Sizing a Water Feature's Pump: Turnover, Nozzle Pressure and Somewhere for the Relief to Go

Six quantities under one cobble deck

What the client sees when the job is finished is stone. What gets bought is a stack of separate items, each measured a different way, and each one wrong on its own terms if it is scaled off the bowl instead of off the chamber. The reservoir is either poured in situ or dropped in as a proprietary basin, and above it sits a load-bearing grate on a frame that has to carry whatever crosses the courtyard — including a scissor lift on the day the glazing gets cleaned.

The number every later figure depends on is the chamber's internal wetted volume, and it is not the chamber's nominal capacity. Support crates, ballast blocks, the pump body, the strainer housing and the cobble that inevitably works its way through the grate all displace water. So does the sump the strainer sits in, which has to stay flooded. Take the volume from the inside faces at the level the water will actually sit at when the display is running, and write it on the drawing, because it is about to be divided by a period and multiplied through the rest of the job.

Turnover is the housekeeping number, not the display number

Nobody swims in a fountain, so turnover here is not the regulated quantity it becomes on a public pool. What it buys is prosaic: an evenly mixed reservoir, a strainer that sees the whole body of water instead of one lane of it, and enough movement that debris reaches the sump rather than settling in the corners of the chamber. Pick a period, divide the wetted volume by it, and the recirculation duty falls out.

The period is a judgement, and the two ends of the range fail differently. A short period on a small chamber pushes the pump into a duty the reservoir cannot feed without drawing air off the surface, and the running cost is continuous because a fountain runs whenever the building is occupied. A long period in a sheltered south-facing courtyard produces exactly what a sheltered south-facing courtyard is good at: warm, still, sunlit water, and a green film on the cobble within a fortnight of handover.

There is a health dimension to the same decision that a landscape contract rarely mentions and a facilities manager will eventually raise. A fountain producing a plume produces aerosol, and a decorative fountain is named as a device to be assessed in ASHRAE Standard 188 on legionellosis risk management for building water systems, with ASHRAE Guideline 12 covering the same ground in more practical terms; in the United Kingdom the duty runs through the HSE Approved Code of Practice L8 and the HSG274 guidance beneath it. Circulation is not a control measure on its own — cleaning frequency, the treatment regime and the distance to openable windows and air intakes are — but a reservoir that stagnates is the condition every one of those documents is written about, and the turnover period is where that starts.

Then read the result for what it is. Volume divided by a period is an average recirculation rate. It is not the instantaneous flow the display asks for, and on almost any fountain with a jet in it the display figure is the larger of the two. The pump is bought against the larger one, and the turnover figure survives as the check that says whether the chamber is being properly worked at the flows the display leaves it running at.

The nozzle publishes pressure; the pump publishes head

A display nozzle is specified as a pair of numbers for every plume height it can produce: a flow, and a pressure measured at the nozzle inlet. Those pairs are not transferable between nozzle families. An aerated or foam jet entrains air and swallows several times the flow of a smooth-bore jet standing at the same height, because most of what you can see is not water. A cascade or weir is specified per metre of lip rather than per jet, so its flow scales with the length of the edge and a long lip is the thirstiest thing in the courtyard — and past a couple of metres the awkward part stops being the pump and becomes holding that edge level enough for the sheet to break evenly along all of it. Take the pair off the manufacturer's table for the exact model and orifice, at the height the client signed off, and treat any generic figure as a placeholder to be replaced.

The two datasheets in front of you will not be in the same units, and that is where the arithmetic quietly goes wrong. Nozzle tables are published in bar or in psi at the inlet. Pump curves are published in metres or feet of head. A foot of water column weighs 62.428 pounds on every square foot beneath it, which spread over 144 square inches is 0.4335 psi, so a psi is 2.3067 feet of head at that reference temperature, and a bar is very close to 10.2 metres. Convert one side before either is read off a chart, and record which of the two figures the pump was selected against, because the next person to look at this will assume the units they use themselves.

Wind decides whether any of it survives contact with the courtyard. An enclosed yard between tall elevations does not produce still air; it produces a downdraught along one wall and a channel along another, and a plume that behaves on a calm commissioning day drifts onto the paving in October. The catch radius the cobble deck has to cover is conventionally set well outside the plume height as a rule of thumb rather than as a code requirement, so take the spray envelope from the nozzle manufacturer's own data, and specify a wind switch on the controller where the plume is tall enough for drift to reach a doorway or a car.

Building the duty head out of the run you actually have

Total head for this pump is a short list, and each item on it has to come off the installation rather than off a habit. Static lift is measured from the lowest operating water level in the chamber to the nozzle exit, not from the still level, because the reservoir draws down the moment the display starts and water is held in the air, on the bowl and in the delivery pipe. Then friction through the delivery run, the equivalent length of every bend and tee in it, the strainer at its dirty condition rather than clean, any isolating or regulating valve, any ultraviolet or filtration unit in the circuit, and finally the pressure the nozzle itself demands. That last item is a demand, not a loss, and it is the one people leave out because it does not look like resistance.

The delivery run on a courtyard job is short and unusually convoluted, which is a bad combination. It leaves the chamber, turns to clear the grate frame, runs under the paving to the plinth, and then goes vertically up through a stone column with two more turns in it to reach the nozzle. That is a handful of metres of pipe carrying eight or ten fittings, and on runs of that shape the fittings are frequently worth more head than the pipe. Convert them to equivalent lengths, add them to the measured run, and only then work the friction — a bore chosen to fit the plinth's core hole rather than the flow is the standard way this ends badly.

