This open letter sets out Ofgem’s understanding of the requirements of BS EN ISO 4064-2014 (“ISO 4064-2014”) and the Measuring Instruments Directive (“MID”) Annex MI-001, and whether heat meters that use flow meters that meet the requirement of these standards also meet the requirements stipulated in regulation 2 of both the GB Regulations and NI Regulations for a “class 2 heat meter”. The open letter is seeking views on this understanding, as well as feedback on Ofgem’s proposed course of action in relation to the these meters.