One box on the application, and the fee schedule behind it
The service application is a short document and most of it is addresses. Then there is a box for meter size, a box for service line size and material, a box asking how the premises will be classified for cross-connection control, and a signature. Everything the design team argued about for a month arrives at the utility as those three entries.
Write a size larger than the building needs and you have bought two things you cannot return. The capacity charge — the connection fee, impact fee, tap fee, whatever the utility calls it — is scaled by meter size, and the ratio between a domestic-sized meter and the one two sizes up is not gentle. The standing monthly charge is scaled the same way, so the mistake keeps invoicing itself. Downsizing later, where a utility permits it at all, usually means a new tap and a second fee.
Write a size smaller than the building needs and the penalty is physical rather than financial: pressure loss through the meter climbs steeply once flow passes the top of its range, and the complaint arrives from the highest fixture in the building at the busiest hour. Between the two errors the trade overwhelmingly commits the first, because oversizing feels like caution and its cost is invisible on the day the form is signed. AWWA Manual M22 exists to talk people out of it.
Counting fixture units the way the utility's worksheet counts them
Water supply fixture units are a loading index and nothing else. A WSFU value carries a fixture's flow rate, how long it runs and how often somebody uses it, folded into one number so that a probability curve can be applied to the total. Nobody has ever measured a fixture unit at a tap. Adding the manufacturers' flow ratings together instead produces a figure two or three times too large, and that figure is the single most common reason a meter comes out oversized.
Take the values from the fixture unit table in the code the utility enforces, and take them from the right column. Codes separate private use from public use, and the same lavatory scores differently in a hotel bedroom than in a lobby washroom. They also separate a flushometer water closet from a tank-type one by a wide margin, because a flush valve draws its whole flush in a few seconds while a cistern refills slowly, and a building full of flushometers loads a service far harder than its fixture count suggests. Hot, cold and total are three columns, not three quantities to be summed.
Some loads have no diversity to average out and must not be converted into fixture units at all. A hose bibb somebody leaves running, an irrigation zone on a controller, cooling tower makeup, boiler makeup, a filling loop on a process tank — each of these can run flat out for hours and none of them cares what the rest of the building is doing. The method is to diversify the fixture load, then add the continuous loads on top at their full rated flow.
One more habit worth keeping: total the schedule twice, once as designed and once as the building will plausibly be fitted out. A shell-and-core office with a tenant fit-out clause is not a fixed fixture count, and the meter you install today has to serve whatever the lease permits later. Where the ceiling is unknown, say so on the application rather than guessing high in silence.
Peak demand comes off a probability curve applied to the fixture unit total, not off a sum of flow ratings, so total the schedule from the code's table and let the curve do the diversifying.
The sum of water supply fixture unit values for every fixture served by this meter.
A calibration constant that scales the square-root approximation to your building's fixture mix.
Estimated peak demand
22.6 GPM
A meter is sized on peak demand and on the pressure it costs you, not on the size of the pipe it sits in. Fitting a meter one size up because the main is large is a common and expensive error — an oversized meter under-registers low flows.
They open the calculator with your figures already in it
Water Meter Sizing Calculator (Fixture Units): 22.63 GPM — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 22.6 GPM — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Does not compute the meter's pressure loss at the design flow, which must be subtracted from the available pressure before sizing the distribution.
- Excludes fire demand entirely. Where a supply serves both domestic use and a fire system, the sizing basis and often the meter type are different.
- Meter accuracy classes have a minimum registrable flow. Continuous small leaks below that threshold pass unmetered, which matters for leak detection rather than for billing.
Where the square root stops describing the building
The demand curve everyone still uses traces back to Roy Hunter's work at the National Bureau of Standards, published as Report BMS65 in 1940. It assumed the fixtures of that decade: water closets flushing several gallons, showers pulling far more than modern heads, and taps without aerators. Every one of those assumptions has since been legislated away, and the probability that a given fixture is in use changed along with the volume it draws.
