Backflow Preventer Pressure Drop Calculator — RPZ and DCVA Loss, Minimum Pressure Loss, and Pressure Budget

Overview

What to Look at First

How to Use This Calculator

1. Choose the unit system with the calculator's own selector. Every field, label, result and export follows that selector, and switching it converts the values you already entered rather than reinterpreting them.

2. Choose the assembly type. This version models reduced pressure assemblies, known as RPZ or RP, and double check valve assemblies, known as DCVA or DC. Other device classes are listed so you can see where the boundary is, and the calculator returns an out of scope result for them rather than a figure with no verified curve behind it.

3. Choose the assembly size and enter the design flow through it. A water supply fixture unit calculation is the usual source for the flow. Where no verified curve exists for that type and size, the calculator says so and asks for the manufacturer figures instead of inventing one.

4. Enter the minimum or standby flow if you want the loss at low demand. Left blank it is taken as zero, which returns the spring floor, and that is often the more revealing number.

5. Enter the upstream pressure. There are two fields because they are two different numbers: the static pressure with nothing running, and the pressure still available upstream while the system is passing design flow. Each one feeds a different output, and the calculator will not substitute one for the other.

6. Enter the allowable loss for this device if the pressure budget assigns one, and the calculator will test the result against it.

7. If you have the manufacturer loss chart, enter the minimum loss at zero flow, and a published loss with the flow it was stated at. The estimate is then fitted through your own point instead of the representative one. Published curves differ between makers, so this is worth doing whenever the budget is tight.

8. For a reduced pressure assembly, enter the differential across the first check if you have a test reading taken with a field test kit. That unlocks the relief discharge check. Without it the check is reported as not evaluated.

9. Read the loss at design flow, the minimum loss, the downstream pressures, the budget share, the curve status, and the flags for the budget, the relief and the closed system consequence.

Only the assembly type, the size and the design flow are required, and no optional field blocks the Calculate button. Blank and an explicit zero are different states: a blank minimum flow is taken as zero and reported as the spring floor case, while a blank upstream pressure leaves the downstream figures reported as not evaluated rather than passed.

Inputs & Outputs

Backflow Preventer Pressure Drop Formula

FORMULA BLOCK START

GUARDS
  IF assembly_type, size_in or q_gpm is blank        THEN INCOMPLETE
  IF the type is a vacuum breaker, a dual check
     or a detector assembly                          THEN OUT OF SCOPE,
     no loss figure is returned for a device class with no verified curve
  IF any entered value is negative                   THEN OUT OF RANGE
  IF q_gpm is at or below zero                       THEN OUT OF RANGE,
     zero flow is the minimum loss case, not a design duty
  IF no verified floor exists for that type and size
     and published_static_psi is blank               THEN CURVE NOT VERIFIED
  IF published_loss_psi is below the floor           THEN the point is not
     fitted: the coefficient would be negative. The representative curve is
     used instead and the result says why
  IF published_loss_psi or published_at_gpm is
     entered alone, or published_at_gpm is at or
     below zero                                      THEN the point is not fitted
  IF neither upstream pressure is entered            THEN the downstream figures
     and the share of supply read NOT EVALUATED, never passed
  IF only one upstream pressure is entered           THEN only the output it
     supports is produced. One is never substituted for the other
  IF the assembly is not a reduced pressure assembly THEN the relief check reads
     NOT APPLICABLE, never passed
  IF first_check_diff_psi is blank                   THEN the relief check reads
     NOT EVALUATED. It is never inferred from the total loss
  IF q_min_gpm is above q_gpm                        THEN warn, the design flow
     is understated

MINIMUM LOSS, the spring floor that is there before any water moves
  IF published_static_psi is entered
     static_floor = published_static_psi                    MANUFACTURER FLOOR
  ELSE
     static_floor = the verified floor for that type and size
                                                            REPRESENTATIVE CURVE
  verified floors    3/4 in reduced pressure    8.0 psi
                     3 in double check          4.0 psi
  it is check spring tension rather than friction, so it is present at zero flow
  where no verified floor exists for the selected type and size and none was
  entered, no figure is returned at all rather than an unverified one

