Assembly comprising a gas valve for providing gas, a fan for providing air, a gas/air mixer for providing a gas/air mixture, and a burner door for connecting the assembly to a gas burner; wherein the gas valve comprises a gas valve housing providing a gas inlet through which the gas valve is connectable to a gas pipe, and further providing a gas outlet through which the gas valve is connectable to the gas/air mixer; wherein the gas/air mixer comprises a gas/air mixer housing providing a gas inlet through which the gas/air mixer is connectable to the gas outlet of the gas valve, providing an air inlet, and further providing an gas/air mixture outlet; wherein the fan comprises a fan housing and a fan motor carrying a fan impeller, wherein the fan housing comprises a fan plate housing part carrying the fan motor and a fan impeller housing part, characterized in that the fan plate housing part and the burner door are commonly provided by one monolithic element, the fan impeller housing part comprises grooves for receiving a non-return valve; the non-return valve comprises a valve frame made from a rigid material and a valve flap made from an elastic material; the non-return valve is inserted into the grooves of the impeller housing part through the valve frame with the valve flap being attached to the valve frame. a gas valve for providing gas, a fan for providing air, a gas/air mixer for providing a gas/air mixture, and a burner door for connecting the assembly to a gas burner; wherein the gas valve comprises a gas valve housing providing a gas inlet through which the gas valve is connectable to a gas pipe, and further providing a gas outlet through which the gas valve is connectable to the gas/air mixer; wherein the gas/air mixer comprises a gas/air mixer housing providing a gas inlet through which the gas/air mixer is connectable to the gas outlet of the gas valve, providing an air inlet, and further providing an gas/air mixture outlet; wherein the fan comprises a fan housing and a fan motor carrying a fan impeller, wherein the fan housing comprises a fan plate housing part carrying the fan motor and a fan impeller housing part, characterized in that the fan plate housing part and the burner door are commonly provided by one monolithic element, the fan impeller housing part comprises grooves for receiving a non-return valve; the non-return valve comprises a valve frame made from a rigid material and a valve flap made from an elastic material; the non-return valve is inserted into the grooves of the impeller housing part through the valve frame with the valve flap being attached to the valve frame. wherein the gas valve comprises a gas valve housing providing a gas inlet through which the gas valve is connectable to a gas pipe, and further providing a gas outlet through which the gas valve is connectable to the gas/air mixer; wherein the gas/air mixer comprises a gas/air mixer housing providing a gas inlet through which the gas/air mixer is connectable to the gas outlet of the gas valve, providing an air inlet, and further providing an gas/air mixture outlet; wherein the fan comprises a fan housing and a fan motor carrying a fan impeller, wherein the fan housing comprises a fan plate housing part carrying the fan motor and a fan impeller housing part,