Night Flow Analysis: A Daily Leak Test

At 3am, a 20-storey office tower in KL should be one of the quietest hydraulic systems in the country. Nobody is flushing, nobody is washing hands, the pantry taps are shut and the cleaners went home hours ago. Yet on a surprising number of buildings, the incoming water meter is still turning steadily at that hour. That steady turning is the entire basis of night flow analysis: water consumed when nobody is consuming water is, almost by definition, water being lost.

This is not a new idea. Water utilities have used minimum night flow for decades to hunt leaks in buried distribution mains, dividing their networks into district metered areas and watching the small hours. What changed is that the same technique now fits inside a single building, because a pulse or Modbus output on a meter costs very little and a logger can read it every minute, every night, forever.

What night flow analysis actually measures

Take the flow rate through your main incoming meter and find the lowest sustained value during the deep night window. Not the lowest single reading, which will be noisy, but the lowest average sustained over a meaningful block: the smallest 15 or 30 minute average between roughly 2am and 4am works well for most commercial buildings.

That single number, one per day, is your minimum night flow. Plot it as a daily trend and you have a health chart for the whole reticulation system. A building with no legitimate night loads and no leaks should sit close to zero. A building sitting at 1.5 m3/h at 3am is losing something in the order of 1,080 m3 a month through pipework that nobody is using, and it will keep doing that until somebody notices.

The power of the method is that it strips out behaviour. Daytime consumption swings with occupancy, weather, tenant mix, event bookings and school holidays, which makes month-to-month bill comparison a weak diagnostic. Night flow removes almost all of that variation, so what remains is mostly the physical condition of your pipes, valves and tanks.

Why 2am to 4am is the right window

You want the hour when human demand is at its floor but the system is still fully pressurised. In Malaysian commercial buildings that is typically the middle of the night after the last cleaning shift and before the first deliveries, security rounds and early gym users. Midnight is too early, because late tenants and cleaning crews are still active. Five in the morning is too late in a hotel or hospital, where the first shift is already moving.

Pick a window, keep it fixed, and let the trend do the work. Consistency matters more than picking the theoretically perfect hour, because you are comparing tonight against the last ninety nights, not against an absolute ideal.

What legitimate night flow looks like

Almost every building has some honest 24-hour water use, and part of setting up night flow analysis is writing that list down before you start raising alarms. Common ones in Malaysia include:

- Cooling tower make-up, if chillers run overnight for data rooms, trading floors or hotel back of house
- Automatic urinal and landscape irrigation timers, which often fire in the small hours by design
- Water features, ice machines, laundry and kitchen prep in hotels
- Water treatment backwash cycles, which run as short, large, repeating pulses
- Genuine occupant use in residential blocks, serviced apartments, hospitals and hotels

That last one is why night flow analysis behaves differently by building type. An office tower, school, mosque or shopping mall should approach zero at 3am, which makes the method extremely sensitive. A 400-unit condominium never approaches zero, because people do get up at night, but the baseline is still remarkably stable from night to night, so you watch the change rather than the absolute value.

Note the difference between a jockey pump topping up the fire main and a legitimate load. If the jockey pump is cutting in repeatedly overnight, that is not a 24-hour load. That is a leak in the fire system telling you about itself.

How much night flow is too much?

There is no single universal figure, and anyone who quotes one for all buildings is guessing. The practical approach is comparative. Establish your own baseline over two to four weeks, agree what portion of it is explained by the legitimate loads above, and treat the unexplained remainder as the leakage figure you are trying to drive down.

As an illustrative rule of thumb, in a building with negligible genuine night use, night flow that exceeds a few percent of the average hourly daily flow deserves a walk-through. Many buildings discover on their first week of logging that their unexplained night flow is a double-digit percentage of total consumption, which is money leaving through drains rather than through taps.

If you want that number apportioned properly across chiller make-up, toilets, kitchens and losses, night flow pairs naturally with a full building water balance audit, which is what turns "we are losing something" into "we are losing it here".

Burst versus creep: reading the shape of the trend

Two failure modes show up completely differently on a daily night flow chart, and both are worth catching.

A burst or a sudden component failure appears as a step. Monday night sits at 0.3 m3/h, Tuesday night sits at 3.1 m3/h, and it stays there. The classic Malaysian version is a roof tank ball float valve that sticks open. The tank overflows continuously into the overflow pipe, the water goes straight down a storm drain, nobody upstairs hears anything, and the tower keeps operating perfectly normally. Without night monitoring, that failure is discovered on the bill, six weeks later. With it, the step is visible the next morning.

A slow creep is the opposite shape: a gentle upward slope over months. Flush valve seats erode, pipe joints weep, isolation valves pass slightly, and each one adds a few litres per hour. No single night looks alarming. Compare this January against last January and the difference is stark. Creep is the reason to keep the daily trend for years and not just for the current month.

Turning it into an alert somebody acts on

A chart nobody opens is not a leak detection system. Two rules cover most of it:

Absolute threshold. If tonight's minimum night flow exceeds an agreed ceiling for the building, notify the M&E team the same morning.

Change against baseline. If tonight's value exceeds the rolling median of the previous fortnight by a meaningful margin, notify, even if the absolute figure is still under the ceiling. This is what catches a burst on a building that already has a high honest baseline.

Send both to where the facility team actually lives, which in Malaysia usually means WhatsApp and email rather than a portal they log into once a quarter. Include the number, the previous baseline and the meter name, so the recipient can decide whether it is a 7am problem or a next-Monday problem.

Alerts on the main incoming meter tell you the building is losing water. Alerts on zone or riser meters tell you where, which is why submetering by zone and by tenant shortens the hunt from days to hours. CobiNeural does this part continuously: it logs each water meter, computes minimum night flow automatically, holds the baselines and pushes the alert out when tonight breaks the pattern, standalone or on top of an existing BMS.

What it is worth in ringgit

The saving is arithmetic rather than estimation. Take the unexplained night flow in m3/h, multiply by 24 and by 30, and price it against your operator's tariff. Water in Malaysia is billed in tiered blocks that differ by state and by domestic versus commercial category, so check the current schedule with your operator, for example Air Selangor for Selangor, KL and Putrajaya, rather than relying on a remembered rate. Then remember that charges linked to consumption ride along with it, and that a leak inside a wet riser or a basement slab eventually costs far more in reinstatement than in water.

The attractive part is the cost of the test itself. Once the meters and the logging are in place, the leak test runs every night with no labour, no shutdown and no valve exercising. You simply read the number in the morning.

If you would like to see what your building's 3am number actually looks like, talk to us about water monitoring.