In homes with a single central return-air grille, return air often struggles to find its way back to the furnace. The result: room-to-room pressure imbalances that 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 return-air ducts to convey air back to the furnace to be heated again, in a kind of continuous loop.
While most HVAC contractors install ducts and registers 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. 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.
Here’s what can happen when a forced-air system doesn’t have adequate return-air pathways:
Jim Cummings, a senior scientist at the Florida Solar Energy Center (FSEC), has measured pressure differences arising from unbalanced forced-air systems in over 200 Florida houses. “The typical positive pressure in a bedroom with a closed door is on the order of 15 pascals — ranging from a low of plus 5 pascals to over 45 pascals,” said Cummings. “That’s a tremendous amount of pressure. Air is being pushed out of these rooms into the attic and through the walls towards the exterior, while the central zone of the house…