My 2010 vintage Dimplex was doing that after it's first winter (I full time). It got a lot of use. I went to the Dimplex web site and d/l the instructions for removing the unit, opening it up, and cleaning out the air flow route including the heating unit itself. The air flow was restricted by lint and other assorted crud. It hasn't needed cleaning since (4 more years) so I attribute the problem to accumulated construction "crud". Their directions for this process were spot on including how many screws had to be removed for each part.