The British Standard BS845: 1987 describes the methods and conditions under which a boiler should be tested to determine its efficiency. For the testing to be done, the boiler should be operated under steady load conditions (generally full load) for a period of one hour after which readings would be taken during the next hour of steady operation to enable the efficiency to be calculated.

The efficiency of a boiler is quoted as the % of useful heat available, expressed as a percentage of the total energy potentially available by burning the fuel. This is expressed on the basis of gross calorific value (GCV) .

This deals with the complete heat balance and it has two parts:

        Part One deals with standard boilers, where the indirect method is specified

        Part Two deals with complex plant where there are many channels of heat flow.

        In this case, both the direct and indirect methods are applicable, in whole or in part.

ASME Standard: PTC-4-1 Power Test Code for Steam Generating Units

This consists of

        Part One: Direct method (also called as Input -output method) U 

        Part Two: Indirect method (also called as Heat loss method)

IS 8753: Indian Standard for Boiler Efficiency Testing

Most standards for computation of boiler efficiency, including IS 8753 and BS845 are designed for spot measurement of boiler efficiency. Invariably, all these standards do not include blow down as a loss in the efficiency determination process.

Basically Boiler efficiency can be tested by the following methods:

        (1)The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.

        (2)The Indirect Method: Where the efficiency is the difference between the losses and the energy input.