On what basis are instantaneous hot water heaters most commonly sized?

Instantaneous hot water heaters are primarily sized based on the number of fixture units that they are expected to serve. This is because fixture units provide a standardized way of calculating the demand for hot water in a plumbing system based on the types and quantities of fixtures connected to the heater.

Each plumbing fixture is assigned a specific number of fixture units that corresponds to the anticipated flow rate and usage patterns, enabling designers to assess the total demand during peak usage. By understanding this demand, the instantaneous hot water heater can be correctly sized to deliver the necessary flow rate and temperature rise efficiently without running out of hot water.

Sizing based on the volume of stored water is not applicable here, as instantaneous heaters do not store water but instead heat it on demand. Similarly, while surface area of pipes can affect system efficiency, it is not a primary metric for sizing the heater itself. The type of heating source is relevant to performance characteristics but does not determine the size in terms of flow rate required by the fixtures. Therefore, fixture units provide the best measurement for ensuring adequate supply and efficiency of the instantaneous hot water heating system.