A return air opening is located inside a closet containing a gas water heater. Why is this reported?

A return air opening is the intake side of the air handler, and it is under negative pressure whenever the blower runs. Placing that intake in a closet with an atmospheric gas water heater sets two systems against each other: the blower is pulling air out of the closet while the water heater needs that same air for combustion and needs a positive draft to carry flue gases up the vent. The blower wins, because it moves far more air. The result is that the closet is depressurised, the water heater's draft can reverse, and combustion products — including carbon monoxide — are drawn into the return and distributed through every supply register in the house. This is why return air is prohibited from spaces containing fuel-burning appliances, and it is one of the higher-consequence HVAC defects an inspector encounters.

Return air must not be taken from a space containing a fuel-burning appliance. The blower depressurises the closet, which can reverse the water heater's draft and pull combustion products into the return for distribution throughout the house.

Return air openings must not be located in rooms or closets containing fuel-burning appliances, because blower-induced depressurisation can cause backdrafting. Combustion air for such appliances is supplied separately through sized openings to outdoors or to adequately large adjacent spaces.

Never Return From a Burner Room. The blower pulls harder than the flue pushes, so the exhaust comes back inside and rides the ducts.

The same reasoning applies to leaky return ducts running through a garage or a furnace closet — they depressurise the space and can draw in exhaust or fumes.

• •Treating the issue as one of closet size or combustion air volume
• •Confusing return location rules with return height guidance
• •Reading the water heater as an obstruction to airflow