If the furnace utilizes natural gas — with a specific gravity that is lighter than air — any gas below the orifice openings will slowly be replaced by air inside the manifold. If the furnace utilizes liquid propane turned into gas — with a specific gravity heavier than air — it’s the gas in the upper half of the manifold that gets replaced by air. For furnaces in the horizontal configuration, it’s all the gas below the top orifice or above the bottom orifice, again depending on whether the gas is natural or LP. At a minimum, half of the residual gas inside the manifold will drift out of the manifold and may get picked up by a person with a sensitive sense of smell and/or an electronic gas leak detector.

That doesn’t indicate a failed gas valve, just a normal transfer of gas/air in the manifold during an off cycle. The amount of odorant (mercaptan) in the gas directly effects the strength of the odor, which can cause a homeowner to think that a problem just occurred when the condition has always been present — it just wasn’t detected. This is not an unsafe condition, as the gas/air mixture created by this process is too diluted to ignite or sustain combustion.

Any furnace with induced draft combustion may occasionally exhibit unburned gas odor near the furnace in the off cycle. If the gas delivery system is not leaking and the furnace is working correctly, it is very likely that the condition described above is the result of the gas odor.