CSLB C-27 Landscaping ContractorIrrigation SystemsHard
A C-27 contractor is troubleshooting a newly installed irrigation system. The client reports that when Zone 3 (spray heads) activates, the water pressure drops significantly across the entire system, causing heads in other zones (even when off) to weep, and Zone 3's heads to mist. The mainline is 1.5-inch PVC, and Zone 3 has 12 spray heads, each rated at 2.5 GPM. The static pressure is 60 PSI. What is the most likely issue?
- AA partially closed main shut-off valve.
- BThe mainline is undersized for the system's total demand.
- CA faulty pressure regulator on Zone 3.
- DExcessive friction loss due to too many elbows in Zone 3's lateral lines.
Show answer & explanationAnswer & explanation
Correct answer: B. The mainline is undersized for the system's total demand.
Significant pressure drop across the entire system when one zone activates, affecting other zones and causing misting in the active zone, strongly indicates that the mainline (or the water source itself) is undersized to handle the system's total instantaneous flow demand. This results in excessive velocity and friction loss in the mainline.
Why the other options are wrong
- A. A partially closed main shut-off valve would cause low pressure across ALL zones, even when static, or when any zone is active, but the question implies the problem is most pronounced when Zone 3 activates.
- C. A faulty pressure regulator on Zone 3 would affect only Zone 3's pressure, not cause system-wide pressure drop or weeping in other zones.
- D. Excessive friction loss in Zone 3's lateral lines would cause pressure issues only within Zone 3, not system-wide pressure drop or weeping in other zones.
Undersized Mainline
An irrigation mainline pipe that is too small in diameter to adequately deliver the required water volume at acceptable pressure to meet the system's total instantaneous flow demand.
- Leads to excessive water velocity and friction loss
- Causes significant pressure drop across the entire system
- Results in poor sprinkler performance (misting, short throw)
- Requires pipe replacement or redesign to fix
Memory trick: Whole system drops? Mainline's too small, can't handle the call.