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From Night-Flow Alarm to Verified Repair: A Worked Leak Case
A worked water leak investigation in our example office tower: rule out tank refill and legitimate night uses, check the data, find the leak with a step test, repair it and prove the fix over three nights. Includes a second case where the data cannot settle the answer.
Tan Kok XinWater Fundamentals
Part 11 of 12 in Cobler's Water Fundamentals course. New here? See the course page.
It's 7:00am on a Wednesday, and the facility manager's phone shows a message. The night flow on one of the office tower's water meters is more than seven times its usual figure. It might be a leak. But it might also be the tanks refilling, a cleaner working late or a faulty meter.
This is the moment Part 1 promised. We said we'd be able to look at the tower at 3am, say where its water is going, and prove a leak is fixed. So let's do it, as a water leak investigation in seven tasks, following one of the alerts from Part 10.
It uses almost everything in the course. We'll need tanks and level readings (Part 3), meters (Part 6) and the cost of a cubic metre (Part 7). We'll also need the storage-corrected balance (Part 9) and night flow (Part 10). And at the end there's a second, shorter case where the data isn't good enough to reach an answer, because that happens too.
The water leak investigation
Each task gives us some evidence and asks a few questions. Try the questions first, with a calculator and a notepad. Then open the worked answer under each task to check your thinking.
All the readings are made up, but they're consistent with each other. Prices use Part 7's figure of RM4.28 for each extra cubic metre the tower buys, water plus sewerage.
The building and its meters
First, a quick reminder of the layout from Parts 1 to 3. The office tower takes water from Air Selangor through a 100 mm bulk meter into a 300 m³ suction tank. Transfer pumps lift the water to two 100 m³ roof tanks. From the roof tanks, water flows down to the floors in two zones:
- The upper zone serves levels 11 to 20.
- The lower zone serves the basement and levels 1 to 10, where level 1 is the ground floor. Each floor branch has a pressure reducing valve (PRV, Part 2) to bring the pressure down to a safe level.
The cooling tower make-up line also draws from the roof tanks. The fire system has its own tank, pumps and pipes, and the incoming supply fills it after the bulk meter.
The building has four water meters, all logged every 15 minutes. They're the bulk meter, a meter on each zone downfeed, and the cooling tower make-up meter. Level transmitters on the suction tank and the roof tanks are logged as contents in cubic metres. And each floor's branch off the riser has its own isolation valve.
The tower's meters and tanks, with the readings from the case night. The booster pump set for the top floors isn't shown.
The tower's normal minimum night flow is the lowest 30-minute average between 2am and 4am. It's 0.08 m³/h on the upper-zone meter and 0.12 m³/h on the lower-zone meter. That water goes to the ground-floor security post, the lobby washrooms and the urinal flush timers. The chillers are off at night, so the tower make-up meter reads zero.
The building's alert rule, set by its team, checks each zone meter every morning. It sends a message if the minimum night flow is more than 0.10 m³/h above the median of the previous 14 nights.
Task 1: the tanks
The alert (Wednesday 7 October 2026, 7:00am): "Lower-zone meter: minimum night flow 0.90 m³/h. Baseline (14-night median) 0.12 m³/h." The upper zone was normal, at 0.08 m³/h.
The readings from 2am to 4am that night:
Meter or tank | Reading over the two hours |
|---|---|
Bulk meter | 3.0 m³ |
Suction tank | 276.0 m³ to 279.0 m³ |
Roof tanks | 45.0 m³ to 43.0 m³ (transfer pumps off) |
Fire tank | Level steady |
Upper-zone meter | 0.16 m³ |
Lower-zone meter | 1.80 m³ (steady at 0.90 m³/h throughout) |
Tower make-up meter | 0 |
Questions:
- Could the suction tank refilling explain the lower-zone alert?
- Using the storage correction from Part 9, how much water did the building use between 2am and 4am?
- Does that agree with the zone meters?
Show the worked answer
1. No. The lower-zone meter sits after the roof tanks. Water refilling the suction tank passes through the bulk meter only, so it can't reach the lower-zone meter. The bulk meter's reading does include refilling, and that's why we can't use it on its own at night (Part 10).
