Pressure Loss Calculation

[kybob]

In the midst of determining 3/4 vs 1/2 for a two shower head situation where both would be on at the same time and assuming the higher limits of the permitted flow (2.5 GPM each). This will be a home run from a manifold. There are small things such as the shower line that are not accounted for yet.

- Am I in the ballpark for the pressure loss estimations?
- Will 3/4" with 3/4" valve make a noticable difference?
- Will 3/4" with 1/2" valve (reducing at shower valve) make a noticable difference?

 

[John Gayewski]

In the midst of determining 3/4 vs 1/2 for a two shower head situation where both would be on at the same time and assuming the higher limits of the permitted flow (2.5 GPM each). This will be a home run from a manifold. There are small things such as the shower line that are not accounted for yet.

- Am I in the ballpark for the pressure loss estimations?
- Will 3/4" with 3/4" valve make a noticable difference?
- Will 3/4" with 1/2" valve (reducing at shower valve) make a noticable difference?

How many feet of pipe?

 

[Sponsor]

 

[John Gayewski]

At 2.5gpm (your shower head likley won't even give you that much) 1/2" pex looses 14ft of head per 100 ft. of pipe. 3/4" looses 4 ft of head per 100 ft.

So your gonna get roughly 3.5 times more pressure with 3/4".

Will it make a noticeable difference? Probably not, not unless the run is long excessively long or your pressure is excessively low.

 

[kybob]

How many feet of pipe?

I've uploaded an image with details on each section. I estimated ~8 PSI loss up to the manifold then my optionality. I redid the calculations with uponor chart for 120 degreee (https://www.uponorengineering.com/~...ressure loss charts.ashx?version=062520180400) and got these numbers. Using 4gpm based on 80/20 hot to cold ratio.

Initial: 60 PSI - 8 PSI loss
Option A (1/2): 4gpm 6.59 psi/100ft * (40ft/100ft) = 2.64 psi loss est final 49.4 PSI
Option B (3/4): 4gpm 4.29 psi/100ft * (40ft/100ft) = 1.72 psi loss est final 50.28 PSI

Does that look correct? If so this seems negligble as long as I can reasonably get that flow from 1/2.

 

[wwhitney]

I redid the calculations with uponor chart for 120 degreee (https://www.uponorengineering.com/~...ressure loss charts.ashx?version=062520180400)

Broken link.

Initial: 60 PSI - 8 PSI loss
Option A (1/2): 4gpm 6.59 psi/100ft * (40ft/100ft) = 2.64 psi loss est final 49.4 PSI
Option B (3/4): 4gpm 4.29 psi/100ft * (40ft/100ft) = 1.72 psi loss est final 50.28 PSI

One of those numbers is wrong, the ratio should be over 5.

Does that look correct? If so this seems negligble as long as I can reasonably get that flow from 1/2.

I get 4 gpm in 1/2" PEX = 7.2 ft/sec, under the Uponor recommendation of maximum 8 ft/sec.

Cheers, Wayne

 

[kybob]

Broken link.

One of those numbers is wrong, the ratio should be over 5.

I get 4 gpm in 1/2" PEX = 7.2 ft/sec, under the Uponor recommendation of maximum 8 ft/sec.

Cheers, Wayne

Shoot - here is the correct link https://us.v-cdn.net/5021738/uploads/FileUpload/70/d215f960a1b9a3ad6208c62fc26891.pdf.

Am I misreading the table? for 1/2... 4 gpm is 4.29 PSI /100 feet @ 120F.I have a 40 ft run so 6.59 *.4 = 2.64.

Side note - based on this calculator I could in theory draw up to 4.61gpm via a 1/2 line before seeing issues?

Velocity, in feet per second (fps) to Gallons per Minute(gpm)

Velocity, in feet per second (fps) to Gallons per Minute(gpm)

pigsunlimited.com

 

[wwhitney]

Shoot - here is the correct link https://us.v-cdn.net/5021738/uploads/FileUpload/70/d215f960a1b9a3ad6208c62fc26891.pdf.

Am I misreading the table? for 1/2... 4 gpm is 4.29 PSI /100 feet @ 120F.

Yes, you are misreading the table. 4 gpm in 1/2" PEX at 120F is 19.21 PSI / 100 ft. While 6.59 PSI/100 ft is the entry for 4 ft/sec or 2.21 gpm in 1/2" PEX at 120F.

Cheers, Wayne

 

[kybob]

Yes, you are misreading the table. 4 gpm in 1/2" PEX at 120F is 19.21 PSI / 100 ft. While 6.59 PSI/100 ft is the entry for 4 ft/sec or 2.21 gpm in 1/2" PEX at 120F.

Cheers, Wayne

Thanks Wayne - I've been starring at too many tables for too long.

That appears to put 1/2 at final velocity ~42 psi +/- 5 and 3/4 at 48.2 +/- 5. My guess is no, but will that be a noticeable difference when running two shower heads on a 1/2 valve?