Sizing Domestic Water Without Oversizing the Building

The problem with Hunter's curve in 2026

Hunter's curve was developed in 1940, when a typical lavatory used 4 to 5 gallons per minute and flush valves moved 4 to 5 gallons per flush. Today's WaterSense lavatories run at 0.5 gpm, water closets flush at 1.28 gpf, and urinals can be pint-flush or waterless. Applying the original curve directly produces water services that are routinely 2 to 3 times larger than the building will ever demand.

The consequences are not academic. Oversized mains mean longer dwell times, which means warmer cold water, colder hot water at the fixture, more stagnation, and a real Legionella risk in healthcare and multifamily projects.

How V3 approaches sizing

We use IAPMO's Water Demand Calculator (WDC) for any project with more than a handful of fixtures. The WDC is built on peer-reviewed field data from low-flow buildings and produces peak demands that are typically 30 to 60 percent lower than Hunter's curve for the same fixture count.

On projects where the AHJ still requires Hunter's, we run both calculations and submit the WDC result as supporting documentation for a reduction. Most jurisdictions accept it once they see the math.

What this changes in the field

• Smaller water services reduce tap fees, meter costs, and backflow assembly size.
• Smaller risers and mains free up shaft and ceiling space for other trades.
• Shorter dwell times improve hot water delivery and reduce the need for recirculation pump runtime.
• Domestic booster pumps, when needed, can be selected for the actual demand instead of a phantom peak that never occurs.

Where we still stay conservative

Medical, lab, and food service occupancies get separate analysis. Process loads, dishwashers, autoclaves, and emergency fixtures do not follow residential probability and we size them on actual manufacturer data plus a documented diversity factor.

Right-sizing is not under-sizing. It is sizing for the building that will actually be built and occupied.