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(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. 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.
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 is depressurized and pulls air in from the attic. … A lot of this uncontrolled airflow is actually an exchange between the house and the attic.”
During the winter, the air that is pulled from the attic is cold, and the cold air increases the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load.. During the summer, the air that is sucked indoors is hot and humid; this infiltration increases both the sensible and latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. 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 gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss., 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.
Sizing crossover ducts
A variety of methods have been proposed for sizing transfer grilles and crossover ducts. According to Cummings, a transfer grille should provide 70 square inches of free area per 100 cfm of supply air ducted to the room; Cummings’ recommendation is sometimes called the “FSEC guideline.” Cummings says, “That guideline is based on calculations, confirmed by experiments in the field.”