2. First we find the change in storage. The suction tank gained 279.0 − 276.0 = +3.0 m³. The roof tanks lost 43.0 − 45.0 = −2.0 m³. The fire tank didn't change. So the total change in storage is +3.0 − 2.0 = +1.0 m³.
Water used = inflow − change in storage = 3.0 − 1.0 = 2.0 m³ in two hours, or 1.0 m³/h.
3. Yes. The zone meters recorded 0.16 + 1.80 = 1.96 m³, or 0.98 m³/h, and the tower make-up meter recorded nothing. The two methods agree to within 0.04 m³, which is smaller than the rounding of the tank readings (0.1 m³).
So what does that agreement tell us? Two things. The bulk meter's 1.5 m³/h (3.0 m³ in two hours) was all going into the suction tank, which is normal refilling. And no water is being lost between the bulk meter and the zone meters, because a zone meter measured everything that left storage. So the extra water is being used, or lost, inside the lower zone.
Task 2: known night uses
The alert says the lower zone used 0.90 − 0.12 = 0.78 m³/h more than normal. But before we go looking for a leak, we should rule out the uses we already know about.
The evidence the team collected by 9am on Wednesday:
- The cooling tower make-up meter read zero all night. The chillers stopped at 7pm.
- The security log shows no contractors or tenants in the building overnight. The cleaners finished at 11:15pm.
- The building management system shows no change to the urinal flush timer schedule.
- The fire tank level was steady, and the jockey pump didn't start overnight.
- On a normal Tuesday evening, the lower-zone flow falls to about 0.12 m³/h by 8pm. This Tuesday it stopped falling at 0.90 m³/h, and stayed there all night.
- The maintenance log shows a plumber replaced a pantry tap on level 6 at 2pm on Tuesday.
- The leasing team showed the vacant level 8 to a possible tenant from 3:30pm to 4:30pm on Tuesday.
Questions:
- Does any known night use explain the extra 0.78 m³/h?
- When did the extra flow start?
- The level 6 tap and the level 8 viewing both happened on Tuesday afternoon. Which one caused it?
Show the worked answer
1. None does. None of the listed night uses changed, and the extra flow is in the lower zone, not on the fire system or the cooling tower. So we treat it as a leak until something shows otherwise.
2. The data can only say "by 8pm on Tuesday". During office hours, the lower zone uses several cubic metres an hour, so an extra 0.78 m³/h can't be seen in the daytime readings. It only became visible once the evening use fell away.
3. The evidence can't tell yet, because both events fit the timing. It's tempting to blame the new tap, since plumbing work happened there. But guessing here would send the plumber to the wrong floor half the time. So the next step is a test that separates the floors.
Task 3: the data
Before asking a plumber to work at 2am, we should make sure the alert isn't a meter or logging fault.
The checks:
- All the loggers and level transmitters take their time from the building management system clock. On Wednesday morning, every clock was within one minute of it.
- Every meter had all eight 15-minute readings between 2am and 4am, with no gaps.
- The lower-zone meter's total for Wednesday was 19 m³ higher than the average of the previous four Wednesdays.
- The storage-corrected bulk figure for Wednesday was also about 19 m³ higher than usual. That's the bulk meter minus the change in all tank contents over 24 hours.
Questions:
- Why does it matter that the two daily figures agree?
- What would go wrong if the lower-zone logger's clock had been one hour behind?
Show the worked answer
1. Because they come from different instruments. The lower-zone meter measures the water directly. The storage-corrected bulk figure comes from the bulk meter and the tank transmitters. A fault in one instrument wouldn't make both show the same extra 19 m³. So since they agree, the extra water is real.
It also matches the flow, because 0.78 m³/h × 24 hours = 18.7 m³, about 19 m³ a day.
2. The meter's "2am to 4am" readings would really be from 1am to 3am, while the tank readings covered 2am to 4am. So the refill check from Task 1 would mix two different time periods. It could disagree when nothing is wrong, or agree when something is. That's the same read-timing problem Part 9 described for monthly readings, on a smaller scale.
