This British Standard gives recommendations and guidance for the sampling of water from closed-circuit heating and cooling systems in buildings and related infrastructure to maintain water quality and the integrity of pipework, plant and fittings, including: hydronic space heating and cooling systems; heat pump ground and water loops; thermal storage systems; closed-circuit condenser water systems; low-temperature district heating and cooling mains; generator cooling systems; and similar closed-circuit water systems. The standard gives recommendations for the design of the sampling programme during each phase of the system lifecycle, including the location and suitability of sampling points, frequency of sampling and record keeping, to facilitate the control of closed system water quality throughout the life of the building. It also gives guidance on: procedures for the collection, preservation, handling and storage of samples prior to analysis (by reference to BSENISO5667‑3); analytical methods (by reference to other British Standards and approved methods) and the statistical treatment of results; and the selection of the analytical parameters appropriate to the sampled system and life stage. This standard does not provide guidance on the interpretation of results or associated action levels. These matters are discussed in other standards and industry guides, e.g. BSRIA BG29. This standard is not applicable to sampling water from open circuit systems such as: cold water services (see BS7592 and BSISO5667‑5); open circuit cooling towers and cooling ponds; steam boiler plant (see BS6068‑6.7 and BS2486); or sprinkler systems. NOTE AnnexA gives recommendations for sampling points. AnnexB gives guidance and recommendations for appropriate laboratory analytical methods for water quality sampling, while AnnexC describes the application of a statistical method for detecting sulfate reducing bacteria (SRBs). AnnexD summarizes the minimum performance requirements for on-site analysis of common parameters using test kits and portable instruments. Guidance on the interpretation of sample results for pre-commission cleaning with respect to the guidelines in BSRIA BG29 is given in AnnexE.