The consequence is systematic and it runs one way. Applied to a modern multifamily or hotel building, the classic curve over-predicts peak demand, sometimes badly, which is exactly how a building ends up with an oversized meter sitting in an oversized service and a bill to match. The metering studies behind the Water Research Foundation's Residential End Uses of Water were the evidence that forced the issue, and the IAPMO Water Demand Calculator published as an appendix to the Uniform Plumbing Code is the current answer for residential and multifamily work. Whether you may use it is a question for the authority enforcing the code, not a preference — so ask before the drawings are issued rather than after the application is refused.
The service line is not the meter, and the meter is not the service line
These two numbers get married on the form and they are sized by different criteria. The meter is sized on the demand profile and on the head it costs at that demand. The service line is sized on flow, on the head available to spend on friction, and on a velocity ceiling that exists to keep the pipe from eroding itself. It is entirely normal and entirely correct for a one-inch meter to sit inside a larger service.
Fitting the meter up to match the pipe is the reflex to resist. Every meter has a lower limit below which it stops registering accurately, and a domestic building spends most of its hours down at that end — a single tap, a cistern refilling, a dishwasher taking water. An oversized mechanical meter under-registers those flows for its whole service life, which is a revenue problem for the utility and a leak-detection problem for the building owner, since a slow loss that never shows on a bill never gets investigated.
The velocity ceiling deserves its own thought on the service. Copper in continuous service has an erosion-corrosion limit that the Copper Development Association's Copper Tube Handbook sets out, and it is lower on hot lines than cold; plastic and ductile iron have their own limits for their own reasons. A service run fast is also a service that transmits every surge downstream with more energy behind it. Size the bore, then check what the water is doing inside it.
The service carries the diversified load at a velocity the pipe material will tolerate for decades, which is a different question from what the meter should be — settle the bore on its own terms before matching anything.
The sum of water supply fixture unit values for every fixture served by this pipe segment.
A calibration constant that scales the square-root approximation to your building's fixture mix.
The maximum velocity allowed in the pipe to limit noise and water hammer risk.
The pressure available where the supply enters the building.
The length of pipe from the point of supply to the furthest fixture, following the run.
How much extra length is added to represent the elbows, tees and valves in the run.
The height of the highest fixture above the point of supply.
The pressure the metering assembly loses at your design flow.
The flow pressure the furthest fixture needs at its inlet to work properly.
Which Hazen-Williams roughness coefficient the friction calculation uses.
Minimum pipe diameter
0.862 in
Velocity governs this run. The diversified demand would fit through a smaller bore on pressure alone, but sustained velocity above the ceiling erodes the pipe and is audible through the building, so the velocity limit is the binding one and the pressure budget has room to spare.
- Estimated peak demand
- 14.31 GPM
- Diameter set by the velocity ceiling
- 0.86 in
- Diameter set by the pressure budget
- 0.77 in
- Pressure lost to static lift
- 58,286.8 Pa
- Pressure lost to friction at this diameter
- 122,120.36 Pa
- Pressure left at the furthest fixture
- 198,803.83 Pa
- Equivalent length used for friction
- 147 ft
They open the calculator with your figures already in it
Domestic Water Pipe Sizing by Fixture Unit Calculator: 0.8616 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.862 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Fixture-unit methods work because fixtures are used intermittently and rarely together. That diversity is the whole basis of the sizing, and it is why the pipe is far smaller than the sum of the fixture flows would suggest.
- Hunter's curve is old, and its age biases this in one direction. It was derived in the 1940s from fixtures using several times the water modern ones do, and it is well documented as oversizing systems built with low-flow fittings — the demand factor is the handle for that, and lowering it is a judgement with consequences rather than a correction.
- Excludes continuous-demand loads such as irrigation and hose bibs, which do not benefit from diversity and are added at their full flow.
- The friction calculation assumes one diameter for the whole run. A real system steps down as branches leave it, so the true loss is somewhere between this figure and the loss of a system sized entirely at the smallest branch — this is the screening answer, and a segment-by-segment calculation is the design one.
