Couldn't you determine whether the sanitary sewer system was connected to the A/C ductwork by some other method?
Where I used to work, our facilities manager chunked a smoke bomb into the A/C return air handler. Immediately HUGE volumes of smoke shot up through the bottom of the three mainframes in the computer room. Smoke detectors went nuts. Operators pulled the emergency power-down handles. About a zillion fire trucks surrounded the building and demanded the release of the hostages. Confusing and contradictory instructions were bellowed out over the PA system. People screamed "Run for your life" and "We're all gonna die!". Some sat at their desk and wept.
All in all, a fun afternoon.