Pump Sizing Basics
Sizing a pump correctly is one of those things that seems complicated but really comes down to two numbers: flow and pressure. Get those right, and the pump practically picks itself.
The Two Numbers You Need
1. Flow (GPM)
This is how much water your biggest zone needs. Add up the GPM of every head on your largest zone. That's your minimum flow requirement.
Example: Zone 3 has 6 rotors at 3.5 GPM each = 21 GPM needed.
2. Total Dynamic Head (TDH)
This is the total pressure the pump needs to produce. It includes:
- Operating pressure at the heads (typically 45-55 PSI for rotors, 30 PSI for sprays)
- Friction loss in pipe (depends on pipe size, length, and flow rate — use a friction loss chart)
- Elevation change (add 0.433 PSI per foot of elevation rise)
- Losses through valves, fittings, and backflow preventer (typically 5-10 PSI total)
Example:
- Head operating pressure: 50 PSI
- Friction loss in 200 feet of 1" PVC at 21 GPM: 8 PSI
- Elevation rise of 15 feet: 6.5 PSI
- Fittings and valves: 7 PSI
- Total: 71.5 PSI = about 165 feet of head
Reading a Pump Curve
Every pump has a performance curve that shows flow (GPM) on the x-axis and head (feet or PSI) on the y-axis. You want your operating point (your GPM and TDH) to fall in the middle of the curve — not at either extreme.
- Too far left (low flow): The pump is working too hard against pressure. It'll run hot.
- Too far right (high flow): The pump can't maintain pressure. Your heads won't work properly.
- Sweet spot: In the middle third of the curve, where efficiency is highest.
Common Sizing Mistakes
- Sizing for average instead of peak demand. Always size for your largest zone.
- Forgetting friction loss. Long pipe runs eat up a lot of pressure.
- Ignoring elevation. A 20-foot hill costs you almost 9 PSI.
- Oversizing the pump. A pump that's too big wastes energy and can over-pressurize your system. Bigger isn't always better.
Quick Reference for Residential
| Zones | Typical Flow | Typical Pump Size |
|---|---|---|
| 4-6 | 10-15 GPM | 3/4 - 1 HP |
| 6-10 | 15-25 GPM | 1 - 1.5 HP |
| 10-15 | 25-35 GPM | 1.5 - 2 HP |
These are ballpark figures — always do the actual calculation for your specific system.
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