Task 4: the step test
So the extra water is real, and it's somewhere in the lower zone. But which floor? For that we use a step test. It finds which part of a pipe system is using water by closing its sections one at a time while we watch a meter. When we close the section with the leak, the meter drops.
The plan. The team ran the test from 2am on Thursday 8 October, when the building was empty. The facility technician and the plumber followed this procedure:
- Before you start, tell the security post.
- Before any step test, check that closing domestic water valves won't affect the sprinklers or hose reels.
- Close each floor's valve slowly, to avoid a pressure surge in the pipes (water hammer).
- Keep each valve closed for 10 minutes.
- Read the lower-zone meter's dial at the start and end of each closure.
- Reopen the valve fully, and write down the time.
In the office tower, step 2 is easy, because the fire system has its own tank, pumps and pipes. But check it for your own building every time.
The results (litres through the lower-zone meter in 10 minutes):
Valves closed | Litres in 10 minutes |
|---|---|
None (all open) | 150 |
Levels 2 to 5 | 150 |
Levels 6 to 10 | 20 |
Level 6 only | 150 |
Level 7 only | 150 |
Level 8 only | 20 |
Level 9 only | 150 |
Level 10 only | 150 |
Only closing level 8 brings the flow down to the normal night figure of 20 litres in 10 minutes.
Questions:
- Convert 150 litres and 20 litres in 10 minutes to m³/h. Do they match the alert?
- Where is the extra flow?
- What do the 20 litres that remain with level 8 closed represent?
Show the worked answer
1. There are six 10-minute periods in an hour. So 150 L × 6 = 900 L an hour = 0.90 m³/h, which matches the alert. And 20 L × 6 = 120 L an hour = 0.12 m³/h, which is the lower zone's normal night flow.
2. It's on level 8. Closing levels 6 to 10 together removed 130 litres in 10 minutes, so the extra flow is somewhere on those five floors. Closing each of them in turn shows that only level 8 makes a difference. Closing level 6 changed nothing, so the new pantry tap isn't the cause.
3. They're the normal night uses on the ground floor: the security post, the lobby washrooms and the urinal flush timers. Those sit below all the floor branches that were closed, so they kept running.
So the extra flow is 150 − 20 = 130 litres in 10 minutes. That's 0.78 m³/h, or about 0.22 L/s, and it's all on level 8.
Task 5: the fix
But the step test only narrows it to a floor. So the plumber went up to level 8 at 2:50am and listened at each washroom and pantry.
What they found. In the men's washroom, one WC's flush valve was stuck part-open. A flush valve is the push-button valve that flushes a WC straight from the water supply, without a cistern. Water was running quietly into the pan and down the drain. There was nothing on the floor, and because level 8 is vacant, nobody had been there to hear it. The WC was last used during Tuesday's viewing.
What they did. At 3:00am, the plumber closed the small isolation valve on that WC's supply pipe, which stopped the loss straight away. The rest of level 8 stayed in service. Then they reopened the level 8 branch valve and read the lower-zone meter for 10 minutes. It showed 20 litres, back to normal.
On Thursday afternoon, the plumber replaced the flush valve's worn seal and reopened the WC's isolation valve. They flushed it five times, checking each time that the flow stopped. And the facility manager recorded the fault, the times and the repair on a work order.
Question: Why close the WC's own isolation valve first, instead of leaving the whole of level 8 shut off until the repair?
Show the worked answer
Closing the smallest possible section stops the loss while keeping the rest of the floor in service. It also confirms the diagnosis. Say the lower-zone flow hadn't returned to 20 litres with only that WC isolated. Then the plumber would have known there was another leak on the floor.
Task 6: the proof
Now for the second half of Part 1's promise. A repair isn't finished until the data shows the problem has gone. And we decide what "fixed" means before we look, so the result can't be bent to fit.
The team's test, set on Thursday. The lower-zone minimum night flow must be back within the range of the four weeks before the fault, 0.10 to 0.14 m³/h. It must stay there for three nights in a row. The storage-corrected bulk figure must also be back to normal. The Thursday night of the step test doesn't count, because valves were being opened and closed.
