Adding Rotors to a Zone That Cannot Afford Them
A 12-year-old subdivision lot in Bend has a 5/8-inch meter rated at 20 gpm and 72 psi static pressure at the meter, with a 1-inch copper service and a 1-inch PVC main. You measure 54 psi at the far end of the main while a zone is running. The backflow preventer is a PVB at the house. Zone 4, which serves the backyard, sits 8 ft above the PVB and currently has five spray heads at 1.5 gpm each. The client wants better coverage in the backyard, so you propose adding six rotors rated 3.0 gpm at 45 psi on 40-ft spacing to zone 4. The client currently runs 20 minutes per zone, three days a week. The zone-4 side is thin, with dry spots on the high corner. Soil is sandy loam.
1. Will the proposed zone work hydraulically? Show the numbers and state what you would install instead.
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Flow check:
- Existing sprays: 5 × 1.5 gpm = 7.5 gpm
- Added rotors: 6 × 3.0 gpm = 18 gpm
- Total = 25.5 gpm
Against a meter rated 20 gpm - and against the usual design ceiling of about 75% of meter capacity (≈15 gpm) - the zone is roughly 70% over design flow. Velocity in the 1-inch main goes well past the usual 5 ft/s limit, friction loss climbs steeply, and every head on the zone sags in pressure together.
Pressure check:
- 54 psi at the end of the main under flow
- minus elevation: 8 ft × 0.433 = 3.5 psi
- minus valve and backflow losses, roughly 10 psi
- minus lateral friction at high flow, roughly 5 psi or more
That leaves roughly 35 psi at the head against a 45-psi nozzle requirement. The high corner dries out first because it loses elevation and pressure simultaneously.
Mixing heads is the second error. Sprays and rotors have very different precipitation rates; on one zone the sprays soak and the rotors starve. Never mix them.
Recommendation: split into a spray zone (5 sprays, 7.5 gpm) and a rotor zone. Put 5 rotors on the new zone = 15 gpm, which fits the 75% design flow and drops friction loss enough to recover pressure. If the client insists on 6 rotors, upsize the lateral to 1¼ inch and re-measure, or select a lower-flow nozzle set. Verify with a pressure gauge at the last head before you backfill.
2. Calculate a defensible irrigation schedule for the new rotor zone and show how far the client's current 20-minutes-per-zone program is off.
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Precipitation rate of the rotor zone: PR = 96.3 × gpm ÷ (spacing × spacing) = 96.3 × 3.0 ÷ (40 × 40) = 288.9 ÷ 1600 = 0.18 in/hr
Plant water use (July, Bend): ETo ≈ 0.25 in/day; cool-season turf Kc ≈ 0.8 → ET ≈ 0.20 in/day, or about 1.4 in/week.
Runtime required: 1.4 in ÷ 0.18 in/hr = 7.8 hours per week ≈ 467 minutes per week.
Client's current program: 20 min × 3 days = 60 min/week × 0.18 in/hr = 0.11 in/week - roughly 8% of demand. The dry high corner is not a mystery.
Delivering it without runoff on sandy loam: Sandy loam takes water fast, so long sets are acceptable, but sloping ground and the thin areas argue for cycle-and-soak. One defensible pattern:
- 3 start times per week, each set split into 3 cycles of about 52 minutes with a 45–60 minute soak between cycles.
- Re-check after two weeks: probe the soil at 6 inches at the high corner and adjust runtimes, not the frequency, first.
- Install a smart controller with a local ET source and a rain sensor, and program a seasonal percentage adjustment so July's 100% becomes roughly 40–50% in October.
Key point: the schedule is derived from the head's precipitation rate, not from how long the neighbor waters. If you change nozzles, the PR - and every runtime - changes.
3. The client asks what else the zone needs before winter and whether the PVB can stay where it is. Answer both, with reasoning.
Read the worked answer
Backflow: the PVB is the correct device type for an irrigation system on a residential potable supply in most Oregon jurisdictions, but it must be installed per the plumbing code - 12 inches or more above the highest downstream point of the system and never where it can be submerged or where water can freeze in the body. It must be tested at installation and annually by a certified backflow tester, and the test report goes to the water supplier. If the PVB sits in a low spot or in a valve box that floods, relocate or protect it; a submerged PVB is not a functioning air gap.
Freeze protection: Bend winters will split a PVB, a plastic valve body, and any above-grade pipe. Options are:
- Blowout with a properly sized compressor - regulate so pressure at the blowout point stays low (do not exceed roughly 50 psi at the fitting, and never use a large compressor unregulated, which is how lateral fittings get shattered), going zone by zone from the farthest zone back to the valve, until only a fine mist remains.
- Manual drain if the system has drain valves at low points and the pipe was installed with fall.
- Remove or insulate the PVB per manufacturer instructions and the local code; many manufacturers specify removal of the poppet assembly for winter.
Slope and drainage items for zone 4: because it is 8 ft above the PVB and drains back downhill, install check valves in the heads (or heads with built-in checks) so the 8 ft of head (about 3.5 psi) does not drain the lateral and puddle the low corner after every cycle. Set the controller to rain-off and verify the rain sensor is wired and functioning before the first fall storm.