A technician working in a new 200-unit Fort Collins apartment project encountered heat spilling into one apartment from its water heater flue. The apartments were stacked: the ground-floor units were built over vented crawl spaces; the upper units had vented attics above. Furnaces were located in closets in the apartments. Leaky supply ducts running through the crawl spaces and attics were the root cause of the problem. The heating distribution system was no longer a closed loop. Each time the furnace came on, some warm air delivered through supply ducts was blown into the crawl space or the attic. This meant the single return, located right at the furnace, was trying to draw more air from the apartment than the supply ducts delivered; as a result, the apartment became depressurized. Whenever any one of the seven supply registers was closed by occupants, negative pressure within the home grew even stronger, strong enough to overpower the water heater’s draft and continuously backdraft all combustion byproducts. Once re-burned by the water heater, those byproducts can create carbon monoxide, which can then be drawn into the furnace and recirculated within the living space. Health complaints by residents in other apartments suggested this problem was likely present in other units as well.