This Technical Specification provides guidance on the selection, use and maintenance of portable electrical apparatus conforming to EN 50379-1 [4] and EN 50379-2 [5] or EN 50379-3 [6] to: measure combustion flue gas parameters of appliances in domestic premises burning 1st, 2nd or 3rd family gases of the following description: Type A, Type B and Type C gas-fired appliances, except those appliances where the appliance instructions (or design, see 7.3.2.1), prohibit combustion sampling, and, all gas-fired appliances for which the appliance manufacturer has provided a purpose-designed combustion sampling point or specific sampling instructions, use as a diagnostic instrument to assist an operative: in confirming satisfactory combustion at the time of commissioning, in accordance with appliance instructions or national or local regulations or standards; in confirming satisfactory combustion at the time of servicing in accordance with national or local regulations or standards or following servicing in accordance with appliance instructions; in confirming satisfactory combustion following maintenance, in accordance with appliance instructions or national or local regulations or standards. NOTE 1 Type A, Type B and Type C classification of gas-fired appliances are defined in 3.1.2 and more fully in CEN/TR 1749 [2]. NOTE 2 Existing national or local regulations or standards conflicting with the guidance in this Technical Specification have precedence over this guidance. NOTE 3 It is not the intention of this Technical Specification to suggest that portable electrical combustion flue gas analysers are to be used as a substitute for normal service and maintenance carried out in accordance with the gas appliance instructions. Clause 9 describes how analysers can be used in conjunction with the appliance instructions. NOTE 4 EN 50379–1 [4] specifies general requirements for the construction, testing and performance of portable spot reading apparatus designed to check the combustion performance of appliances in domestic premises using commercially available fuels. NOTE 5 EN 50379–2 [5] is for apparatus intended to be used for statutory measurements. In several European countries, legal requirements exist for the performance of heating appliances (see EN 50379–1:2012 , informative Annex A [4]). Legal consequences resulting from performance measurements makes for strict requirements for the apparatus used (see EN 50379–1:2012 , normative Annexes B and C [4]). NOTE 6 EN 50379–3 [6] is for apparatus intended to be used for non-statutory applications, which allows for reduced performance requirements for the portable electrical apparatus. NOTE 7 This Technical Specification deals with the determination of levels of combustion gases carbon monoxide (CO), carbon dioxide (CO2) and/or oxygen (O2) in combustion products from gas-fired appliances. Combustion products from gas-fired appliances will contain nitrogen oxides (NOX), predominantly nitrogen monoxide (nitric oxide, NO) and nitrogen dioxide (NO2). This Technical Specification does not deal with the measurement of combustion products such as NOXand aldehydes.