An instantaneous boiler

• A. Heats water as needed.
• B. Stores hot water in a tank.
• C. Runs only on solar energy.
• D. Maintains a constant high temperature.

Answer: (A)

Instantaneous boilers heat water only when there’s a demand, turning on as you open a hot water tap and heating the water as it flows through the unit. Since there’s no storage tank, there’s little or no standby heat loss, and hot water is provided on demand. That’s why the best description is that it heats water as needed.

Storing hot water in a tank describes a storage water heater, which keeps a reservoir of hot water ready. Running only on solar energy isn’t inherent to an instantaneous boiler, which is typically gas or electric and operates regardless of solar input. Maintaining a constant high temperature would imply keeping water hot all the time, which contradicts the on-demand, on-flow nature of an instantaneous system.