Commercial Backflow Sizing: 1.5" vs 2" RP on 2" City Main

Backflow Preventer Sizing: Pressure Loss vs. Flow Capacity

City main: 2". Irrigation main: 1.5". Should you upsize the RP backflow to 2"?

Pressure Loss Reality

• RP backflows lose 8–12 PSI regardless of size.
• 2" RP on 1.5" pipe: no pressure gain—loss is internal to the device.
• 2" RP allows higher flow volume at same pressure loss.

Decision Matrix

Key Insight

Pressure and flow are linked but distinct. If heads perform poorly due to low pressure, a larger backflow won't help. If they fail due to insufficient GPM (too many heads per zone), 2" helps.

Pro Move

Get the manufacturer's friction loss chart for the exact RP model (Zurn, Watts, Febco). Compare 1.5" vs 2" at your design GPM. The difference is usually 1–3 PSI—rarely decisive.