A hammer assembly including a hammer housing and a work tool movably supported in the hammer housing is provided. A gas chamber is defined in the hammer housing and contains a compressible gas. An accumulator assembly includes an interior space. A barrier divides the interior space into a first interior portion containing a compressible gas and a second interior portion configured to receive a pressurized fluid. The barrier is configured to be movable in response to changing the amount of pressurized fluid in the second interior portion and such that movement of the barrier varies the volume of the first interior portion. The first interior portion is in communication with the gas chamber. A control valve assembly is configured for selectively placing the second interior portion of the accumulator assembly in communication with a pressurized fluid source.