Top-floor shower pressure and temperature swings
A 40-room hotel in Negombo has five floors. Cold water is fed from a rooftop tank whose outlet is about 3 m above the fifth-floor ceiling. Hot water is generated in a basement calorifier at 70°C and delivered through a thermostatic mixing valve (TMV) set to 48°C. Rooms on floors 1–3 are satisfactory. In two fifth-floor rooms, the shower flow falls sharply when a nearby washbasin tap is opened, and the temperature swings between about 40°C and 55°C. The cold static pressure at the fifth-floor shower is measured at 0.2 bar; the hot static pressure at the same point is 1.6 bar. The TMV has no strainers or pressure gauges fitted, and its cold inlet is fed by a 15 mm branch taken 12 m from the riser. The maintenance team proposes replacing the shower head with a larger one. The hotel is fully booked and the manager wants a one-hour fix this afternoon.
1. Interpret the pressure readings and explain why fitting a larger shower head is unlikely to solve the complaint. Include an approximate static-pressure calculation.
Read the worked answer
Static pressure from a tank outlet 3 m above the fifth-floor ceiling is about 3 m × 0.098 bar/m ≈ 0.29 bar. The measured 0.2 bar is lower because of pipe friction, fittings and the 15 mm branch, but it is the same order. The hot reading of 1.6 bar comes from the basement calorifier or its PRV and is about 1.4 bar higher than the cold. A thermostatic mixing valve controls temperature by adjusting hot and cold ports; it needs both supplies within the manufacturer's pressure and temperature limits, commonly close to balanced, often within about 10% or as stated by the maker. With cold starved and hot at 1.6 bar, the TMV cannot hold 48°C: opening a nearby tap drops cold further, the valve hunts, and hot can pass. A larger shower head increases flow demand and makes the cold pressure worse, so it may reduce performance and increase scalding risk. The one-hour response should therefore be diagnosis and isolation or balancing, not a new head.
2. Outline the tests you would carry out and the immediate and permanent remedies, stating what you would do if the imbalance cannot be corrected today.
Read the worked answer
Fit calibrated gauges at both TMV inlets and measure static and dynamic pressure with the shower running and with the nearby basin tap opened. Measure shower flow rate and temperature stability. Isolate the TMV, inspect and clean strainers, check for scale or debris, and test the non-return valves and thermostatic element. Confirm the cold branch size and length: a 15 mm branch 12 m long is likely undersized for the shower.
Immediate remedy: reduce the hot pressure to match the cold with an approved PRV, or provide a temporary local cold boost; replace the TMV if it fails its test; restore the original shower head. If pressures cannot be balanced, take the two rooms out of service and label them rather than leave a scalding risk. Permanent remedy: upgrade the cold distribution to the fifth floor with a larger branch or booster set, fit a PRV on the hot supply, install strainers and gauges, and commission the TMV to 48°C with a written test record. Record all readings and actions.