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.
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 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.
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.”
Some engineers prefer larger grilles. According to Paul Raymer, the chief investigator at Heyoka Solutions and former president of Tamarack Technologies in West Wareham, Massachusetts, a manufacturer of through-wall transfer grilles, it’s best to add 40% to the FSEC guideline. “We figure that with two grilles, instead of 0.7 square inch per cfm of supply air, you need one square inch, to account for the resistance of the two grilles,” said Raymer.
Armin Rudd, an engineer at Building Science Corporation in Westford, Mass., advises that transfer grilles need only 54 square inches of free area per 100 cfm of supply air. While Cummings prefers to keep room-to-room pressure differences at 2.5 pascals or less, Rudd is willing to accept room-to-room pressure differences of 3 pascals. (The 2.5-pascal standard has been mandated by the Florida Building Code since March 1, 2002; in almost every other state, building codes are silent on the issue.) Rudd’s rule of thumb assumes that the average bedroom door has a 1/2-in. by 32.-in crack at the bottom — a crack that contributes to solving the pressurized-bedroom problem.
According to Rudd, a 6-in. round jumper duct is adequate for a bedroom with 75 cfm or less of supply air. An 8-in. duct can balance a room with 120 cfm of supply air; a 10-in. duct balances 175 cfm of supply air; a 12-in. duct balances 225 cfm of supply air; and a 14-in. duct balances 300 cfm of supply air.
Why can’t I just undercut the door?
Undercutting a bedroom door won’t solve the pressurized bedroom problem. Raymer points out that 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.
There’s another problem with door undercuts: they’re not under the control of the HVAC contractor. The final gap depends on the choice of finish flooring and the way the door is hung; these details are usually determined by the finish carpenter, long after the HVAC contractor has left the job site.
First, design the system; then install it; then commission it
Of course, even well-designed jumper ducts will only work properly if the forced-air system is set up properly. When the HVAC system is commissioned, the supply airflow to every register in the house should be verified and individual balancing dampers adjusted, to be sure that the system meets the design air flows shown in the Manual D calculations. Although residential HVAC system commissioning should be routine, it is, alas, still rare.
For further discussion of ways to solve room-to-room pressure imbalances, see Peter Yost’s article, “Return to Sender – HVAC Return Pathway Options.”
Last week’s blog: “Hot-Climate Design.”