Duct Losses Hurt Forced Air Heating System Performance
Despite the advances in heating and cooling equipment, heat loss from the average ducted air distribution system reduces the overall system efficiency by 30 percent. Research conducted around the U.S. leaves no doubt that forced air systems have serious problems.
Ken Baker, manager of the New Construction Section with the Energy Division of the Idaho Water Resource Department, ticks off a list of research findings that indicate the severity of the problem:
- In one study, homes with forced air distribution systems used 16 percent more energy than homes with zonal electric heat. All the homes were built to the Model Conservation Standards, so the insulation levels were similar.
- Ducted air distribution losses cut heating and cooling efficiency by 25 to 40 percent.
- In one study, the cracks and openings in ductwork represented 13 percent of the house leakage area. But when the furnace blower operated, ducts accounted for 70 percent of the air leakage.
- Duct leakage commonly reaches 350 cubic feet per minute during blower operation.
- During operation, air pressure inside ducts reaches 50 pascals (0.2 in. w.g.). That pressure can create 25 times more air leakage through a hole in the duct than the same size hole in the building shell. So, a one square inch hole in a duct is equivalent to a 25 square inch hole in a wall.
- A buildings air leakage rate can triple when the furnace blower is turned on.
Some of these findings come from the Residential Construction Demonstration Project, which conducted several studies over the years. RCDP is operated by state energy offices in Washington, Oregon, Idaho and Montana with funding from the Bonneville Power Administration (BPA). Other studies have been sponsored by the Florida Solar Energy Center and Pacific Gas and Electric.
Researchers reported excessive duct leakage in virtually every house they investigated. The problem is so widespread that energy utilities and agencies across North America are starting duct sealing programs.
Leaks in these ducts allow conditioned air (which has been heated or cooled) to escape or unconditioned air to slip in. Misguided air flow also upsets the pressure balance of the house, which generates leakage through the building shell, too.
Preliminary results from one RCDP study shows heat losses from a ducted air distribution system fall into three categories: conductive losses, direct air leakage and pressure imbalances.
- Conductive loss through the duct material itself represents about 38 percent of the losses from the forced air system. You can reduce these losses by insulating the duct to at least R-8.
- Direct air leakage causes another 38 percent of the loss. Liberal use of duct mastic cuts these losses. Specific recommendations for how and where to seal appeared in the June 1992 issue.
- The remaining 24 percent of the heat loss results from pressure imbalance inside the building created by improper air distribution. Proper duct sealing will help reduce the pressure imbalance. A bigger problem could be the layout of the supply registers and return grilles. This aspect of air distribution may be the most difficult to understand and then to correct.
Leaks are a problem whenever air escapes to the outside. Ducts running through attics, crawlspaces, garages, basements and other unconditioned spaces are common sources of these leaks.
One of the biggest problems is building cavities used as ducts, says Ken. Air easily slips out around a rim joist or wall plate. You probably dont think that a panned joist cavity in a basement would leak to the outside, but it often does.
Rooms with more return than supply flow experience a suction (called negative pressure) that pulls outside air into the building. That increases energy costs. But negative pressure also pulls airborne hazards into the house: radon from the soil, car exhaust from the garage, combustion gases from furnaces, water heaters and fireplaces, and breathable dust from attics and crawlspaces.
Rooms with more supply than return flow experience positive pressure that drives conditioned air to the outside. The air carries moisture into the building structure, which could be a serious problem in cold climates.
There are several possible solutions. Undercutting doors doesnt help much, unless the bottom of the door is knee high. Transfer grilles between rooms can help, although they allow noise to travel with the air. Another option is placing a return in every room that has a supply. Finally, a short duct can be run between zones, creating a by-pass around the closed door.
How can duct leaks disrupt air pressure in a building? The three drawings should save you some head scratching. This simplified presentation illustrates the basic effect of leaky ducts. The numbers represent generic units of air flow. Of course, real buildings experience all these effects at the same time, but one condition usually dominates in each room or zone.
If the duct system was perfectly designed and sealed, the same amount of air would enter the return grille and leave the supply registers. This is the ideal situation, and so it rarely happens.
When supply ducts leak, air escapes before it gets into the building. However, the blower still tries to pull the full amount of air out of the building. This creates negative pressure that pulls outside air into the building.
The return side of the system is under suction, so leaks here pull air into the ducts. The blower then forces more air into the building than its taking out (supply flow exceeds return flow), creating a positive pressure. Until recently, duct installers routinely skipped the taping of return ducts and plenums. It was considered a waste of time, since it was the supply ducts that delivered the goods. Perhaps this is one reason researchers find the most significant leaks on the return side.
Some experts claim closed doors have can a greater impact on pressure balance than duct leaks. Most houses have one or two centrally located returns with supply registers dispersed throughout the house. Closing a door blocks the air flow from the supply register to the return. This increases the air pressure in the room with the supply register. The room with the return has a negative pressure.
This article appeared in Energy Source Builder #28 August 1993