- Fitting losses are an allowance, not a count. Each elbow, tee and valve has its own equivalent length and a push-fit insert reduces the bore as well, so the percentage is a stand-in for a takeoff nobody has done. On a run with many tight bends it will understate the loss.
- The meter figure is yours to supply and this page cannot check it. Meter loss rises steeply as flow approaches the meter's rating and a backflow preventer or pressure-reducing valve often costs more than the meter itself; entering a nominal figure where the real assembly loses far more is the quiet way this calculation goes wrong.
- Velocity limits govern independently of flow: sustained velocities above roughly 2.4 m/s (8 ft/s) cause erosion and noise regardless of what the fixture units allow.
- The result is an internal diameter, not a pipe size. Nominal designations are not bores — copper, PEX and CPVC of the same nominal size have materially different internal diameters, and a push-fit system loses more at every joint — so take the next size up whose actual bore meets this figure in the material you are using.
The pressure budget, spent in the order the water meets it
Ask the utility for its guaranteed minimum static pressure at the main, in writing, and ask what hour of what season that minimum applies to. A pressure quoted from a hydrant test on a Tuesday morning is a measurement, not a guarantee, and a design built on it has no margin when the neighbourhood's demand peaks. The low end of the published range is the number the design has to survive.
From that figure, subtract in turn: the static lift to the highest fixture, at roughly 0.433 psi for every foot climbed or 9.81 kPa for every metre; the meter's head loss at the design flow, read off the manufacturer's curve for that model rather than assumed; the strainer ahead of the assembly; the backflow assembly's own loss at the same flow; friction through the service and the distribution including every fitting; the differential a pressure reducing valve needs before it can regulate at all; and finally the residual flow pressure the worst fixture requires to work. What is left over is the margin, and on a tight site it is frequently negative.
The backflow assembly is usually the largest single deduction and the one discovered last. A reduced-pressure assembly does not lose pressure by accident — it maintains a deliberate differential across its first check, because that differential is the mechanism by which the device knows it is working. ASSE 1013 governs how the assembly is proved; the manufacturer publishes the curve of loss against flow for each model and each size, and those curves are what belongs in the calculation.
When the budget will not close there are four honest moves and one dishonest one. Increase the service bore to buy back friction. Increase the meter size and pay the fee. Fit a booster set and accept the plant, the power and the maintenance. Or, if the hazard classification genuinely permits a lower-loss assembly, change device class with the approving authority's agreement. The dishonest move is to design against the utility's best-case pressure and let the top floor discover the difference.
Choosing a meter you will still be honest about in twenty years
Meter classes differ in what part of the flow range they measure well, and the choice follows the shape of the building's demand rather than its size. A residential block spends its life at low flow with brief peaks; an irrigation service or a cooling tower makeup line runs steadily near the top of its range. Those two buildings should not receive the same instrument even where the peak demand figure is identical.
The metrology is worth understanding because it is what the utility is buying. The ISO 4064 series describes a meter by four flows — the minimum flow Q1 at which it must still be accurate, a transitional flow Q2, the permanent flow Q3 it can run at continuously, and a short-duration overload Q4 — with the ratio Q3 to Q1 quoted as an R value. A high R number is a wide honest range, and it is the specification that decides whether a small continuous leak registers or passes free. The older Class A to D designations describe the same idea more coarsely and still appear on older stock.
