low water pressure supply to sprinkler system

[ultraman]

i'm having problems finding the root cause of the low water pressure supply to my sprinkler system to the backyard. the front yard and backyard share the same water line. the front yard sprinkler system has no problems. i can't find any areas of flooding to indicate cracked/leaking pipe. water is being supplied to all the sprinkler heads in the backyard, but just not enough to make them pop up.

recently, i had to raise the solenoids because of the drainage issues in my yard. any help/ tips would be greatly appreciated to helping me find the low water pressure. it's driving me crazy!!

thanks in advance.

[LazyPup]

The post states that both the front yard and back yard are being supplied from the same line so the commonly accepted inference there is that the pressure would also be equal at both the front yard and the back yard but in reality such is not the case.

Under the International Residential Code the minimum permissible "static head" pressure to the structure is 40psig. (Uniform Plumbing Code min. 15psig). The operative words in that statement are "Static Head" pressure. Static head pressure is the pressure throughout the system when all valves are closed and no flow is occurring, however the moment that a valve or faucet is opened and flow begins the pressure will immediately drop to "Dynamic Head" (working head) pressure.

Dynamic head pressure will be Static head pressure less "Vertical Static head" pressure losses and "Friction head" pressure losses.

If you could post an illustration of your layout showing where your sprinkler heads are and the length and size of the lines maybe we can find a simple solution to your problem.

Vertical static head loss is a constant and is equal to .434psi for each foot of vertical rise. For example, let us assume that we are looking at a two story house and the main water line enters the basement at floor level while the bathroom in on the second floor. In this case the water enters the structure at floor level in the basement and must rise 8' to the first floor, another 8' to the second floor and an additional 6' to the shower head. The vertical rise from the water main to the shower head is then 8+8+6=22'. The vertical static head (physical weight of the water in the line an opposing flow) is then 22' x .434psi/ft = 9.54lbs. Therefore while you have 40psig at the main the resultant pressure at the shower head is now 40psig - 9.54psig = 30.46psig from which we must also deduct friction head loss.

Friction head loss is a pressure loss which results from friction between the flowing water and the interior of the pipe wall. While vertical static head loss was a constant Friction Head Loss is a variable that is directly proportionate to the size of the line and the velocity of flow. In fact, friction head loss is so significant that we have a design standard which states we must remain under 5.6fps. (At velocities greater than 5.6fps pipe wall erosion may occur)

Designing an irrigation system is especially problematic due to the high volume of flow and the long length of lines involved.

For illustration let us assume a house where the water line enters the front of the house at floor level in the basement and the irrigation lines are exiting the structure 4' above the floor. Let us further assume that the house is 40' from front to back and the irrigation line in the back yard is 100' long with 4 sprinkler heads.

Consulting the "Rainbird" catalog we find that the average rate of flow for a pop-up sprinkler head is 3gpm and they require 25psi.

If we have 4 heads and each has a flow rate of 3gpm the combined flow rate on the line is 12gpm.

Consulting the Sched. 40 PVC friction head loss table we find that the smallest diameter line that will handle 12gpm and remain within the recommended 5.6fps is a 1" line which would produce a velocity of 4.44fps and a friction head loss of 3.36psig per 100' of pipe (.033psig/ft).

From the main the water line runs 40' across the house and another 100' to the furthest sprinkler so the resultant length is 140'. If we were to then run a 1" line the full distance the resultant pressure drop to the last head would be 140' x 0.033psig/ft= 4.62psig.

In this example we have a vertical head loss of 4' x .434psig/ft = 1.736psig
If the static head pressure is 40psig the dynamic head at the irrigation line is now 40psig - 1.736 = 38.26psig.

With a 1" line the resultant pressure at the last head is now 38.26psig - 1.74= 36.5psig.

With a 3/4" line the at 12gpm the velocity of flow would be 7.21fps and the Friction Head Loss would be 0.108psig/ft.
140' x 0.108= 15.12psig loss.

If we then deduct the Friction head loss from the irrigation line Dynamic head we have 38.26psg - 15.12psig friction head loss = 23.14psig at the sprinkler head however the manufacture specifications stated the sprinkler head
requires 25psig.

