In my 2009 article, “Return-Air Problems,” I described a common flaw with many forced-air heating and cooling systems: the lack of a good return-air pathway from bedrooms. In homes with this flaw, return air often struggles to find its way back to the furnace or air handler. The result: room-to-room pressure imbalances that may lead to uneven room temperatures, comfort complaints, higher energy costs, and even moisture problems in walls and ceilings.

When a furnace comes on, heated air is pushed through supply ducts to registers in each heated room in a house. If the forced-air system is properly designed, the house includes pathways to convey conditioned air back to the furnace to be heated again, in a kind of continuous loop.

While most HVAC contractors install ducts to deliver conditioned air to every room in a house, they often neglect to provide an adequate return-air path from each room back to the furnace. Typically, most rooms don’t have a return-air grille; instead, there’s often just a single large return-air grille in the living room or a central hallway to serve the whole house. That means that all of the air needed by the home’s forced-air system has to be pulled through that single grille before it can be heated by the furnace or cooled by the air-conditioning system.

Homes with return-air problems

When a bedroom door is closed in a house that lacks adequate return-air pathways, the bedroom becomes pressurized, forcing air into cracks in the bedroom walls and ceiling. During the winter, this humid indoor air can contact cold surfaces in the wall, leading to hidden condensation and even mold. Meanwhile, the big return-air grille in the hallway is probably starved for air. Since the hallway and living room are now depressurized, air is pulled from…