Whatever class you choose, size against the meter's safe continuous operating capacity rather than its maximum rating. The maximum is a short-duration figure; a meter parked against it registers well enough and wears out early, and the head loss up there is the number that produces the complaint from the top floor.
| Class and standard | Registers well | Wrong choice where |
|---|---|---|
| Positive displacement, nutating disc or oscillating piston — AWWA C700 | Low and moderate flows, which is most of a domestic building's year | Sustained flow near the top of the range; head loss climbs steeply and wear accelerates |
| Multi-jet — AWWA C708 | Small services with good low-flow accuracy | Dirty or debris-laden supplies, which foul the jets |
| Turbine — AWWA C701 | High, steady flow with low head loss | Buildings that idle at low flow, where it under-registers or stops registering |
| Compound — AWWA C702 | Wide range, by pairing a low-flow register with a high-flow element | Where the crossover region sits at the building's normal demand, and where maintenance access is poor |
| Fire service — AWWA C703 | A combined fire and domestic supply, sized on the fire demand | Sizing domestic demand alone; the fire basis governs and the domestic side is metered separately |
| Electromagnetic or ultrasonic, solid state | Very wide turndown with no moving parts and no wear | Sites with no reliable power or no data path back to the utility |
Hazard picks the assembly; nobody's preference does
Two separate protections are usually required and they are frequently confused. Containment sits at the service entrance and protects the utility's main from the whole premises. Isolation sits at the individual hazard inside the building — the boiler fill, the irrigation zone, the darkroom, the trap primer — and protects the building's own occupants. Fitting a reduced-pressure assembly at the meter does not discharge the isolation duty, and a building full of correct isolation devices does not always satisfy a utility that requires containment.
The device class follows the degree of hazard, which is a classification made by the authority enforcing cross-connection control, not by the installer. In North America the split is between health and non-health hazards, with the approved-model lists maintained under the Foundation for Cross-Connection Control and Hydraulic Research's Manual of Cross-Connection Control widely adopted; in the United Kingdom the same logic runs through fluid categories one to five under the Water Supply (Water Fittings) Regulations 1999 and BS EN 1717, where the reduced-pressure device is the Family B, Type A unit — the type BA — to BS EN 12729. Get the classification confirmed in writing before ordering, because the model has to be on the enforcing authority's list and lists differ between neighbouring jurisdictions.
One practical consequence for the form: the assembly is often specified a size larger than the meter, because its head loss at the meter's nominal size would eat the pressure budget. That is a legitimate choice and it needs checking rather than assuming — the flow through the assembly's connection at peak demand has to sit inside a sensible velocity, and an assembly two sizes up is a large, expensive, slow-registering way to solve a problem that a bigger service would have solved better.
| Device | Protects against | What the installation demands |
|---|---|---|
| Dual check, ASSE 1024 | Low-hazard backflow on a residential service | Not testable in service; replaced rather than repaired |
| Double check valve assembly, ASSE 1015 / AWWA C510 | Backpressure and backsiphonage, low hazard only | Testable, and may sit in a vault since it has no relief discharge |
| Pressure vacuum breaker, ASSE 1020 | Backsiphonage only — not backpressure | Must stand above every downstream outlet it protects |
| Reduced pressure principle assembly, ASSE 1013 / AWWA C511 / BS EN 12729 Type BA | Backpressure and backsiphonage at high hazard | Above grade, never in a pit, with a relief port discharging through an air gap into a sized receptor |
| Air gap, ASME A112.1.2 | Everything, absolutely, by breaking the pressure path | A break tank and a booster set, since the supply pressure ends at the gap |
An assembly specified a size up from the meter changes the flow through its connection, and velocity through the body is the check that says whether the choice was reasonable or just cautious.
The design flow rate through the backflow preventer.
The internal diameter of the pipe at the backflow preventer connection.
Flow velocity
484.1 ft/min
They open the calculator with your figures already in it
Backflow Preventer Discharge Velocity Calculator: 484 ft/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 484.1 ft/min — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The relief port is the flow that floods plant rooms, and it is not this one. A reduced-pressure assembly is built to dump water to atmosphere when a check fouls or downstream pressure rises, and the rate at full relief can be a large fraction of line flow — far more than a routine floor drain and air-gap fitting will swallow. That receptor is sized from the manufacturer's published relief discharge for the specific model and size, and it is what decides whether a failure is a puddle or a ceiling coming down on the floor below.
