Top-floor shower starved by a loft cistern and an old float valve
A three-storey Victorian house has been converted into six flats. Cold water to the three top-floor flats and a communal shower room comes from a single 180 L galvanised cistern in the loft, filled through a 15 mm copper branch and an old pattern 2 float valve. The incoming 25 mm MDPE main gives 3.2 bar static pressure at the ground-floor stop tap. The second-floor shower delivers about 6 L/min at the rose, and the occupant wants at least 9 L/min with 0.5 bar minimum at the outlet. The cistern lid is missing, the overflow pipe is unscreened and terminates 150 mm above the flat roof, and the only insulation is a loose jacket laid across the top. Water hammer is heard at about 23:00 most nights, and the caretaker reports the overflow running for short periods in the early morning. The cistern water line is roughly 6.8 m above the second-floor shower rose.
1. Explain the likely causes of the night-time water hammer and the intermittent overflow, and describe the tests you would carry out to confirm each before recommending any work.
Read the worked answer
The two symptoms have different causes and must be confirmed separately.
Night-time hammer. The 3.2 bar figure is a daytime static pressure. Overnight demand falls, so district pressure rises, often by 0.5 to 1.0 bar. The old float valve closes abruptly against that higher pressure and the sudden stop sends a surge back along the 15 mm copper branch, which is almost certainly under-clipped. Confirm by fitting a pressure logger at the incoming main for 48 hours, then isolating the cistern branch overnight and refilling from a tap to hear whether the noise moves. A worn jumper washer or a loose float produces chattering rather than a single bang; listening at the valve with a screwdriver stethoscope separates the two.
Intermittent overflow. Watch the warning pipe outlet. A steady trickle rather than a surge means the float valve is passing, usually grit under the seating or a perished washer. A surge only overnight points to the same pressure rise lifting the float against a badly set arm. Test by shutting the cistern service valve last thing at night and checking the level the next morning; if the level has fallen normally and the overflow is dry, the valve is at fault. Also prove which pipe is discharging, because a WC inlet valve can be mistaken for the cistern overflow.
Recommendation: 22 mm feed, a type 2 equilibrium float valve with a servicing valve, re-clipping the branch, and a pressure-reducing valve only if the logger confirms sustained pressure above 4 bar. None of this should be decided without the logger data.
2. Assess whether the existing arrangement can deliver 9 L/min at 0.5 bar to the second-floor shower, and specify what you would change, with reasons.
Read the worked answer
Work the pressure budget before specifying anything.
Static head: 6.8 m to the rose, so 6.8 × 0.0981 ≈ 0.67 bar.
Losses at 9 L/min:
- 15 mm copper feed and the old float valve: roughly 0.3 to 0.4 bar. A 15 mm feed is already near its practical limit at 9 L/min.
- 22 mm distribution from cistern to second floor, say 8 m with fittings: about 0.05 bar.
- 1 m rise from pipe to shower rose plus the shower fitting: about 0.2 bar.
That leaves roughly 0.1 to 0.2 bar at the rose, well below the 0.5 bar required. The arrangement cannot meet the duty even with perfect pipework, because the available static head is only just above the requirement to begin with.
Specify:
- Replace the 180 L galvanised cistern with an insulated polypropylene cistern sized from a demand calculation. Three flats plus a communal shower is likely to need 400 to 500 L for useful storage; record the final figure on the handover.
- Fit a byelaw 30 kit: close-fitting lid, screened warning pipe with 3 mm mesh, insect screen, and insulation to all faces, not just the top.
- Increase the feed to 22 mm and fit a type 2 equilibrium valve.
- For the shower, install a twin-impeller booster set drawing from the stored water, never directly from the mains, giving 2.5 to 3 bar at 9 to 12 L/min. This future-proofs the other flats as well.
- Confirm the new cistern is separated from the mains by a type AA air gap and remove the redundant galvanised tank.
The booster is the only reliable route to 0.5 bar at the second floor with a cistern only 6.8 m above it, and it avoids over-pressurising the older pipework.