Pex line pressure drop

[W. Bernard]

Hello,

At my cottage the main supply line from the pressure tank (30-50 psi range) is 1/2" pex which runs about 30 feet, no elbows etc., before it splits to hot and cold supply lines. In the current setup I am happy with the pressure at all faucets, shower etc. I am considering a renovation which would require me to change the location of the supply line (it would be about the same length/type, 30 feet and 1/2" pex) but would require 90 degree elbows, three of them. Just wondering if this will drop my pressure too much ? I can go to copper if necessary, but would prefer to use pex. Thanks in advance for the help !

 

[Diehard]

Well I suppose it depends on your flow needs.
For 1/2" PEX...(pressure loss values vary slightly and I've rounded them up.)
At 4 gpm...30 feet x say 0.2 psi/ft = about 5.5 psi loss.
At 3 gpm...30 feet x say 0.12 psi/ft = 3.6 psi loss.
At 2 gpm...30 feet x say 0.05 psi/ft = 1.5 psi loss

Use 16.5 equivalent ft of loss for every 90 degree plastic elbow.(These values vary a bit between sources.)
So 16.5 x 3 = say 50 equivalent feet of pipe.
Or an addition loss of 50 feet
At 4 gpm...50 x 0.2 psi/ft or 10 psi
At 3 gpm...50 x 0.12 psi/ft or an added loss of 6 psi.
At 2 gpm...50 x 0.05 psi/ft or 2.5 psi
(These losses are just for the ftgs and would be added to the pipe loss.)

If you could loop the PEX rather than using a fitting that would be good.
Or otherwise, if you feel the pressure loss is too high you could consider using 3/4" PEX.
Friction loss, and resulting pressure loss, for 3/4" PEX tubing would be about 1/2 as much.

In fact if you went with 3/4" you would end up gaining pressure over what you have now. A small amount!

 

[frodo]

1/2 '' pex is barely big enough for 1 fixture
how many fixtures are you trying to feed with a 1/2''line ?

at 100' you are getting 0.92 volume gal

minimum...bare minimum you should have coming into your cottage is 3/4''
I would actually use 1'' from pump to first fixture, then 3/4 all the way

 

[Mikey]

View attachment 20209
at 100' you are getting 0.92 volume gal

I think what the chart is saying is that 100' of 1/2" pipe holds 0.92 gallons. That chart doesn't deal with flow, just volume.

Doing the math, 1 gallon = .133681 cu ft = 231.008 cu in.

A cylinder 0.485" in diameter 100' long is 221.6943 cu in, or 0.96 gallons. Close enough for government work.

 

[frodo]

I think what the chart is saying is that 100' of 1/2" pipe holds 0.92 gallons. That chart doesn't deal with flow, just volume.

Agree,

 

[frodo]

http://shop.solardirect.com/pdf/water-heaters/solar-hot-water/active-solar/pumps/grundfos-guide.pdf

 

[Diehard]

Did you get your threads mixed up?

 

[frodo]

View attachment 20214

Did you get your threads mixed up?

i think i did...

 

[W. Bernard]

Well I suppose it depends on your flow needs.
For 1/2" PEX...(pressure loss values vary slightly and I've rounded them up.)
At 4 gpm...30 feet x say 0.2 psi/ft = about 5.5 psi loss.
At 3 gpm...30 feet x say 0.12 psi/ft = 3.6 psi loss.
At 2 gpm...30 feet x say 0.05 psi/ft = 1.5 psi loss

Use 16.5 equivalent ft of loss for every 90 degree plastic elbow.(These values vary a bit between sources.)
So 16.5 x 3 = say 50 equivalent feet of pipe.
Or an addition loss of 50 feet
At 4 gpm...50 x 0.2 psi/ft or 10 psi
At 3 gpm...50 x 0.12 psi/ft or an added loss of 6 psi.
At 2 gpm...50 x 0.05 psi/ft or 2.5 psi
(These losses are just for the ftgs and would be added to the pipe loss.)

If you could loop the PEX rather than using a fitting that would be good.
Or otherwise, if you feel the pressure loss is too high you could consider using 3/4" PEX.
Friction loss, and resulting pressure loss, for 3/4" PEX tubing would be about 1/2 as much.

In fact if you went with 3/4" you would end up gaining pressure over what you have now. A small amount!

 

[W. Bernard]

Thanks this is very good advice. I guess I'll look into going to 3/4" pex to the first fixture.

 

[W. Bernard]

View attachment 20209 1/2 '' pex is barely big enough for 1 fixture
how many fixtures are you trying to feed with a 1/2''line ?

at 100' you are getting 0.92 volume gal

minimum...bare minimum you should have coming into your cottage is 3/4''
I would actually use 1'' from pump to first fixture, then 3/4 all the way

Thanks for the reply.
I have a kitchen sink faucet, Bathroom faucet and shower, hot water tank (not sure if that counts or not) and toilet. In the summer I run a garden hose off the drain valve/tap at tank and that keeps the garden watered.

The flow with the current setup for my needs is not too bad or perhaps its just that I've gotten used to it. But you make an excellent point, why not go to at least 3/4" preferably 1" at least from the pressure tank to the first fixture. Cheers!

 

[W. Bernard]

Thanks this is very good advice. I guess I'll look into going to 3/4" pex to the first fixture.

 

[frodo]

 

[W. Bernard]

View attachment 20227

Thanks! This looks like the way to go.