formulas out there that are reliable. Thank you Sylvan. Where did you get that info, by the way?

And as far as my main question, do you have any thoughts on what I planned to do? I initially figure I could just use the flow rate for each fixture to be used concurrent with the shower, sum them up, then subtract from the total .
i.e., let's say the kitchen faucet, bath faucet, washer, and DW, were like 2.5gpm, 2.5gpm, 2gpm, and 1.5gpm, respectively.
Then 2.5 + 2.5 + 2 + 1.5 = 8.5
Therefore, would I then expect to have 11.1 - 8.5 = 2.6 gpm left for my shower.

It's also worth mentioning, the shower would be "last" on the pecking order for the supply. Every other fixture gets supplied first before the supply reaches the shower. So there's a lot of "assuming" that goes into these calculations, not to mention everything else that affects the supply that I'm not even aware of. That's why I just figured running the other fixtures then performing the test at the shower would be the best way to determine what the specs will actually be when it reaches the fixtures in question (the rain head and handheld).

 

[Davud]

Actually, it just occurred to me that the DW is probably rather irrelevant, since they basically use about 4-5 gallons of water during the whole process. They mainly just draw water during a short period then recycle it (unless I’m wrong?).
But if I hopped in the shower at the exact same time my wife started a load of dishes, then it would matter during the first few minutes or so. So with my planned test, I would measure my shower flow right after starting the washer.

Either way, it's another reason why I think doing an "actual" test rather than calculating based on assumptions is a good idea.

And sorry if I'm sounding like someone who already has their mind made up and is just looking for validation. That is partially true. . But it will require a decent amount of work to set this up, since I've yet to run any of the supply lines, and I don't have a plumber's knowledge, so in the event there was something completely off in my thinking I figure it would be good to ask before going through all the hassle.

 

[Jadnashua]

Google the Copper INstitute Handbook. IT has all of the info you need about sizing, available and recommended flow rates and potential issues. Their recommendation for long-term reliability, quiet operation, and minimized frictional losses has a flow rate of 5fps for hot and 8fps for cold. A 1/2" copper pipe at 5fps is only 4gpm while a 3/4" copper pipe is 8gpm at the specified maximum flow rate. That (free) manual describes the consequences of exceeding those values.

 

[Davud]

Google the Copper INstitute Handbook. IT has all of the info you need about sizing, available and recommended flow rates and potential issues. Their recommendation for long-term reliability, quiet operation, and minimized frictional losses has a flow rate of 5fps for hot and 8fps for cold. A 1/2" copper pipe at 5fps is only 4gpm while a 3/4" copper pipe is 8gpm at the specified maximum flow rate. That (free) manual describes the consequences of exceeding those values.

Thanks a lot.

I looked through the handbook and did some calculations.

I believe I should have just under 24 psi at 6 gpm when I reach the fixtures.

Now I understand most shower heads output 2.5 gpm, so if I have 6, then I should be fine(?)

But what about my pressure? This is where I get confused. If the shower itself is limited to 2.5 gpm, then isn't it only able to achieve that flow if I have a minimum psi such that the water can be "pushed" through hard enough?

 

[Sylvan]

1- Wow, that almost exactly the same as my actual measurement. Good to know there are formulas out there that are reliable. Thank you Sylvan. Where did you get that info, by the way?

Answer

Serving a 5 year (10,000 hr) apprenticeship and 744 hours of code and formulas the mathematics are very seldom forgotten

2- let's say the kitchen faucet, bath faucet, washer, and DW, were like 2.5gpm, 2.5gpm, 2gpm, and 1.5gpm, respectively.
Then 2.5 + 2.5 + 2 + 1.5 = 8.5
Therefore, would I then expect to have 11.1 - 8.5 = 2.6 gpm left for my shower.

Answer
Plumbing supply as and drainage are not designed to have all fixtures working at the same time as not only would there be a severe pressure drop the loss of volume while taking a shower could cause scalding

For example one of my accounts is a 27 story residential building with a 4" water main can you imagine if every single fixture was used at the same time the 10 dia sewer would be cause sewage to pour out of the lower floors

To find velocity for example PSI X 12.1 Sq

65 PSI x 12.1 Sq = 28.04

It is hard to put the formulas on here

 

[Jadnashua]

Most people would find 24psi pretty anemic. If you're willing to add the expense for the additional performance, you might consider adding a booster pump. That will only work well if the supply is not limited in volume and the utility would allow you to pull volume rather than them pushing it to you. There's a short video of one that This Olde House did a number of years ago that shows them installing one on their website in the archives. Basically, it's pretty simple - small storage tank, check valve, and pump.

 

[Davud]

Most people would find 24psi pretty anemic. If you're willing to add the expense for the additional performance, you might consider adding a booster pump. That will only work well if the supply is not limited in volume and the utility would allow you to pull volume rather than them pushing it to you. There's a short video of one that This Olde House did a number of years ago that shows them installing one on their website in the archives. Basically, it's pretty simple - small storage tank, check valve, and pump.

Thank you jadnashua. I tested 11.1 gpm and 36 psi where the city supply enters my home. I’ve been researching booster pumps. Do you think this pump will work?
https://www.waterpumpsdirect.com/Bur-Cam-506532SS-Water-Pump/p7177.html

I have two showers on the second floor, at about 24’ feet above where the city supply enters my home. I also have 4 sinks and 2 toilets on the second floor, then a kitchen, 1/2 bath, and laundry room on the first floor. I don’t have an irrigation system. I’m really not looking for fantastic performance, just something adequate enough where my wife and I can choose a shower setup based on what we want, not what we’re limited to.

I also see the pump takes a 1” supply. Mine is 3/4”. Is that an issue, or can I simply use reducers?

Thanks in advance for any further advice you’re willing to provide. I really do appreciate it sir.

 

[Davud]

Never mind. I read through the manual, it doesn’t look like it’ll work. It specifies incoming pressure needs to be between 20-25 psi, and that it is only well suited for smaller homes with only one or two bathrooms.