Heavy smokers lived in the house. All duct work, return and supply plenums, returns and supply grills have been replaced. Walls wood trim kilzed and painted. Carpet ect. replaced. No problem with smoke odor during heating season, all work done during last cooling season. During spring when heat or cooling not needed odor returned. The only thing not replaced or kilzed is the supply boots and I have recommended that be done. I wonder if the furnace (80% gas) cabinet and cooling coil box insulation or heat exchanger is causing the smell. PS the blower assembly was removed, cleaned and reassembled and the cooling coil was washed also. What can the insulation be cleaned and or treated with other than replacing it. Seems when the humidity increased in the house the smell returned. Owner is running the AC now to see if this reduces the odor but does not want to sell the property without being able to be satisfied the odor will not return. It is an ethical thing to him in the sell of the property.