Cold Water Supply and Storage Systems
This domain covers how drinking water reaches fixtures: service connections, storage, distribution, pressure and backflow protection. Because supply in many Ghanaian areas is intermittent, the exam expects you to know indirect (tank-fed) systems well, including tank sizing, stand heights and pump arrangements.
System types
- Direct: mains pressure feeds every fitting; only suitable where supply is continuous and pressure adequate.
- Indirect: the mains fills a ground or roof tank, and gravity or a pump feeds the building. Most houses use this pattern.
- Boosted: break tank plus pump set with a pressure vessel, used where the roof head cannot give enough pressure.
Core rules
- Storage: allow about 100 litres per person per day and 1–2 days' storage; 1000, 1500 and 2000 litre polytanks are standard sizes.
- 1 litre of water weighs 1 kg, so a 1000 litre tank weighs over a tonne; the stand must be designed and braced for the full load.
- The tank must be covered, insect-screened, ventilated and fitted with an overflow that discharges visibly outside and a drain-off at the base.
- Maintain an air gap or category 5 backflow arrangement; never connect a pump suction directly to the mains.
- Pressure: 1 m head = 9.81 kPa. Keep velocity between about 0.5 and 1.5 m/s; 15 mm to individual taps, 22 mm to bathroom groups, 28 mm distribution mains.
- Fit a boundary stop valve, branch isolating valves and a drain cock at the lowest point.
Worked example
A tank base sits 4 m above a shower rose. Static pressure = 4 × 9.81 = 39.2 kPa ≈ 0.39 bar. Subtract about 0.15 bar of friction loss over the pipe run, leaving roughly 0.24 bar - below the 0.5 bar a mixer shower needs. Options: raise the stand to give at least 6.5 m head, or fit a booster pump. Reason: convert head to pressure first, then deduct losses; head and pressure are not interchangeable.
Traps
- Undersized or unbraced tank stands that sag or collapse when full.
- Uncovered tanks breeding mosquitoes and algae.
- Any temporary cross-connection between mains and stored water.
- Dead legs, no drain-off and no isolating valve, making future repairs impossible.
- Quoting pump pressure when the question asks for static head from a tank.