I have a Coleman self contained a/c and it's tied into the ductwork shared by a propane furnace. My problem is that it doesn't blow hard out of the vents. But as soon as I open the service panel, it works fine and will continue to after the panel is replaced. It lasts 2-3 weeks, and then reverts back to low speed. I'm guess there's some sort of damper in the ductwork to keep heat from escaping in the winter. Would this be in the a/c unit, or in the ductwork underneath?