Duplex booster pumps in a mechanical room

Booster Pump Sizing for Ontario Buildings: Method and Code

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Booster pump sizing is the calculation of two values at a single duty point: the peak demand flow the building requires, and the pressure the pump must add to the incoming municipal supply so that the most remote fixture receives adequate residual pressure. The inputs are the fixture-unit load, the vertical height to the highest fixture, the friction losses in piping and valves, and the lowest suction pressure the municipal main will ever provide. The pump arrangement (number of pumps, speed control, redundancy) is then selected to follow the building’s load profile within the pressure limits of the Ontario Building Code.

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TL;DR

  • Size the pump on peak demand flow, taken from the fixture-unit load, and on total dynamic head: static height plus friction losses plus required residual pressure, minus the lowest suction pressure.
  • Use the minimum suction pressure the main will ever supply, not the typical pressure measured on one afternoon.
  • Provide at least two pumps so that design demand is met with the largest unit out of service, and use variable-speed control where demand varies through the day.
  • Keep static pressure at any fixture at or below 550 kPa under the Ontario Building Code, using pressure zoning or pressure-reducing valves where the pump pressure would exceed it.
  • A pump drawing from a public main needs backflow protection or an air gap upstream, and the design should be reviewed and stamped by a licensed engineer.

Table of Contents

What Booster Pump Sizing Must Establish

A booster system is justified when the municipal supply cannot deliver the required pressure at the top of the building during peak demand. The sizing exercise answers four questions, and each one corresponds to a measurable input.

  1. How much water is required at peak, in litres per second or US gallons per minute?
  2. What pressure must exist at the pump discharge to deliver the required residual pressure at the highest or most remote fixture?
  3. What is the lowest pressure the incoming service will supply to the pump suction?
  4. How should the capacity be divided among pumps, and how should they be controlled?

Manufacturer sizing guidance, such as the Armstrong Fluid Technology booster selection document, lists the same requirements: the total demand flow derived from fixture units, the required discharge pressure, the system losses, and the minimum incoming pressure. A calculation that omits any one of them produces a pump that either fails at peak or runs far from its best efficiency point.

Step 1: Establish the Design Flow Rate

The design flow is not the sum of every fixture’s rated flow, because fixtures are rarely used simultaneously. The standard method assigns each fixture a fixture-unit value, totals the units for the building or the zone, and converts the total to a probable peak flow with a demand curve. The Armstrong document describes this conversion through a demand-unit graph, with separate curves for systems dominated by flush tanks and those dominated by flush valves. The curve selection matters: a building with predominantly flush valves produces a considerably higher peak flow for the same fixture-unit total.

In Ontario the fixture loads and the pipe sizing follow Part 7 of the Ontario Building Code, and the engineer should confirm which fixture-unit table the design uses before reading the curve. Three practical points reduce error at this stage:

  • Include continuous loads separately, such as cooling tower make-up, irrigation, and process water, and add them to the probable peak flow rather than converting them to fixture units.
  • Separate fire-protection water from domestic demand. Sprinkler and standpipe supply is a different design with its own pump requirements, covered in our note on fire pump rooms in Ontario.
  • Check the building at partial occupancy. A tower that is 30 percent leased in its first years will not operate at design flow for some time, which affects the control strategy discussed below.

For municipal systems, the Ontario design guidelines for drinking water systems direct designers to analyze several flow conditions, including average day, maximum day, peak hour and minimum hour demand, so that the pump operates satisfactorily across the whole range. Although that document addresses municipal pumping facilities, the principle transfers to a building: the pump is selected for the peak but must also behave acceptably at the minimum.

Step 2: Calculate Total Dynamic Head

The required pressure boost is the total dynamic head expressed as a pressure. According to the Armstrong method, system pressure equals static height plus system losses plus residual pressure, minus suction pressure, with static height defined as the difference between the suction water level and the highest discharge point. The same formulation appears in engineering guidance on the International Plumbing Code, which also gives the conversion that 1 foot of head equals 0.433 psi. In metric units, 1 metre of water column is approximately 9.81 kPa.

Static height

Measure from the pump suction to the highest fixture or outlet served, not to the roof slab. For a building with a mechanical penthouse, the cooling make-up or the top-floor service sink often governs.

Friction losses

Calculate the loss through the riser, the horizontal distribution, the backflow preventer, the water meter, strainers, valves and any pressure-reducing stations along the critical path. The backflow assembly is frequently the largest single loss and is easily forgotten; consult the manufacturer’s published pressure-drop curve at the design flow. Our guides on backflow preventer types under Canadian codes and on backflow prevention requirements for commercial buildings describe which assemblies are applicable and where they are located.

