What to check in tracing odor in heating system

Q. Our church's heating cooling system, in summer only, emits an offensive odor. Two different experienced plumbers, city water and sewer people, and the regular air-conditioning servicemen have checked everything they know and have come up with neither cause nor cure of the odor.

Return air ducts are under the concrete slab, supply registers are in the ceiling or walls. We are at wits' end.

What do you recommend to check the offensive smell in summer? William Finley Rehoboth Beach, Del.

A. A pinpoint solution at this distance is a tough assignment.

Oil heating systems are odor-producing suspect. Further, underground return air ducts below the slab tend to pick up moisture in summer. Humid stagnation may then promote mildew-caused odor. In winter these underground return ducts are kept dry by heated air. Hence, a possible explanation for a seasonal system odor.

I'd make absolutely sure that there is nary a drop of oil gaining access at any spot to the supply or return-air piping system. Is the church near a damp coastal shore?

Though we have never installed nor used one, Honeywell, 1885 Douglas Drive North, Minneapolis, Minn. 55422, makes a device called Odor Control System Y495. This system controls disagreeable odors by evaporating a small amount of odor counteractant into the air.

How about that? The fragrance interacts with the odor to neutralize it, lending only a faint trace of fragrance.

Another obvious option is to consult with a professional firm listed in the Yellow Pages under ''Deodorizing and Disinfecting.'' My guess: The culprit triggering the summer-only odor in the air system is mold.