The first-floor tap that only trickles: head, loss or blockage?
A newly built two-storey house has a 1000 L PVC tank on the roof slab, with the water level 5.0 m above ground-floor level. The first-floor bathroom basin outlet is 3.6 m above ground-floor level. The branch to the bathroom is 15 m of 15 mm CPVC with six elbows and two tees, serving one basin tap and one WC cistern float valve. The ground-floor tap has strong flow. At the basin, measured flow is about 0.1 L/s and it drops to almost nothing when the cistern refills. There is no visible leak, the tank is full, the stop valve is fully open and the aerator is clean. The owner plans to add a storage geyser above the basin next month and wants 'a small pump' installed; the plumber's pump quote is ₹6,500 plus ₹1,500 for an electrical point. The owner's budget is ₹5,000 and he is convinced the pipe is blocked with construction debris.
1. Calculate the pressure actually available at the basin and explain, with numbers, why the tap behaves as it does. Is the owner's 'blocked pipe' theory supported?
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Static head available: water level 5.0 m minus outlet height 3.6 m = 1.4 m of water column, i.e. about 0.14 bar (14 kPa). Good practice wants roughly 0.5–1.0 bar at a fixture for a satisfactory flow, so the installation is already far below the desirable figure before any friction.
Why flow collapses: the 15 mm CPVC has an internal diameter of about 13 mm. At 0.1 L/s the velocity is around 0.75 m/s, and the friction gradient at that velocity is high - of the order of 0.15–0.2 m per metre. Over 15 m plus six elbows and two tees, the losses exceed the 1.4 m available, so the system simply chokes itself to an equilibrium flow of about 0.1 L/s. When the float valve opens, the branch shares that tiny head with a second outlet, so the basin tap nearly stops.
Blocked pipe? Not supported: a debris blockage would give low flow at all times and often intermittent sputtering, and it would not explain why the problem depends on which fixtures are drawing. The ground-floor tap is strong because it is closer to the tank with a shorter, larger-bore run.
Confirm with a gauge at the basin: measure static pressure with all taps closed, then dynamic pressure with the basin tap fully open. A large static-to-dynamic drop is friction and head, not blockage.
2. Give three remedies with approximate costs, recommend one, and justify the choice using the calculated figures.
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Option A - re-pipe the bathroom branch in 25 mm CPVC. Material about 15 m × ₹160 = ₹2,400, fittings about ₹1,200, labour about ₹800, so roughly ₹4,400.
Option B - booster pump. ₹6,500 pump plus ₹1,500 electrical point = ₹8,000, plus running cost and maintenance, and it adds noise and a failure point.
Option C - raise the tank by about 2 m to gain 0.2 bar. This is a structural change and typically costs far more than A for a small gain.
Recommend A. Geometry shows why: going from roughly 13 mm to 21 mm internal diameter cuts velocity at the same flow by about two-thirds and cuts friction loss by a factor of roughly 7–10 for the same flow, while leaving adequate margin for the geyser. At 0.2 L/s the 25 mm pipe runs near 0.6 m/s with losses of about 1 m over the run - within the 1.4 m head, so a normal tap flow is restored without electricity.
If the existing branch is buried and cannot be re-piped within budget, choose B and size the pump for about 0.2–0.3 L/s at 1.0–1.5 bar with a pressure switch, non-return valve and a dry-run cut-out.
3. What would you check before selling the owner a pump, and what must the owner understand about the geyser he plans to add?
Read the worked answer
Checks before any pump:
- Disconnect the branch at the basin and flush it to prove there is no construction debris or scale; check the float valve strainer and the tank outlet for a rag or bag.
- Measure static and dynamic pressure at the basin with a gauge, not by feel, and record both readings.
- Confirm the stop valve opens fully and is not a low-quality gate valve half-sheared.
- Check the branch route and bore; if it is 15 mm concealed, quote the re-pipe first.
A pump sold before these checks can end up pressurising a blocked or undersized line, giving noise and cycling without solving the complaint.
Geyser point: a storage geyser will not fill and will not deliver a steady shower if the inlet pressure and flow are this poor, and most manufacturers state a minimum inlet pressure for smooth operation, commonly around 0.3 bar. So the pipe upgrade should be done before the geyser is fitted, not after. Also insist on a non-return valve and an expansion arrangement as per the geyser instructions, and if a pump is used, keep its cut-off pressure below the geyser's rated inlet pressure.