- The velocity limit that bites first usually belongs to the pipe, not the assembly. Copper thins by erosion-corrosion above roughly 8 ft/s (2.4 m/s) cold and about half that on hot recirculating lines, and it thins at the elbow and tee immediately downstream where the flow is still disturbed by the device — with plumbing codes capping velocity for noise on top of that. A figure comfortably inside the assembly's rating can still be outside what the pipe carrying it will survive.
The relief valve is a rated flow, not a fault
The reduced pressure principle works by holding the zone between two check valves at a pressure meaningfully below the inlet. A differential relief valve watches that gap, and the moment it narrows past the setting, the valve opens and dumps the zone to atmosphere. That is the assembly doing its job, not failing.
Four ordinary events open it. Debris on the seat of the first check lets inlet pressure through into the zone. A drop in supply pressure — a hydrant flowing, a main under repair, a pump starting in the street — collapses the differential from the inlet side. Backpressure from downstream, most often thermal expansion off a water heater in a system that has just become closed, pushes on the second check. And a supply that fluctuates rapidly makes the valve spit repeatedly without any single fault being present, which is the intermittent puddle that gets reported as a leaking assembly.
What comes out is not a dribble. A relief valve opening on a large assembly discharges at a rate measured in litres per second, immediately, and it keeps discharging until the fault clears — which on a fouled check means until someone attends. Sizing the drainage on what a dripping valve would do is the mistake that floods plant rooms, and it is worth stating plainly on the application that the receptor was sized on the full relief rate.
Take that rate from the manufacturer's published data for the exact model and size: Watts publishes discharge figures for its Series 909, Zurn Wilkins for the 375 and 975XL families, Febco for the 825Y and 860. An orifice calculation is a sanity check on the order of magnitude and a way to test what happens when the driving head is higher than the datasheet assumed. It is not a substitute for the rated figure, and no approving authority will accept it as one.
Orifice area, discharge coefficient and driving head give the order of magnitude a relief port can produce, which is the sanity check to run against the manufacturer's rated discharge before the receptor is sized on it.
The open cross-sectional area of the relief valve's discharge orifice.
An empirical factor accounting for orifice flow contraction and friction losses.
The pressure head driving flow through the relief valve orifice, expressed as a water column height.
Relief valve discharge flow
36.8 gal/min
Confirm the actual relief valve orifice area and discharge coefficient from the specific backflow preventer manufacturer's data — this is a general orifice-flow estimate, not a substitute for the manufacturer's rated relief capacity.
- Discharge velocity
- 913.37 ft/min
They open the calculator with your figures already in it
Backflow Preventer Relief Valve Discharge Calculator: 36.77 gal/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 36.8 gal/min — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Takes the head as given and does not derive it. During a genuine relief event the driving head is the supply-side pressure at that instant, not a static column — a 70 psi (483 kPa) main is about 50 m (164 ft) of water, not the 3 m (10 ft) the field opens on — and because flow goes with the square root of head, an assumed head that low understates the discharge by roughly a factor of four.
- Says nothing about how the discharge is piped. Relief from a reduced-pressure assembly must reach the drain through an air gap and never a direct connection — hard-piping it into the waste system rebuilds the exact cross-connection the assembly was installed to prevent, and it also means a relief event pushes into the drain instead of showing itself on the floor where someone notices it.
The air gap, and the drain that has to swallow all of it
The relief port must discharge to atmosphere across an air gap. Hard-piping it into a drain converts the port itself into a cross-connection — the assembly's own defeat, installed by the person who installed the assembly — and it is the single most common reason an installation fails its first inspection. ASME A112.1.2 governs the gap and ASME A112.1.3 the manufactured fittings; the adopted code expresses the dimension as a multiple of the effective opening with an absolute minimum, and the multiple is not the same everywhere, so read the code that will be enforced rather than the one you learned on.
Use the air gap fitting the assembly's maker supplies for that model. It bolts to the relief body, holds the gap at the dimension the certification assumed, and directs a violent discharge downward instead of across the room. A funnel improvised from a fitting off the van loses the gap dimension, loses the mounting, and usually loses the argument with the inspector.
