Thin Stream on the First Floor and a Pump That Cycles at Night
A three-storey house has a 1,000 L overhead tank on the roof; the tank outlet is 12 m above ground level. A 25 mm service pipe runs down the rear wall and 15 mm branches feed the taps. The ground-floor tap gives good flow. The first-floor bathroom tap, 4.5 m above ground, gives a thin stream, and the second-floor shower is unusable. A pressure gauge at the ground-floor tap reads 1.1 kg/cm² when the tank is full. At night, with every tap closed, the booster pump in the sump starts for 20–30 seconds every 3 minutes; during the day it runs almost continuously. The first-floor branch is about 12 m long and has four elbows and a partly closed gate valve.
1. Calculate the static pressure at the first-floor tap when the tank is full, and decide whether low pressure alone explains the thin stream.
Read the worked answer
The tank water level is 12 m above ground; the first-floor tap is 4.5 m above ground.
Head available = 12 − 4.5 = 7.5 m.
Pressure = ρgh = 1000 × 9.81 × 7.5 = 73,575 Pa ≈ 0.74 bar ≈ 0.75 kg/cm².
The ground-floor gauge reads 1.1 kg/cm², equivalent to about 11 m of head, which is consistent with 12 m static head minus normal losses.
A pressure of about 0.75 kg/cm² at the first-floor tap is adequate for a tap; it is not a low-pressure situation. Therefore low pressure alone does not explain the thin stream. The problem is more likely to be flow restriction: a partly closed gate valve, scale or sediment in the 12 m branch, a blocked aerator or strainer, or an undersized branch shared with other fixtures. The second-floor shower at about 4 m head (0.4 bar) may genuinely be marginal, but the first-floor symptom needs a flow investigation first.
2. List the checks you would carry out in a logical order, and state what each result would prove.
Read the worked answer
- Measure dynamic pressure at the first-floor tap with the tap fully open and with a nearby tap also open. A large drop proves a restriction upstream, not a pressure problem.
- Remove the aerator or strainer and retest. Restoration of flow proves debris in the outlet, not the pipe.
- Check the gate valve is fully open and the ferrule is not choked. A half-closed valve is a common cause after maintenance.
- Isolate the branch and compare flow at the branch inlet and at the tap. A difference localises the blockage to the branch.
- Inspect for scale in the 15 mm GI branch, especially at elbows. Hard-water scale builds at the four elbows and reduces the bore.
- Check the tank outlet and float valve for debris or a partly closed outlet valve.
- Check for a concealed leak on the branch if flow is poor but pressure is normal.
Record each reading; do not change two things at once, or the cause cannot be identified.
3. Explain the night cycling of the pump and its likely effect on the overhead tank supply.
Read the worked answer
At night there is no draw, yet the pump starts every 3 minutes. Something is allowing pressure to fall: a leaking foot valve or suction line, a passing non-return valve on the discharge, a waterlogged pressure tank, or a continuously leaking cistern float valve that is not counted as an open tap.
A passing non-return valve lets water run back from the tank or pressure line to the sump; the pressure switch then calls the pump. A leaking foot valve lets the suction line lose prime. Either way, the pump short-cycles - about 20 starts per hour - which overheats the motor and wastes energy.
Effect on the tank: if the non-return valve passes, water may return to the sump and the tank may never reach a stable level; if the foot valve leaks, the pump may lose prime and fail to fill the tank at all. The correct next step is to close the pump discharge valve and observe whether cycling stops, then isolate the cisterns one by one. Repair the faulty valve before adjusting the pressure switch.