11.   The best way to assess the condition of the duct system is with a blower test performed by a qualified professional.

12.   The popping is telegraphed throughout the duct system, so it is heard close up and far away.

13.   Unless there is a way of cleaning the ducts of oil along their full length, I think that the entire duct system must be replaced.

14.   What is happening perhaps is that there is an inadvertent crossover in the duct system.

15.   With a forced-air system, the central heating unit heats air that moves through a duct system around the house.