What Happens When the Bedroom Doors Are Closed
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.
Isn’t One Big Central Return Good Enough?
Here’s what can happen when a forced-air system doesn’t have adequate return-air pathways:
- When the furnace is operating, it pushes conditioned air into each bedroom.
- If the bedroom doors are closed, there’s no easy way for the air to get back to the return-air grille in the hallway. As a result, each bedroom becomes pressurized, forcing air into cracks in the bedroom walls and ceiling. During the winter, this humid interior 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 starved for air. Since the hallway and living room are now depressurized, air is pulled from the attic into the hallway through cracks in the ceiling.
During the winter, the air that is pulled from the attic is cold, and the cold air increases the heating load. During the summer, the air that is sucked indoors is hot and humid; this infiltration increases both the sensible and latent load on the air conditioner.
There are three possible ways to solve the pressurized-bedroom problem. Each bedroom needs either:
- A return air grille ducted back to the furnace;
- A through-the-wall transfer grille connecting the bedroom and the adjacent hallway; or
- A crossover duct (a jumper duct) connecting a ceiling grille in the bedroom with a ceiling grille in the hallway.
The best of these three solutions is the most expensive: install a return grille ducted back to the furnace in every conditioned room of the house. Through-the-wall transfer grilles are a cheaper solution, but they have a major drawback: noise transmission. Homeowners don’t want people in the hallway to hear what goes on in the bedroom.
Most contractors who bother to address the pressurized-bedroom problem install crossover ducts. As typically installed, a crossover duct connects a ceiling grille in each bedroom with a nearby ceiling grille in the hall. Although a crossover duct will transmit sound, it provides more muffling than a through-the-wall transfer grille.
However, crossover ducts have all of the disadvantages of any attic duct. To prevent unacceptable heat loss or heat gain, it’s best to limit the installation of crossover ducts to homes with cathedralized attics — that is, homes with insulation that follows the roof slope.
Why Can’t I Just Undercut the Door?
Undercutting a bedroom door won’t solve the pressurized bedroom problem. Even a very large gap — a gap of 1 inch between the finish floor and a 30-inch-wide door — can handle only 47 cfm of return air at a maximum 2.5 Pascal pressure difference. That’s adequate for only a tiny room measuring about 75 square feet.
So if your room just doesn’t seem to be getting enough air, check for a return, and if you have, with the furnace running place a tissue over the grill. If it doesn’t pull the tissue in, there could be a problem.