[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.