Abstract: A power vented water heater has a radiant gas burner located in a combustion chamber disposed beneath the storage tank portion of the water heater, and a draft inducer fan supported above the top end of the storage tank by a one piece molded plastic adapter fitting having a tubular body interconnecting the upper end of the water heater flue to the inlet of the fan, and a transverse outlet leg. A combustion gas discharge pipe is connected to the fan outlet, and an air intake pipe has a first portion connected to the outlet leg of the adapter fitting to deliver combustion gas cooling air thereto, and a second portion coupled to the burner inlet to deliver combustion air thereto along with gaseous fuel introduced into the second portion of the air intake pipe from an externally mounted gas valve. Integral orifice structures within the adapter fitting automatically maintain a predetermined ratio among the combustion gas, cooling air and combustion air flows.