air flow problem. unit kicking out on overheat limit switch because air is not circulating well enough. make sure all path for air flow is open from the output of the heat exchanger, through the drum, through the filter system, and back to the intake of the heat exchanger and make SURE all covers are in place. Pay careful attention to the housing that holds the filter. Lint is bad to build up behind the filter and go un-noticed. If air does not flow freely the heat will build up inthe heat exchanger and trip the limit switch open.