MANUFACTURER POINT OVERRIDE
  IF published_loss_psi and published_at_gpm are both entered
     AND published_at_gpm is above zero
     AND published_loss_psi is at or above static_floor
       b = (published_loss_psi - static_floor) / square root of published_at_gpm
                                                     MANUFACTURER POINT FITTED
  ELSE
       b = the coefficient of the representative dataset for that type and size
                                                     REPRESENTATIVE CURVE
  representative coefficients are derived from the published anchor points
  rather than typed in, so those points reproduce exactly:
       b = (anchor loss - static_floor) / square root of the anchor flow
  a single fitted point improves the estimate. It does not reproduce the full
  product curve, because the shape between points belongs to the product

LOSS AT FLOW, a floor plus a square root term, not the square law of a meter
  loss_design = static_floor + b multiplied by the square root of q_gpm
  loss_min    = static_floor + b multiplied by the square root of q_min_gpm
  q_min_gpm blank is taken as zero, where loss_min equals static_floor exactly
  IF q_gpm is past the flow range the dataset was checked against
     THEN the figure is an extrapolation rather than a fitted estimate
  IF the assembly is a double check
     THEN loss_design is reported against the 10 psi maximum loss permitted
     through a double check assembly, a device requirement on the listed
     product rather than a project budget

DOWNSTREAM PRESSURE, each figure from its own upstream pressure
  p_down_static  = upstream_static_psi - static_floor
  p_down_flowing = upstream_flowing_psi - loss_design
  the static figure never uses the flowing pressure, and the flowing figure
  never uses the static one
  IF either result is at or below zero THEN it reads None remaining and the
  shortfall is reported as a positive deficit, never as a negative pressure

BUDGET SHARE, screening bands, a presentation choice and not a code limit
  computed only where upstream_flowing_psi is above zero
  share = loss_design / upstream_flowing_psi
  share at or below 0.15                    MODEST
  share above 0.15 and at or below 0.30     SIGNIFICANT
  share above 0.30                          DOMINANT
  no flowing upstream pressure entered      NOT EVALUATED

BUDGET TEST, only when allowable_loss_psi is entered
  IF loss_design is above allowable_loss_psi THEN LOSS OVER BUDGET
  ELSE                                            WITHIN ALLOWANCE
  an assembly can be operating well inside its own rating and still not fit
  the budget it was given: what fails here is the budget, not the device

RELIEF DISCHARGE, reduced pressure assemblies only
  the relief watches the differential across the first check, which is a
  different quantity from the total loss and is never derived from it
  relief_threshold_psi blank is taken as 2 psi, the published opening point
  relief_margin = first_check_diff_psi - relief_threshold_psi
  relief_margin at or below zero            RELIEF DISCHARGE RISK
  relief_margin above zero                  MARGIN ABOVE THE RELIEF SETTING
  IF first_check_diff_psi is below 5 psi while the margin is still positive
     THEN raise FIRST CHECK BELOW THE FIELD TEST CRITERION. The AWWA field
     test criteria ask the first check to hold at least 5 psi, which keeps a
     buffer of at least 3 psi above the relief opening point

CLOSED SYSTEM
  IF a storage water heater is served downstream
     THEN THERMAL EXPANSION CONTROL REQUIRED, IPC 607.3 and UPC 608.3
  IF that was not stated
     THEN ADVISORY, HEATER NOT STATED
  ELSE NO REQUIREMENT FROM THIS DEVICE
  the check function closes the branch either way. The requirement follows the
  water heater, not the assembly on its own
  location and heater_downstream change the wording only, never a loss figure

UNIT CONVERSIONS
  L/min = GPM multiplied by 3.785411784
  kPa   = psi multiplied by 6.895
  feet of head = psi multiplied by 2.31, taken from the psi figure as displayed,
                 so the same loss reads the same feet of head in both unit
                 systems
  every entry is held internally in GPM and psi at full precision and is
  converted only when it is drawn into a field, so the math never works from
  a rounded figure and a unit switch never changes what you entered