The results (lower-zone minimum night flow, 2am to 4am):
- Friday 9 October: 0.12 m³/h
- Saturday 10 October: 0.11 m³/h
- Sunday 11 October: 0.12 m³/h
The storage-corrected bulk figure agreed with the zone meters on all three nights.
Questions:
- Is the repair verified?
- Why wait for three nights instead of one?
- About how much water did the leak waste, and what did it cost?
- What would it have cost if it had waited for the next monthly bill, or for a year?
Show the worked answer
1. Yes. All three nights are inside the 0.10 to 0.14 m³/h range, and the two methods agree. So the alert can be closed, with the work order attached.
2. One quiet night could be a coincidence, for example if the leak came and went. Three nights in a row inside the normal range is much stronger evidence.
3. The leak ran from Tuesday afternoon until 3:00am on Thursday. The data only proves "by 8pm", so take 4:00pm on Tuesday as the start. That's 35 hours, so the water lost is 0.78 m³/h × 35 hours = 27.3 m³. At RM4.28 per cubic metre, that's about RM117.
4. If it had been found on the next monthly bill instead, about 30 days later, that's 0.78 × 24 × 30 = 561.6 m³. That's about RM2,404. Left for a year, it's 0.78 × 24 × 365 = 6,832.8 m³, about RM29,244.
The exact start time makes little difference. Starting two hours earlier would add 1.6 m³, about RM7. And in October's water balance (Part 9), the 27.3 m³ shows up inside the lower zone's measured use.
So that's the promise kept. Catching this leak the next morning held its cost to about RM117. The same stuck flush valve, on a vacant floor where nobody would hear it, would have cost about RM2,404 by the next monthly bill. And it would have cost about RM29,244 if it had run for a year.
Task 7: a case the data can't settle
But not every morning ends with an answer. Here's a second case, twelve days after the first alert.
Monday 19 October 2026, 7:00am. No leak alert was sent, but the morning data check shows two problems:
- The upper-zone meter has no readings from 1:00am to 5:00am. Its logger went offline and came back at 5:10am.
- The suction tank's level has read exactly 88.0 percent since 11:40am on Saturday. A real tank level moves all day, so the transmitter has almost certainly frozen.
The other readings, 2am to 4am:
- Bulk meter: 4.8 m³ (2.4 m³/h).
- Suction tank: 88.0 percent, frozen.
- Roof tanks: 46.0 m³ to 45.6 m³, with the transfer pumps off.
- Lower-zone meter: 0.24 m³ (0.12 m³/h).
- Tower make-up meter: zero.
- Fire tank: level steady, and the jockey pump didn't start.
Questions:
- What can we conclude about the floors?
- What can't we conclude?
- What should the team do today?
Show the worked answer
1. The roof tanks fell by 0.4 m³ in two hours, which is 0.20 m³/h. With the transfer pumps off and the tower make-up at zero, that water went down the two zone downfeeds. The lower zone took 0.12 m³/h, so the upper zone took about 0.20 − 0.12 = 0.08 m³/h, its normal figure. So the floors look normal. It's an estimate, because it relies on the roof tank transmitter instead of the missing upper-zone meter, but it's a fair one.
2. We can't say where the 4.8 m³ through the bulk meter went. If the suction tank was refilling, its level should have risen by 4.8 m³, which is 1.6 percent. With the transmitter frozen, we can't tell refilling apart from water running out of the overflow. Nor can we tell it from a leak on the buried pipe between the bulk meter and the tank.
The flow itself isn't unusual, since a normal night in Part 10 showed 3.0 m³/h of refilling. So the night can be neither cleared nor called a leak.
3. Record the night as "unresolved, missing data". The floors look normal, so don't start a floor-by-floor step test. Instead:
- Raise work orders to repair the suction tank transmitter and the upper-zone logger.
- Today, walk to the suction tank's overflow outlet and look for running water.
- Check the tank's level against the gauge or markings on the tank, and compare it with the frozen 88 percent.