The receptor beneath it is sized on the relief flow, not on drainage fixture units. A DFU allowance describes fixtures that discharge briefly and rarely, with the probability of coincidence already built in; a relief event is a sustained full-bore flow from a single opening with no diversity in it at all. The floor drain and its branch have to carry that flow at the fall the run can actually achieve, and in a retrofit basement — where the sewer invert is already close to the slab — fall is exactly what has run out. Work the drop over the developed length before committing to the assembly's position, because moving the assembly is cheap on paper and expensive after the pipework is up.
Then look at the room rather than the pipe. The assembly cannot go in a pit or a vault where a flood could submerge the relief port. The floor should fall toward the receptor, not away from it toward the switchgear. Nothing electrical belongs below the assembly, and a float alarm wired to the building management system costs very little against a night of unattended discharge. Where the only receptor available is undersized and cannot be enlarged, that is a design finding to raise before the application is submitted, not a defect to discover during commissioning.
What sits between the utility main and the first tee
- Building distribution downstream — sized on the diversified fixture load and on whatever pressure survives the meter and the assembly ahead of it Domestic Water Pipe Sizing by Fixture Unit Calculator
- Reduced pressure assembly — two checks, four test cocks and two shutoffs, holding a deliberate differential that is also its largest head loss Backflow Preventer Discharge Velocity Calculator
- Service line, meter and register — sized on peak demand and on the head it costs there, which is a separate question from the bore it sits in Water Meter Sizing Calculator (Fixture Units)
- Relief port and air gap funnel — the discharge is atmospheric by design, and hard-piping this joint turns the assembly's own defence into a cross-connection Backflow Preventer Relief Valve Discharge Calculator
- Floor receptor — sized on the assembly's full rated relief flow rather than on any drainage fixture unit allowance
- Indirect waste branch — carries the relief discharge to the sewer at whatever fall the slab and the existing invert still allow Drain Pipe Slope Calculator
Fall is what runs out first in a plant room retrofit, so work the drop over the developed length from the receptor to the connection before the assembly's position is fixed.
The horizontal length of the drain pipe run.
Larger drain pipes are allowed a gentler minimum slope.
Minimum required drop
4.875 in of drop (minimum)
This is the code minimum slope — always check your specific local plumbing code, which may set stricter requirements in some jurisdictions.
- Pipe run length
- 19.5 linear ft
They open the calculator with your figures already in it
Drain Pipe Slope Calculator: 4.88 in of drop (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 4.875 in of drop (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The size question is answered with two bands, so a 4 in branch and a 300 mm building sewer both land in the same band and get 1/8 in per ft. Real code tables are not that coarse: several add a gentler band for the largest sizes, and some jurisdictions require 1/4 in per ft at every size unless the reduction is specifically approved, so both the band boundary and the permitted figure are jurisdictional.
- Nothing here checks the drop you actually have against a maximum, and nothing checks that the fall will physically fit: a 20 m run of large pipe needs about 208 mm of drop at this slope, and that depth has to be available between the fixture connection and the invert of the sewer or septic tank at the far end.
- Slope is not capacity. Pipe size is set by the drainage fixture unit load on the run and the maximum permitted flow depth, and this page never asks how many fixtures discharge into it — the size band is something you tell the calculator, not something it works out for you.
- The length is treated as one straight horizontal run. No allowance is made for bends, offsets or developed length through fittings, and the page does not size or locate the vents, traps and cleanouts the drain also needs; a run at exactly the right slope will still siphon its traps if the venting is wrong.
- The figure is a total end-to-end drop and assumes the fall is delivered evenly along the run. It says nothing about hanger spacing or pipe bedding, and a single sag deep enough to hold standing water will collect solids even though the two ends of the pipe are the compliant distance apart.