FORMULA BLOCK END

Key Facts

• A reduced pressure assembly has a minimum pressure loss from its check springs. In the representative dataset used here, a 3/4 in assembly shows at least 8.0 psi, which is 55.2 kPa, in the static test state at zero flow.
• The same representative 3/4 in assembly shows about 12.5 psi, which is 86.2 kPa, at 20 GPM or 75.7 L/min, and about 14.4 psi, which is 99.0 kPa, at 40 GPM.
• In the same dataset a 3 in double check assembly shows about 4.0 psi, which is 27.6 kPa, at zero flow and about 5.0 psi, or 34.5 kPa, at 150 GPM.
• The loss curve is not a square law. Fitting one to the reduced pressure points predicts 26.0 psi at 40 GPM against a published figure near 14. A floor plus a square root term reproduces those points within 0.36 psi.
• The curve is not always rising either. A published 6 in reduced pressure assembly gives 12.0 psi at 140 GPM and 9.5 psi at 840 GPM, so no simple formula covers the full range and the manufacturer chart governs.
• The first check valve in a reduced pressure assembly is held closed by a spring of about 5 psi and the second by about 1 psi. In a double check assembly each check spring must be at least 1 psi, and makers keep them near 1.5 psi to hold loss down.
• AWWA field test criteria: the first check must hold at least 5 psi differential, maintaining a buffer of at least 3 psi above the relief opening, and the relief must open at no less than 2 psi differential. Each check in a double check assembly must hold tight at 1.0 psid or greater.
• The maximum loss through a double check assembly must not exceed 10 psi, which is 68.9 kPa. A double check is rated to 175 psi maximum working pressure over 34 F to 180 F.
• Under AWWA C511 a reduced pressure assembly must withstand at least 150 psi, which is 1034 kPa, and cold water assemblies are designed for 33 F to 140 F, or 1 C to 60 C.
• A reduced pressure assembly must not be installed in a pit and its relief must never be submerged, because the relief discharges to atmosphere through an air gap. A double check does not carry that restriction.
• A reduced pressure assembly must not be installed vertically unless specifically tested and approved for that orientation.
• Any check function makes the downstream system closed. IPC Section 607.3 and UPC Section 608.3 require thermal expansion control where a storage water heater is supplied through a check valve, a pressure reducing valve or a backflow preventer.
• The 2021 revision of the reduced pressure and double check standards removed the double check fire protection category, because no manufacturer produced that low loss assembly. Fire service uses ASSE 1047 or ASSE 1048 detector assemblies.
• Standards define device performance and listing. None of them publish a pressure loss figure for a specific product; loss curves are manufacturer data.

Applications

• A designer building a pressure budget needs a number for the backflow line and has nowhere to get one. This page produces it, and the budget is assembled on the water pressure page.
• A contractor investigating low pressure in a building with a reduced pressure assembly checks how much of the missing pressure is the assembly, and specifically how much of it is present even at low demand.
• A service technician called out to an assembly that is discharging water uses the relief section to understand what the relief actually measures, and to see that a field test rather than a guess settles whether the cause is supply or device.
• An engineer comparing a marginal supply against a required assembly checks whether the pressure left downstream still supports the system, and hands the figure to the booster pump calculation if it does not.
• A designer comparing two manufacturer charts for the same nominal size and type finds that the product choice alone decides whether the pressure budget closes, which is the case for entering a published point rather than accepting a generic figure.
• A plans reviewer checks that the loss assumed in a submitted pressure calculation is consistent with the assembly actually specified, and that manufacturer data was used where the margin is tight.

