The gasifying operation of the furnace causes the primary air supply holes in the furnace to plug up fairly quickly. This is because the smoke has no place to go when the furnace shuts off after having reached its upper setpoint. With no air being blown into the firebox, the hot wood fumes expand in the firebox and are drawn into the now-cooling primary air ducts all the way back to the air supply valves (mainly the primary air ducts). A sure sign of plugged-up primary air ducts are slow-to-heat flue temperatures and a smoky output from the chimney. Besides the unpleasantness of being downstream of a smoky chimney, plugged ducts cause increased wood consumption from inefficient operation.
It is important to regularly clean out the Reaction Chamber because excessive ash build-up could:
- prevent the flue gases from flowing out of the firebox
- prevent the thermocouple from reading the reaction chamber temperature correctly
- reduce the reaction time of the flue gases with the secondary air, thereby exacerbating smoky exhaust
- cause the steel plate covering the insulation in the reaction chamber door to warp
- cause ash build-up around the bypass door and chimney base.
Regularly cleaning the primary air ports is a very important a and should be done whenever the firebox is cool enough to enter. It's much easier to rod-out the ports when the creasote build-up is light. It gets progressively harder with increasing blockage. Severely blocked primary air passages can take hours to clean and several broken rodding-out wires - definitely not a fun job.
I originally did not shut down during the winter because is possible to shovel out the Reaction Chamber without first shutting down the furnace. My current practice to only start the furnace before 7 pm (when time-of-use electrical rates become Off-Peak) and only put in enough wood so that it will only last until 7 am (when TOU On-Peak rate begins). No more than every few days or so, the primary air ports should be rodded-out with a stiff wire. I use coat hangers cut up into even lengths but, while convenient, a coat hanger, isn't ideal because the rodding operation causes the wire to flex a lot and the cheap steel in coat hangers will break from metal fatigue. I've had to retrieve a broken piece of coat-hanger wire from inside the primary air passages more than once.
In addition to cleaning the primary air ports around the perimeter of the furnace, the primary air valve (made from a solenoid lifting a cap from a mitre-elbow) also needs cleaning. The creosote will collect in this passage and will either be a tarry mess or a glassy residue, depending upon its temperature and residence time. You will need a knife and chisel to scrape out the residue and a drill bit to clean out the air flow measuring tubes. A shop vac would be useful to vacuum out the creosote particles loosened by the knife and chisel.