NOTE See Clause 3 for a definition of the terms used below. Heat cost allocators in accordance with this standard are used for the registration of the proportionate thermal output of radiators in consumer units. If an account unit comprises consumer units of different types (e. g. technically different types of heating systems or differences due to the consumer behaviour, e. g. industrial plants as opposed to private apartments), it could be necessary to divide this account unit into groups of users. Heat cost allocators enable the determination of the heat consumption only of each radiator in a consumer unit as a share of the total heat consumption of the account unit or user group (see Clause 4); it is therefore necessary to determine this total heat consumption either by measuring the consumed fuel quantity or the amount of heat delivered (the latter by means of a heat meter, for example). For the appropriate use of the heat cost allocators in accordance with this standard, the heating system needs to: correspond to the state of the art at the time of installation of the heat cost allocators; be operated in accordance with the state of the art (see A.1). Heat cost allocators in accordance with this standard shall not be used for heating systems where the temperature of the heating system falls below or exceeds the temperature limits of the heat cost allocators, where the rating factor for the thermal output, KQ, cannot be clearly specified or where the heating surface is inaccessible. This applies usually to the following heating systems: floor heating; radiant ceiling heating; flap-controlled radiators; radiators with ventilators; fan-assisted air heaters; heating systems with steam-operated radiators.