Standards & References

• ASSE 1013-2021, Performance Requirements for Reduced Pressure Principle Backflow Prevention Assemblies The device standard for reduced pressure assemblies, revised in 2021 from the previous edition. It describes two independently acting check valves, internally force loaded to a normally closed position, separated by an intermediate zone with a hydraulically operated relief means force loaded to a normally open position. Read-only access through the IAPMO codes library.
• ASSE 1015-2021, Performance Requirements for Double Check Backflow Prevention Assemblies The device standard for double check valve assemblies. The 2021 revision removed the double check fire protection category, because no manufacturer produced that low loss assembly; fire service now uses detector assemblies under ASSE 1047 and ASSE 1048. Read-only access through the IAPMO codes library.
• ASSE International, product standards overview The scope statement for each ASSE product standard, including 1020 for pressure vacuum breakers, 1024 for dual check devices, 1047 and 1048 for detector assemblies, 1056 for spill resistant vacuum breakers, and 1064 for the backflow prevention assembly field test kits the first check differential is read with.
• ANSI/AWWA C511, Reduced-Pressure Principle Backflow Prevention Assembly Sets the device envelope used on this page: a working water pressure of at least 150 psi, which is 1034 kPa, without damage or impairment of function, and a cold water design range of at least 33 F to 140 F, which is 1 C to 60 C. A complete assembly is a hydraulically dependent relief valve between two independently operating internally loaded check valves, between two resilient seated shutoff valves, with four test cocks.
• ANSI/AWWA C510, Double Check Valve Backflow Prevention Assembly The parallel AWWA standard for double check assemblies, which carries the maximum working pressure and the loss limit quoted on this page.
• AWWA Manual M14, Backflow Prevention and Cross-Connection Control The source of the field test criteria quoted on this page: the first check holding at least 5 psi differential, the relief opening at no less than 2 psi differential, and each double check valve holding tight at 1.0 psid or greater.
• IPC Section 608 and Section 607.3, backflow protection and thermal expansion control IPC Section 608 lists the assembly types and the standards each must conform to. IPC Section 607.3 requires thermal expansion control where a storage water heater is supplied with cold water through a check valve, a pressure reducing valve or a backflow preventer, installed downstream of all of them.
• UPC Section 608.3, thermal expansion control The Uniform Plumbing Code carries the parallel requirement to IPC Section 607.3. Where a pressure reducing valve is also present on the same service the requirement is raised once, not twice, and the tank is sized with the Expansion Tank Sizing calculator.
• Attribution note on the loss figures The code sections decide which assembly is required and the ASSE and AWWA standards define what the device must do to be listed. Neither publishes a pressure loss figure for a specific product. The loss values on this page are representative figures from published datasets, used as preliminary estimates. For final design the manufacturer loss curve for the actual assembly governs.

Units

Common Mistakes to Avoid

• Treating a backflow assembly as an ordinary fitting loss. It is the one device in the service whose loss does not fall away toward zero at low flow, because the check springs have to be overcome before any water moves. A budget built from friction alone is short by that floor.
• Using a square law for the loss curve. That is the correct model for a water meter and the wrong one here. Anchored on the same points it overstates the loss at 40 GPM by nearly a factor of two, in the direction that buys an unnecessary booster pump.
• Assuming the loss always rises with flow. On larger assemblies it can fall: one published 6 in reduced pressure assembly loses less at 840 GPM than at 140. Extrapolating any formula past the data behind it is guesswork.
• Treating one manufacturer point as the complete product curve. A single published point improves the estimate, but it does not describe the shape between points across the full flow range. Use the full chart for final selection.
• Using one upstream pressure for everything. The static figure and the pressure still available at design flow are different numbers, and each feeds a different output.
• Assuming a discharging relief means a broken assembly, or assuming it never does. A sagging supply and a fouled check produce the same symptom. Measure the first check differential and test before deciding.
• Trying to calculate relief discharge from the total loss. The relief watches the differential across the first check only. That is a different quantity and the total cannot produce it.
• Using the pressure comparison to pick a cheaper assembly. The hazard assessment decides the type. Comparing a reduced pressure assembly against a double check is useful only where the assessment already permits either.
• Forgetting the expansion tank. A check function closes the system, and where a storage water heater is served downstream, thermal expansion control is required by code.
• Checking the pressure at the assembly and calling it done. What matters is the pressure that survives to the worst fixture after elevation, friction and every other device.
• Assuming an assembly can go in a pit. A reduced pressure assembly must not, because its relief has to discharge to atmosphere and must never be submerged.

Frequently Asked Questions