Which boiler type heats water on demand rather than storing hot water?

• A. Instantaneous boiler.
• B. Storage boiler.
• C. Condensing boiler.
• D. Hybrid boiler.

Answer: (A)

The key idea is whether hot water is produced on demand or kept in a tank. An instantaneous boiler heats water as you use it, flowing through a heat exchanger to deliver hot water the moment you open a tap. It has no separate hot-water storage tank, so you don’t keep a reserve of hot water at temperature. This tankless, on-demand approach is why it’s the best fit for “heats water on demand.”

In contrast, a storage boiler keeps hot water in a cylinder for later use; you draw from the stored hot water and it can waste energy through standby losses when the tank is hot but not being used. Condensing boilers describe how efficiently the boiler uses fuel (by condensing water vapor in the flue gases) and can be used with either storage or instantaneous systems. A hybrid boiler combines multiple heat sources, but the essential distinction about on-demand versus stored hot water is captured by the instantaneous type.