This Part of this European Standard specifies a concise procedure for conducting thermal performance tests, using the indirect (losses) procedure for boilers for steam or hot water. Test results are based on either the gross or net calorific value of the fuel. This concise procedure provides a convenient means for assessing boilers which are thermodynamically simple, i.e. having a single major source of heat input and a simple circuit for water, steam or high temperature heat transfer fluid. NOTE 1 The use of the direct method is not advocated, because the estimated measuring error is three to four times greater than with the indirect method. NOTE 2 An acceptance test may be required: a) after the commissioning of new plant or after the recommissioning of modified plant in order to verify compliance with a specification or contractual obligation; b) whenever the user wishes to determine the current performance of the plant either on a routine basis or due to change of load or other operating conditions or when a change of fuel or a modification to the plant is being considered; c) whenever the user wishes to check combustion conditions. Regular tests in accordance with this European Standard will enable boiler plant to be monitored in normal operation for optimum efficiency in the interests of fuel conservation. This procedure does not cover condensing boilers. The application of boilers, where heat is extracted from waste gases is specified in annex B.