- If the transmitter isn't fixed by tonight, read the tank level by hand at 2am and 4am.
- If the rise matches the bulk meter volume, the flow was refilling, so close the case.
- If the rise is clearly smaller, water is being lost before the tank. Do the next step.
- At night, with the fire tank full, close the suction tank's inlet valve for a short time. The bulk meter should stop.
- If the bulk meter keeps turning, water is escaping between the meter and the tank. Call a leak detection specialist.
The specialist can use acoustic equipment to locate a leak on a buried pipe.
We don't know the answer to this case, and on Monday morning neither would you. The right result is a clear record of what we know, what's missing and what we'll check tonight.
The whole building
So we've followed one fault from a 7:00am alarm to a verified repair. We ruled out the tanks, the known night uses and the data. Then a step test found level 8, and three quiet nights proved the fix. And we saw that honest "we don't know yet" is a result too.
But one leak, found and fixed, says nothing about the whole year. Is the tower's monthly water use good or bad for a building of its size and type? That's the last question, and it's Part 12: Water Consumption Benchmark KPIs for Buildings.
Check your understanding
1. During a step test, closing a floor's branch valve makes the zone meter drop from 150 litres to 20 litres in 10 minutes. How large is the leak in m³/h?
Show the answer
The drop is 130 litres in 10 minutes. There are six 10-minute periods in an hour, so that's 780 litres an hour, or 0.78 m³/h.
2. Why didn't the team blame the level 6 pantry tap, even though a plumber had worked there the same afternoon?
Show the answer
The timing fitted both the level 6 work and the level 8 viewing, so the logs alone couldn't choose between them. The step test could. Closing level 6 changed nothing, and closing level 8 brought the flow back to normal.
Recap
- Work through a night-flow alarm in order.
- Rule out tank refill, by using meters after the tanks or the storage correction. Then rule out known night uses.
- Check clocks, gaps and a second, independent measurement before sending people.
- Narrow the search with sub-meters and a step test, isolate the smallest section, and repair it.
- Decide what "fixed" means before you look, and confirm it over several nights.
- When the data is missing or frozen, record the case as unresolved and fix the data. Use cheap manual checks before calling in a specialist.
This is Part 11 of 12 in Cobler's Water Fundamentals course. Previous: Night Flow Analysis: A Daily Leak Test. Next: Water Consumption Benchmark KPIs for Buildings.
Cobler builds CobiNeural, a platform that shows a facility team its building's energy, water and indoor air data as live numbers across the whole site. To see how your building performs, talk to us.
FAQ
Frequently asked questions
Look at meters after the tanks, such as the meters on the zone downfeeds from the roof tanks. Refilling passes through the bulk meter only, so it cannot show on them. If you only have the bulk meter, correct it for the change in stored water over the night. Water used = bulk meter volume minus the change in the contents of every tank it feeds. If the corrected figure is close to the building's normal night use, the bulk meter was only recording refilling.
A step test finds which section of a pipe system is using water. You close its isolation valves one at a time, at night when normal use is lowest, while watching a meter. When the section with the leak is closed, the meter reading drops. For example, a zone meter might show 150 litres in 10 minutes with all valves open, and 20 litres with one floor's branch closed. Then the extra 130 litres in 10 minutes (0.78 m³/h) is on that floor. Check first that closing domestic valves does not affect the fire system, and close valves slowly to avoid water hammer.
Decide before you look what counts as fixed. A good test is that the minimum night flow on the affected meter returns to the range it showed before the fault. It should stay there for several nights in a row, for example three. The storage-corrected bulk meter figure should agree. One quiet night could be a coincidence, but several in a row is much stronger evidence.
Do not clear the night and do not call it a leak. Record it as unresolved because of missing data, and raise work orders to fix the logger or transmitter. Then use cheap manual checks. Look at the tank overflow outlet. Read the tank level by hand at the start and end of the night window, and compare the rise with the bulk meter volume. If water appears to be lost before the tank, a leak detection specialist can locate a buried pipe leak with acoustic equipment.
Ask us about this topic
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