What could be causing water pressure drop with sprinklers?

Question

My first issue is that when the sprinklers come on, there is a significant drop in water pressure in the sink, shower, etc. I checked the sprinkler lines and there are no obvious leaks (i.e. holes in the ground).

Part II of my problems is varying pressure in different sprinkler zones. There is no pressure in sprinkler zone 1 (sprinkler heads do not come up but dribble water), 1/2 pressure in sprinkler zone 2 (sprinkler heads come up but are at 1/2 flow), while sprinkler zone 3 has full pressure.

The sprinkler repair company wants to add a pressure regular to the line to the sprinklers (runs at over 100 psi when it should be 75 psi) and split sprinkler zone 1 into two zones due to the # of sprinkler heads on this zone. However this does not answer why sprinkler zone 2 is at 1/2 pressure.

Ironically, the house was built in 2009 and for the past 5 years, including last year, the sprinklers ran fine with no water pressure drop problems.

Any suggestions?

Answer 1

Water pressure drops come from two sources:

1) A constriction in the line upstream, limiting the maximum inflow you can get.

2) More demand on the line, producing more outflow than the inflow can fully pressurize.

If a sprinkler zone is larger (serves too many heads), or if more zones are active at once, it will demand more water. Dividing the zone could be one solution.

If a sprinkler zone's pipe is kinked or otherwise obstructed, it may not be able to deliver as much water as is needed to fully pressurize the line.

If running the sprinklers reduces pressure at the sink, the point limiting that is going to be upstream of both, and you're probably also seeing pressure drops at the sink when you flush a toilet or run a bath. You can check for restrictions working back toward the source (valves not fully open, for example), but if the shared flow limit is what's coming into the foundation...