What the assembly does to everything behind it
The moment a check valve or a backflow assembly goes into the service, the building becomes a closed system, and several things that used to be self-correcting stop being so. Heated water expands and now has nowhere to expand into; pressure climbs through the day until the water heater's temperature and pressure relief lifts, which the occupant reports as a leaking water heater. The adopted code requires a means of controlling thermal expansion on a closed system, and the expansion vessel is sized on the stored volume, the temperature rise and the relationship between the static pressure and the relief setting.
Surge behaves worse too. In an open system a pressure wave from a fast-closing solenoid has somewhere to go; behind a closed check it reflects. A building that was merely noisy before the assembly was fitted can become loud afterwards, and the arrestor that was never needed suddenly is. This shows up on retrofits far more than on new work, because on new work nobody remembers what it sounded like before.
Where a pressure reducing valve is also in the train, be deliberate about the order and about what each device is responding to. A PRV downstream of the assembly sees an inlet pressure already reduced by the assembly's loss, and the valve needs a working differential before it can regulate at all. A PRV that creeps upward when the building is idle raises backpressure on the assembly's second check, which is one of the standard reasons a relief valve weeps overnight and is dry by the time anyone looks at it.
A fire supply is a different application
Sprinkler demand is not a fixture unit load and must never be added to one. NFPA 13 sets the flow and pressure a sprinkler design requires, NFPA 24 governs the private service main that delivers it, and the backflow assembly on the fire side enters the sprinkler hydraulic calculation with its own head loss curve — a curve the fire engineer needs before the design is complete, not after the assembly is on site.
On the metering side a combined supply usually calls for a fire service meter or a detector check arrangement rather than a domestic instrument, and many utilities require a wholly separate connection instead. The approval path then runs through the fire authority as well as the water utility, and the two do not always ask for the same thing. Establish which application you are actually filling in before totalling any fixture units.
Filling the form in, and what to keep afterwards
Everything above resolves into a handful of entries and a set of attachments. Utilities vary in how much working they want to see, but none of them object to receiving it, and an application that shows the basis of the meter size is approved faster than one that shows only a number.
Keep the working. The assembly will be tested annually by a certified tester for the rest of its life, and the first tester to attend will want to know the model, the size, the rated relief discharge and what the receptor was sized on. So will whoever eventually replaces it. A photograph of the installed train with the air gap visible, filed with the pressure figures and the fixture schedule, is a ten-minute job that answers questions for two decades.
- Get the utility's guaranteed minimum static pressure in writing, and ask which hour and season it applies to.
- Total the fixture schedule in WSFU from the enforced code's table, keeping private and public columns apart.
- List every continuous-demand outlet separately and add it at full flow after the diversified figure.
- Take the peak demand and check it against the candidate meter's safe continuous capacity, not its maximum rating.
- Deduct the meter's and the assembly's published head losses at that flow, along with static lift and friction, before sizing anything downstream.
- Confirm the hazard classification with the authority that enforces it, and pick a model from that authority's approved list.
- Take the manufacturer's rated relief discharge for the exact model and size the receptor and its branch on that flow.
- Submit the meter size, the service size and material, the assembly make and model, and the receptor arrangement as one package.
What has to be settled before the application is signed
Six figures decide this job, and none of them can be filled in from the drawing alone. Gather them first; each one has an owner outside your own office who will take a week to answer.
- Fixture unit total, by column and by use — From the table in the code the utility enforces, with private and public use kept apart and flushometer fixtures scored separately.
- Continuous-demand outlets, listed and rated — Hose bibbs, irrigation, tower and boiler makeup, filling loops — added at full flow after the diversified figure, never converted to fixture units.
- Guaranteed minimum static at the main — In writing from the utility, with the hour and season it applies to; a hydrant test result is a measurement, not a guarantee.
- Static lift to the highest fixture — Measured from the meter to the outlet, and deducted before any friction is considered.
- Head loss curves for the meter and the assembly — Both read at the design flow from the manufacturer's published data for that exact model and size.
- Rated relief discharge, and the fall available below it — The assembly's published relief flow, with the receptor and its branch worked against the drop the existing invert still allows.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.