Residual pressure

The residual pressure is the minimum pressure required at the most remote fixture while the system flows at peak. It is set by the fixture requirements, including flush valves and tempered or thermostatic mixing equipment, and not by a general rule of thumb. Flush-valve fixtures commonly demand a higher residual than tank-type fixtures, which is one reason the demand-curve selection and the residual pressure must be established together.

Suction pressure

The suction pressure is subtracted from the required discharge pressure, so its value changes the result directly. A widely cited manufacturer guide states that the lowest suction pressure that will ever supply the pump is the basis for sizing. Obtain a flow test or the municipal minimum pressure in writing, and consider seasonal and time-of-day variation, since a main that reads 350 kPa at 6 a.m. may fall considerably during a summer peak.

Worked Example: Ten-Storey Residential Building

The following figures are assumptions chosen to demonstrate the arithmetic and do not describe a specific project.

Item Value Basis
Height from pump to highest fixture 33 m 10 storeys with a basement pump room
Static head as pressure 324 kPa 33 m x 9.81 kPa/m
Friction losses at peak flow 70 kPa Riser, backflow assembly, meter, valves
Required residual pressure at the top fixture 140 kPa Roughly 20 psi at the most demanding fixture
Required discharge pressure 534 kPa 324 + 70 + 140
Lowest suction pressure from the main 200 kPa Written municipal minimum
Pressure the pump must add 334 kPa 534 – 200

The duty point is therefore the design flow read from the demand curve at the pump’s 334 kPa differential, or about 34 m of head. Two observations follow. First, a 20 kPa error in the assumed suction pressure changes the pump differential by about 6 percent, which is enough to move the duty point off the intended curve. Second, a discharge pressure of 534 kPa is high enough that the lower floors would exceed the code limit discussed below unless the system is zoned or the lower floors are served through pressure-reducing valves.

Code Limits That Constrain the Result

The sizing result must be tested against the Canadian and Ontario requirements that govern both pressure and cross-connection protection.

Maximum static pressure at fixtures

Article 7.6.3.3 of the Ontario Building Code requires a pressure-reducing valve where static pressure at a fixture may exceed 550 kPa, limiting it to 550 kPa at the fixture, as reproduced in a searchable copy of the Ontario Building Code. In tall buildings this is the reason booster systems are divided into pressure zones, each with its own pump set or reducing station. The designer should verify the current edition of the code and the applicable compendium with the authority having jurisdiction.

Discharge pressure range for pumping stations

For municipal booster pumping stations, the Ontario design guidelines recommend maintaining discharge pressures between 275 kPa (40 psi) and 700 kPa (100 psi) during peak and minimum demand. This is a utility-system guideline and not a building-code limit, but it indicates the range in which public water systems are expected to operate, and it is a useful reference when a building booster connects to a municipal main.

Backflow protection

The same guidelines restrict booster pumps that draw from a public main unless an air gap or backflow preventer is provided upstream, or the water authority owns and operates the pumps. The Ministry’s guide on backflow prevention programs adds that a hazard assessment should identify cross-connections, contamination risk, the probability of backflow and the appropriate preventer. In practice the backflow assembly upstream of the pump also protects the suction from sudden drops in main pressure, so its location and its pressure loss must be settled before the pump is selected.

Materials and approvals

The Ontario design guidelines for drinking-water systems state that specifications should identify equipment ratings, applicable ANSI, AWWA, NSF and CSA requirements, and testing procedures. For potable service the pump wetted parts, seals and piping must carry the appropriate NSF/ANSI certification. Halifax Water’s supplementary standard specifications call for stainless steel (Type 304 or 304L) or cement-lined ductile iron piping within booster stations, with flexible couplings for maintenance access, which is a practical reference for material selection even though that utility is in Nova Scotia.

Materials and approvals — overview diagram

Step 3: Select the Pump Arrangement

Once the duty point is known, the arrangement determines how well the system performs for the 95 percent of operating hours that are not at peak.

Number of pumps and redundancy

The Ontario guidelines and Halifax Water’s 2023 design specifications both describe the need for at least two pumps, so that firm capacity, meaning the ability to meet design demand, is maintained with the largest single pump out of service. For a building the same logic applies where an outage would affect habitability, such as a residential tower, a hospital or a care facility. Armstrong’s guidance favours several smaller pumps over one or two large ones where daily demand varies widely, because the system can then match output to demand more closely.

