Water Heaters: Sizing, Recovery and the Safety Devices That Matter

Water heaters are usually specified by storage volume, but two units of the same capacity can behave very differently in service.

Recovery rate matters as much as volume

Recovery rate is how quickly the heater reheats after the stored hot water is drawn off. A small tank with a high recovery rate can outperform a larger tank with a slow one where demand comes in short bursts. Where several outlets are used at once — a villa's morning peak, for example — recovery rate is often the limiting factor.

Sizing to the demand pattern

Estimate simultaneous demand, not total daily consumption. The question is how much hot water is needed at the busiest moment and how fast the heater replaces it, rather than how much is used over a day.

The safety devices are not optional

A storage water heater must have a temperature and pressure relief valve. Heating water expands, and in a closed system that expansion has nowhere to go. Without a functioning relief path, pressure rises inside a sealed vessel.

The relief valve needs a discharge pipe routed to a safe, visible location. A relief valve that discharges into a concealed void hides the very symptom that warns of a fault.

An expansion vessel is required in many closed systems to absorb the expansion before the relief valve operates. Where a relief valve weeps regularly, the usual cause is a missing or failed expansion vessel — not a faulty valve.

Installation and maintenance points

• Provide isolation valves and a drain for servicing
• Check the relief valve periodically; it can seize if never operated
• Confirm electrical supply, earthing and bonding for electric units
• Set the thermostat with scalding risk and scale formation both in mind
• Descale in hard water areas before performance drops noticeably