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1 ScopeThis Part of BS 845 describes a concise procedurefor conducting thermal performance assessments,using the indirect (losses) procedure, to give resultswithin a tolerance of ± 2 percentage points1) forboilers for steam, hot water or high temperatureheat transfer fluids and for presenting the results intabular form. Test results are based on either thegross or the net calorific value of the fuel.This concise procedure provides a convenient meansfor assessing boilers which are thermodynamicallysimple, i.e. having a single major source of heatinput and a simple circuit for water, steam or hightemperature heat transfer fluid, and that do notcondense moisture out of the flue gases.NOTE The titles of the publications referred to in this standardare listed on the inside back cover.2 DefinitionsFor the purposes of this Part of BS 845 the followingdefinitions apply.2.1assessed lossesany thermal losses established from predetermineddata2.2gross calorific valuethe amount of heat liberated by the completecombustion, under specified conditions, of unitvolume of a gas or unit mass of a solid or liquid fuelin the determination of which the water produced bycombustion of the fuel is assumed to be completelycondensed and its latent and sensible heat madeavailable (see BS 526)2.3net calorific valuethe amount of heat generated by the completecombustion, under specified conditions, of unitvolume of a gas or unit mass of a solid or liquid fuelin the determination of which the water produced bythe combustion of the fuel is assumed to remain asa vapour (see BS 526)2.4heat inputthe heat content of the fuel used during the testbased on the gross or net calorific value plus thesensible heat in the fuel above ambient temperature2.5heat outputthe heat gained by the heat carrier from the boilerduring the period of the test

2.6measured lossesany thermal losses calculated from actualmeasurements made during the test2.7indirect procedurethe determination of thermal performance by theassessment of the thermal losses and the measuredthermal input or output. Major thermal losses aredetermined directly from measured quantities;minor losses are determined directly or assessedand in the case of radiation and convection lossesAppendix B gives values2.8radiation, convection and conduction lossesthe losses from water, steam, combustion air,or gas-backed surfaces prior to the flue gastemperature measurement point and directly fromflame to the floor and surroundings of the unit2.9test errorthe combined error due to sampling, measurements,calculations and assumptions used to obtain testresults. The overall effect may be positive ornegative2.10thermal efficiencythe difference between 100 % and the totalpercentage losses based on either the gross or netcalorific value of the fuel. This is equivalent to theratio of the useful heat output to the heat inputexpressed as a percentage2.11turn-down ratiothe ratio of maximum and minimum fuel inputs forcontinuous firing in unit time specified by themanufacturer. This ratio can also be expressed interms of boiler output provided the appropriateefficiencies are known3 GeneralNOTE Where a thermal performance assessment is to becarried out after the commissioning of new plant or after therecommissioning of modified plant, it is necessary for the partiesconcerned to decide at the plant tendering or ordering stage onthe test data required and on the test accuracy and hence theinstrumentation to be used (see Appendix B).It is also necessary for the parties concerned to decide whetherthe test is to be carried out by the contractor or by an independentbody and by whom it is to be witnessed.3.1 Tests shall represent the intended method andsystem of operation of the plant under the intendedconditions of installation and normal operation.