With the duty flow and the duty head settled, read the curve rather than the headline. A pump delivers where its curve meets the system, and the useful question is whereabouts on its own curve that lands: ANSI/HI 9.6.3 sets out preferred and allowable operating regions for rotodynamic pumps precisely because a machine running far from its best efficiency point is loud, short-lived and wasteful even when the flow number is correct. Throttling a display back with a bypass to the chamber is legitimate and common on fountains, since it gives the commissioning engineer a way to trim the plume, but a pump bought two sizes up and then strangled is an energy decision made by accident. Where the pump is dry-mounted in a side chamber rather than submerged in the reservoir, suction conditions become a live question too, and ANSI/HI 9.6.1 on net positive suction head margin is the document that governs it.

A relief port is a design flow with a start time, not a drip

The reduced pressure principle works by holding the zone between two check valves below the inlet pressure, and a differential relief valve dumps that zone to atmosphere the instant the margin closes. On a fountain top-up the conditions that close it are ordinary and frequent. The supply drops because a hydrant is flowing or the courtyard tap has been opened flat out to fill a bucket. Backpressure arrives from the feature side on a hot afternoon. And, most characteristically for this application, the branch spends almost its entire life at zero flow — a float valve admits a trickle for a few minutes a day — so the first check sits in stagnant water and fouls, and a fouled check is precisely what makes the relief lift.

What comes out when it lifts is not a weep. A relief valve on an assembly of any size discharges immediately at a rate measured in litres per second and keeps discharging until somebody attends, which on a landscape contract with no building management system means until a tenant complains. Take the rated relief discharge from the manufacturer's published data for the exact model and size — Watts and Zurn Wilkins both publish it for their reduced pressure families — and use orifice arithmetic as a sanity check on the order of magnitude, and as a way of asking what happens if the driving head at your site is higher than the datasheet assumed.

Outdoors adds a failure the plant room never sees. A relief that weeps intermittently through January puts a sheet of ice across a courtyard threshold, and the first person to find it will be walking on it. ASSE 1060 grades enclosures by exactly this: whether the enclosure is merely a cover, or is insulated, or is heated to keep the assembly and its discharge above freezing. Choose the class against the site's design low temperature and against how long the courtyard goes unvisited, and check the enclosure's own drain outlet as a hydraulic component rather than as a hole — on more than one enclosure the moulded outlet is the narrowest point in the entire discharge path.

One thing must not be done, and it is the tidiest-looking option available. Do not pipe the relief into the fountain reservoir. It removes a visible discharge from a paved courtyard, it appears to solve the drainage problem for nothing, and it converts the assembly's own defence into the cross-connection it exists to prevent: a chamber filling from a stuck float valve rises to submerge the outlet, and the open path from the feature back toward the main is complete. The discharge is atmospheric by design and has to stay that way.

Finding a courtyard drain that can take it

Courtyards are hostile to this. There is often exactly one outlet, it was sized for rainfall on a small paved area, and it may sit above a podium or a basement deck where the drainage is a designed system with no spare connections in it. A relief event is not rainfall: it is a sustained full-bore discharge from a single opening with no diversity and no duration limit. Sizing the route on drainage fixture units, or on the yard's rainfall allowance, is the mistake that produces a flooded threshold rather than a wet patch.

Fall is what usually runs out first. The enclosure stands above the paving so the assembly can be tested, its outlet is a fixed height above that, and the gully invert is wherever it was set decades ago. Between the two is a courtyard whose crossfalls were designed to be barely perceptible underfoot, which means the developed length is long and the drop across it is small. Work the slope over the actual run before the enclosure position is fixed to a wall, because moving a box on a drawing costs nothing and moving it after the paving is bedded costs a week.

Then get the destination agreed rather than assumed. Discharging clean water to a surface water sewer or to a watercourse is not automatically permitted and is a question for the drainage undertaker; BS EN 752 is the framework for drain and sewer systems outside buildings, and an indirect waste arrangement indoors is governed by the adopted plumbing code rather than by preference. Where a soakaway is proposed, BRE Digest 365 sets out how one is designed and tested — and be honest that a soakaway hides the fault completely, so a fouled check can run for weeks with nothing visible to report. If the discharge cannot be seen by someone, specify a float alarm instead, or accept that the first symptom will be a water bill.

Power to a wet chamber, across a paved yard

A fountain is not a general outdoor circuit and is not treated as one. BS 7671 covers basins of fountains within Section 702, alongside swimming pools, with IEC 60364-7-702 behind it; the National Electrical Code addresses fountains in Article 680, Part V. Between them they govern residual current or ground-fault protection, equipotential bonding of conductive parts around and in the basin, and what may be installed within the zones measured from the water. A submersible pump in a chamber under a public courtyard sits squarely inside that scope, and the requirements are not satisfied by a weatherproof socket and good intentions.

The run itself is the other half. From the panel or consumer unit inside the building, across the courtyard, into a chamber under the paving is a long circuit on a conductor somebody will be tempted to keep small because it has to be drawn through an existing duct. Voltage drop over that length is what decides whether the motor starts cleanly against a full chamber on a cold morning, and the two rule sets treat the limit differently: BS 7671 gives figures in Appendix 4 — three per cent for lighting and five per cent for other uses from the origin of a low voltage installation — while the National Electrical Code carries its three per cent branch-circuit figure as an informational note rather than as a requirement. Check the drop against whichever applies, and check it at starting current rather than running current.