Applies to solar supported gas-fired appliances producing domestic hot water. Also relates to a system marketed as single unit or a system fully specified by a manufacturer that: - has a gas heat input not exceeding 70 kW; and - has a hot water storage capacity not exceeding 500 l; and - is equipped with at least one solar collector; and - is, with regard to the solar hydraulic circuit, considered as a forced circulation system (definition according to EN ISO 9488:1999).