Variable-speed control

Variable-speed drives allow the system to hold a constant discharge or remote-sensor pressure as demand falls, which reduces energy use and pressure fluctuation, according to the Halifax Water specification. The drive selection, harmonics and starting behaviour should be coordinated with the electrical design; our comparison of VFD and soft starter applications in Canada sets out the start-frequency limits that affect this choice. Where the booster is on emergency power, the generator loading must also be confirmed, as described in our summary of emergency generator rules in Ontario.

Position on the pump curve

The Ontario guidelines call for pumps to be selected to operate near their maximum efficiency points, and for the available net positive suction head (NPSH) to exceed the required NPSH to avoid cavitation. With a low suction pressure, which is the very condition that justifies a booster, the available NPSH should be checked at the lowest suction pressure and at the highest expected flow, not at the average.

Consequences of Oversizing and Undersizing

An undersized pump fails visibly: top-floor pressure collapses at peak, flush valves do not reset, and the pumps run continuously. An oversized pump fails quietly. It cycles on and off at low flow, runs away from its best efficiency point, wastes energy and increases wear on seals and bearings. Halifax Water’s specification cautions that oversizing initial equipment has consequences, particularly for metering, and it asks that pumps meet the expected flow range near their optimum efficiency.

The most common cause of oversizing is stacked safety factors: a conservative fixture count, a conservative demand curve, an added margin on head and a larger motor on top. Document each assumption once, apply a single justified margin, and let a properly staged, variable-speed arrangement provide the flexibility that excess capacity would otherwise be asked to supply.

Consequences of Oversizing and Undersizing — overview diagram

Information to Gather Before Engaging an Engineer

The following items shorten the design period and prevent a resizing after the permit submission:

  • Architectural sections showing the pump room elevation and the highest fixture.
  • A fixture schedule with fixture types and flush-valve or flush-tank designation.
  • The municipal flow-test or minimum pressure letter, and the service size.
  • The location of the water meter and the proposed backflow assembly.
  • The available electrical capacity and the emergency power arrangement.
  • Any future expansion that the pump room should accommodate.

Our Perspective

In our work on Ontario residential and commercial buildings, the booster calculation is rarely the difficult part. The difficulty lies in the inputs that arrive late or unverified: a suction pressure assumed from a single reading, a backflow assembly added after the pump was chosen, or a fixture schedule that changes after the demand was calculated. We treat the plumbing, electrical and fire-protection designs as one coordinated package, because the pump’s motor, the drive, the emergency power and the backflow loss all affect one another.

We also advise owners to resist the instinct to buy margin through pump size. A two- or three-pump variable-speed arrangement, sized on a documented duty point and a verified minimum suction pressure, is generally more economical to operate and easier to commission than a single oversized unit. The permit submission is then supported by a calculation that the reviewer can follow line by line.

— Sanjay

Engineering Support for Booster Systems

FutureGen Consulting prepares plumbing system design for residential and commercial projects in Ontario, including domestic water booster sizing, backflow coordination and permit-ready drawings. Where the pump affects the electrical or fire-protection scope, the same team coordinates power distribution and fire protection design, so the drawings reach the building department as a single consistent set. Developers, contractors, architects and property managers can review our plumbing system design services and request a scope for their project.

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Drawings submitted for permit generally require a professional engineer’s seal, and our guide to engineer stamp requirements in Ontario explains when that applies. For related plumbing questions, see also our overview of plumbing venting requirements under O. Reg. 163/24.

FAQ

What is the minimum information needed to size a booster pump?

The design requires the peak demand flow from the fixture-unit load, the elevation of the highest fixture above the pump, the friction losses along the critical path, the residual pressure required at that fixture, and the lowest suction pressure from the municipal main. With those five values, the required pressure boost and the duty point can be calculated.

Which suction pressure should be used?

Use the lowest pressure the supply will ever deliver at the pump, taken from a municipal flow test or written minimum, and not a single spot reading. Sizing on a higher figure produces a pump that cannot reach the top floors when the main pressure drops during peak consumption.

How many pumps does a booster system need?

A minimum of two pumps is the usual arrangement, so that design demand can still be met with the largest pump out of service. Buildings with widely varying daily demand often use three smaller pumps with variable-speed control, which lets the system follow the load more closely and use less energy.

Is there a maximum pressure the booster can produce in Ontario?

The Ontario Building Code requires a pressure-reducing valve where static pressure at a fixture may exceed 550 kPa, limiting it to that value. Tall buildings therefore use pressure zones or reducing stations so the pump can deliver the pressure the upper floors need without overpressuring the lower ones.

Does a booster pump require backflow prevention?

When the pump draws from a public main, Ontario guidance restricts the installation unless an air gap or backflow preventer is provided upstream, or the water authority owns and operates the pumps. The appropriate assembly depends on the hazard assessment and should be confirmed with the local authority and the design engineer.

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