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Backflow Preventer Sizing Calculator

Size RPZ and DCVA backflow prevention assemblies with pressure loss budget per ASSE 1013/1015 and AWWA M14

Free backflow preventer sizing calculator for plumbing engineers, fire protection designers, and cross-connection control specialists. Enter the flow rate (GPM), available inlet pressure, required downstream pressure, and hazard level to determine the correct assembly type (RPZ, DCVA, PVB, or AG/AVB) and size (3/4" through 10"). The calculator computes the pressure loss through the assembly at the design flow using published manufacturer head loss curves and checks that the available pressure budget is adequate. Shows the maximum recommended flow rate for each assembly size, oversizing wastes money, but undersizing causes nuisance relief valve discharge on RPZ assemblies.

Pro Tip: The most common RPZ problem is relief valve discharge during peak demand. This happens when the assembly is undersized and the pressure drop exceeds the relief valve opening differential (typically 2-5 psi). Size the RPZ for the maximum instantaneous flow, not the average, if the sprinkler system and domestic supply are on the same backflow preventer, use the combined peak flow. Also verify that the system pressure at the RPZ inlet is high enough to deliver the required downstream pressure after the assembly pressure loss.

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Backflow Preventer Sizing Calculator
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How It Works

  1. Enter Flow Requirements

    Enter the peak design flow rate in GPM. For plumbing systems, calculate this from fixture units using the Hunter's Curve method. For fire sprinkler systems, use the sprinkler system demand plus hose stream allowance. For combined systems, add both demands.

  2. Set the Pressure Budget

    Enter the available inlet pressure (static minus elevation loss) and the minimum required downstream pressure. The difference is your available pressure budget for the backflow preventer. The calculator checks that the assembly pressure loss at design flow fits within this budget.

  3. Select the Hazard Level

    Choose the cross-connection hazard: high hazard (health threat, requires RPZ), low hazard (non-health, DCVA acceptable), or non-pressurized (PVB or AVB may apply). Local codes and the water utility may have specific requirements that override the minimum.

  4. Review the Assembly Selection

    The calculator recommends the assembly type and size, shows the pressure loss at design flow, and verifies the pressure budget is adequate. It also flags when a smaller size would work at the average flow but would cause problems at peak flow.

Built For

  • Plumbing engineers sizing backflow prevention for commercial buildings per local cross-connection control ordinances
  • Fire protection designers specifying backflow assemblies for fire sprinkler supply connections
  • Cross-connection control program administrators reviewing backflow preventer submittals for adequacy
  • Mechanical contractors selecting RPZ assemblies for boiler makeup water and chemical feed connections
  • Irrigation designers sizing backflow prevention for landscape irrigation connections to potable water supplies

Assumptions

  • Head loss curves are based on published industry data for representative assemblies, actual loss varies by manufacturer.
  • The pressure budget accounts for static pressure minus elevation head at the assembly location.
  • Assembly type selection follows the hierarchy in AWWA M14 based on the degree of hazard, local codes may be more restrictive.

References

  • AWWA Manual M14, Backflow Prevention and Cross-Connection Control: Recommended Practices
  • ASSE 1013, Reduced Pressure Principle Backflow Prevention Assemblies
  • ASSE 1015, Double Check Backflow Prevention Assemblies
  • USC Foundation for Cross-Connection Control and Hydraulic Research, Manual of Cross-Connection Control

Frequently Asked Questions

A Reduced Pressure Zone assembly (RPZ, ASSE 1013) has two check valves with a relief valve between them that opens to atmosphere if the checks fail, it provides the highest level of protection and is required for high-hazard (health threat) connections. A Double Check Valve Assembly (DCVA, ASSE 1015) has two check valves without a relief valve, it protects against backpressure and backsiphonage for low-hazard (non-health) connections. RPZ assemblies have higher pressure loss because of the additional internal components.
The relief valve opens when the pressure in the zone between the two checks drops below atmospheric, this can happen during backflow events (the designed purpose) or during high-flow conditions when the pressure drop through the first check exceeds the relief valve differential (a sizing problem). If the RPZ is properly sized and the relief valve still opens, the first check valve may be fouled or the downstream pressure is cycling above the inlet pressure. Oversized RPZ assemblies rarely have this problem; undersized ones frequently do.
Yes, and in many cases you should. A 2" RPZ on a 3" supply line is perfectly acceptable if the flow rate at design conditions produces acceptable pressure loss through the 2" assembly. Oversizing the assembly to match the pipe size wastes money and can also cause problems: RPZ check valves need sufficient flow velocity to seat properly, and an oversized assembly may not fully close at low flows. Size for the flow rate, not the pipe diameter.
RPZ assemblies typically lose 10-15 psi at rated flow. DCVA assemblies typically lose 5-10 psi. Pressure loss varies significantly by manufacturer and size, always use the specific manufacturer's head loss curves for final design. At low flows (below 50% of rated capacity), the loss may drop to 3-5 psi. The calculator uses published industry data for estimating purposes; confirm with the actual product data sheet before specifying.
Disclaimer: This calculator provides backflow prevention assembly sizing estimates based on industry standards and published head loss data. Local plumbing codes, water utility requirements, and specific manufacturer data may differ. Verify the assembly type requirement with the local water purveyor or cross-connection control authority having jurisdiction. Final sizing should use the specific manufacturer's head loss curves.

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