NOTE:The illustration has been somewhat simplified because it does not take into account the additional Friction head losses which result from the fittings on the lines.

If you could post a description and hopefully a diagram of your installation showing the line sizes & approximate lengths as well as the number & placement of the sprinkler heads we may be able to find a relatively simply solution to your problem.

[596]

Excellent design analysis by lazupup. But let me try to simpify: Did the system used to work OK? My impression from your question was that it DID work , meaning an OK design, and now there is a problem.

So, if I am reading correctly, we need to look for something like a valve between the from yard, and the back yard manifold, that is partly closed or clogged.

You mentioned you moved some valves, and thus could have introduced some dirt in the pipes. Solenoid valves are very sensitive to dirt blocking the orifices which control the pilot operation. You might need to literally cut the pipe leading to the valves, and confirm if you have adequate pressure arriving there. If not, then you need to back up and look for the valve, crushed pipe, etc.

[Biddle34]

low water pressure

Wow lazypup, I just woke up from reading your dissertation on water pressure, how about you speak inexperienced handyman language next time, whew. We want quick and accurate answers without having to take a class at Harvard to learn what is wrong. Thanks, but no thanks

[LazyPup]

Biddle34- If you will look at the upper left hand corner of my post you will note that I have to date posted more than 2000 replies to questions in this forum. Throughout that time I have always endeavored to provide a complete description of the problem as well as the solution, and in fact, in most cases I have drug out my crayola's and created illustrations to support my text. I am sorry if you find 8th grade math a bit difficult to follow, but i can assure you that based upon the hundreds of direct emails i get in response to my posts, the majority of those reading my answers appreciate the in depth detail.

The reason I did not post a simple answer is because based upon the information presented in the original post, there is no simple question however there is the false assumption that because both the front and back yard systems are attached to the same line, they should have equal pressure, which simply is not true. Point of fact, once flow begins no two points in the entire potable water distribution system in the whole house will have equal pressure.

No matter what we are working on, in order to diagnose a problem we must first determine what should be happening under normal circumstances, then check the system for abnormalities and once we have discovered the abnormality we can then try to isolate the cause.

On the other hand we could take the typical "handyman's" simple approach and assume facts not present and begin cutting and piecing into the system in the blind hope that we might stumbled upon the solution. But then in my experience that method generally introduces more problems than it resolves.

If you will read the last line of my previous post I specifically asked for additional information so that I could work out the details and help them resolve the problem. I could have just as easily just posted a link where the reader can download the RainBird Lawn irrigation design manual and let them work it out for themselves.

So now we have three possible methods of resolving this problem:
1. We could use the HandyMan technique, cut and piece and hope we find the solution
2. Download the 140page Rainbird manual and spend the next month or so studying the details
3. Post additional information and i will be happy to work through the problem with you.


RAINBIRD LAWN IRRIGATION DESIGN MANUAL (140 pages in PDF format)

http://www.rainbird.com/pdf/turf/IrrigationDesignManual.pdf

[596]

Whoa, Nelly!.

My point was that as I interpret the question, his problem is not related to a design layout. I am assuming ( if this is not correct, let me know ) that the system WAS working fine , he did some work, and now something is wrong. So I am suggesting the need to troubleshoot from that standpoint, not a deficient initial design.

[vdotmatrix]

Thanks LazyPup for all that!

I am in the process in putting in a 4 zone system at my house! yippee!

1. Get a container enough to measure HOW MANY GALLONS of water come from your hose bib.

MINE IS 12GPM.

Each zone cannot exceed this rate.

Every sprinkler head has a GPM rating like 1.6 or 2.2 etc.....

Write down evey sprinkler head GPM rating from the front yard and add them up (Let's call this your zone 1). Next write down evey sprinkler head GPM rating from the back yard and add them, (uplet's call this your zone 2). Make sure each zone doesn't exceed the amount of water available from your supply.

That's all I have , hope this will help. Also , check for crap in your lines from the winter.

[Needaclue]

Wow lazypup, I just woke up from reading your dissertation on water pressure, how about you speak inexperienced handyman language next time, whew. We want quick and accurate answers without having to take a class at Harvard to learn what is wrong. Thanks, but no thanks

Hmm. I haven't spent a day in college and I understood his post. You should have payed attention in highschool.