Calculating Water Pressure Loss
Summary: Need to determine water pressure loss in your home? Low water pressure is fixable. Follow this formula to find your home water pressure.
Calculating Water Pressure Loss
Years ago engineers developed tables to quickly calculate water pressure loss. They are often referred to as hydraulic friction loss tables. Studies produced "constants" (numbers which you use to help in the calculation) which make the math fairly easy. What you need to know are several things: size of pipe, flow (in gallons per minute GPM) and the distance which the water flows (fittings increase the actual distance, so if you have lots of bends and tee's, 100 feet of pipe might really be 130 feet or so). Flow is the thing that stumps most people. Flow rates can also be found on tables. Here are some standard flow rates for residential fixtures: toilet - 2.5 GPM, kitchen sink (H or C - times 2 if both handles wide open) - 1.25 GPM, washing machine (H or C - times 2 if H & C on at same time) 1.6 GPM, shower 2.5 GPM (both hot and cold on at same time).
Now for our constants. I have listed several to show the relationship as the gallons per minute of flow increases. Listed are 1/2 and 3/4 inch copper pipe Type M (standard used in residential construction):
The constants listed are pressure loss per hundred feet of pipe run.
| 1/2 inch pipe: | 1 GPM = 2 |
| 2 GPM = 7.22 | |
| 3 GPM = 15.3 | |
| 4 GPM = 26.0 | |
| 5 GPM = 39.2 | |
| 6 GPM = 55 | |
| 3/4 inch pipe: | 1 GPM = 0.354 |
| 2 GPM = 1.28 | |
| 3 GPM = 2.71 | |
| 4 GPM = 4.60 | |
| 5 GPM = 6.94 | |
| 6 GPM = 9.75 |
Notice the difference in the constants for different sized pipe, but yet the same GPM flow! It's HUGE. See why 3/4 is better?
Now, here is how you do a pressure loss calculation. By the way, we need one other constant and it is a number that relates to pounds per square inch. It's really pretty complicated, so I would appreciate it if you would just trust me on this one. The number we need to remember is 0.4335.
Here's the problem. Let's calculate what will happen to the pressure if we flush a toilet while a shower is running in a bathroom 100 feet away from where the water enters the house. We are using 1/2 inch pipe.
Toilet flow = 2.5 GPM Shower Flow = 2.5 GPM Total flow = 5 GPM
Constant from table multiplied by 0.4335 = Pressure loss in PSI
39.2 X .4335 = 16.99 PSI
If we substitute 3/4 inch pipe, here is the calculation:
6.94 X .4335 = 3.00 PSI
Quite a difference!
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Comments:
Sincere 31 Jan 2008, 08:38
Yes, I'am trying to learn all the necessary techniques in order to pipe up
a building using hot and cold water piping. Calculating gpm, friction
loss, etc. Is their a book out their that has the basic skills for a
beginner?
AsktheBuilder 31 Jan 2008, 08:46
Sincere,
I am sure there are *many*. Contact a Mechanical Engineer for recommendations.
A Boelow 07 Mar 2008, 12:48
So pretty much this 3 part course should of just said up size to 3/4" pipe.
And what about plastic pipe now being used in almost all new construction?
AsktheBuilder 11 Mar 2008, 06:57
A.
I think that oversimplifies it. Manifold systems need to be concerned about friction loss.
barry 30 Mar 2008, 11:26
Water pressure ok to start. Pump runs sprinkler system. sprinklers run fine
for approx. 3 minutes and then lose at least 60% pressure. same way in
house. HELP!!
AsktheBuilder 31 Mar 2008, 16:02
Barry,
Something is wrong with the pump or the pressure tank.
Mao Voeng 03 Apr 2008, 01:25
I need to know the detail calculation of water supply system
mario 10 May 2008, 12:30
I set my sprinklers for 2 days a week and it works find but when I try to
use it on manual function it is not responding till after 4 minutes running
the pump. Is there any specific
accessory that controls the water to flow thru the pump that would cause this delay when its on manual function or what could be wrong? my sprinkler system is almost 2 years old.
Walt Rau 23 May 2008, 14:37
I have 20' of 1/2" copper feeding a 1st floor powder room. Adding basement
powder room. Can't get access to 3/4" main, but can get with 8" of 'T'
where existing 1/2" runs. So if I transition back to 3/4" for 20' run to
powder rooms I will be left with a 8" long 1/2" bottle neck.... Based on
these calcs at 6GPM, this 8" of 1/2" section only causes 55 x .4335 x
.66ft/100ft = 0.16 PSI pressure drop. In other words, has negligible
effect. I am correct in my thinking???
Royce 27 May 2008, 22:19
Hello, I have just installed in inground sprinkler system in my yard. I
have a total of 6 sprinkler head, with water pressure from the source at
around 17psi, the requirements for the heads is less than 17psi,
unfortunately the resulting spy dose not cover the expect ground. From
reading the above article, it seems that by increasing to 3/4 inch pipe the
flow loss will be significantly less. My question is that the source of
water is a 1/2 inch pipe, if I connect it to 3/4" tubing to the 1/2"
sprinkler heads the bottlenecks will be at the sprinkler head and at the
water source, will changing to 3/4" pipe make a significant difference??
Thanks for any feedback you can provide Royce View all comments |
