A powered nailer includes a power source including a driving element reciprocating within a cylinder between a prefiring position and a fastener driving position, the latter occurring when the driving element engages a bumper disposed at the bottom of the cylinder. A reciprocating valve element defines a combustion chamber in fluid communication with the cylinder, and is configured for receiving a dose of fuel and air prior to a user-generated ignition. A return chamber at least partially surrounds the cylinder and is in fluid communication with the cylinder, being configured for receiving a supply of pressurized air generated by the driving element as it moves from the prefiring position to the fastener driving position, the pressurized air acting on an underside of the driving element for returning it to the pre-firing position.