My 10 year old Trane XE80 in my woodworking shop lately cannot put out enough heat to satisfy the thermostat, which yesterday was set at 72 degrees on a 37 degree day. The steel building is completely insulated, and this has never happened before at this outside temperature.

The furnace seems to function properly; it ignites, preheats, and the blower kicks in, but the output air does not seem as warm as in the past. The inside temperature would not go above 67 degrees, and the furnace ran constantly for seven hours.

Any ideas?

The burners and orifices are probably restricted for all the wood dust. Do you know the hazards of running a gas furnace in a wood shop?