This past year "winter" we have answered calls for no heat on residential customers. On two of these calls the gas furnace would not come on. So the tech would trouble shoot by jumping out sensors on the furnace to see if they were malfunctioning.
All sensors were working. However when placing the hand near the intake at the flue you could feel a large draft pushing in "not out" for CO gases to vent. The technician discovered, in the back room of the house the owner had a cannabis grow operation. In that room a large charcoal ventilation system "motor" was installed. This is to reduce the cannabis smell and vent outdoors. This ventilation system was approx 1000 CFM.
On many new and not so old gas furnaces a pressure switch is installed "safety" . If the furnace cannot vent or sees to much pressure it will not fire up. So these growers are not considering health and safety. Big surprise right! Turn off the charcoal vent motor and the furnace would come on. So do you want heat or the smell of cannabis?