Harman's manual specificly states to not install a manual damper in the flue pipe. Depending on how well your chimney draws, how consistant the draw is, and several other variables, you may not need a baro damper, you may be able to control your fire very well just with the air inlet controls [spinner knobs] on the ashpan door. [as long as the gaskets on the doors are in good shape]
A baro is a good idea, and once set up is a good device to even out the changes in draft from temperature changes, wind, and fire output changes.
Hope this helps. Greg L