With all the woodworking and spackeling I’ve been doing lately, I was worried that the furnace was getting covered in dust.  It only takes a thin layer of dust to wreck the thermal efficiency of the heat exchanger.  I do have a filter in there of course, but It doesn’t seal particularly well, so I was though it might be a good time to give it a cleaning.  It’s easy enough to take apart, although remembering where all of the wires went would have made putting it back together easier.  The fan was pretty dirty, but surprisingly the heat exchanger was clean as a whistle.