My furnace is behind a door with a screen on it. I opened the door and removed the furnace front cover (2 screws), which exposed the heat exchanger, along with the plenum (sheet metal box with 5 holes for the 5 round heat duct going out). The plenum was under the floor, but I could just squeeze a shop vac hose past the heat exchanger and into each of the holes to blow it out. Watch your hand as the edges of the heat exchanger are sharp.
I put the shopvac hose into each of the ducts, then put an upside down box over the floor vent to keep stuff from blowing all over.
I have had to do this every year. One time there was a big mouse nest in one of the ducts which blocked it completely. I had to use a piece of romex cable with the end looped back so there was no sharp edges to break up the nest. The ducts are VERY thin so they will puncture easily.
Steve
