Duct Losses Hurt Forced Air Heating System Performance and Cost You Money
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.
• Ducted air distribution losses cut heating and cooling efficiency on average 33%.
• 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 building’s air leakage rate can triple when the furnace blower is turned on.
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 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 increasing the duct insulation R rating.
• Direct air leakage causes another 33 percent of the loss. Liberal use of duct mastic cuts these losses.
• 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.
Duct Leaks
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, Air easily slips out around a rim joist or wall plate. You probably don’t think that a panned joist cavity in a basement would leak to the outside, but it often does.
Pressure Imbalance
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.
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.
No Leaks
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.
Supply Leaks
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.
Return Leaks
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 it’s 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.
This article appeared in Energy Source Builder #28
Jim Fleming Consultant
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jim@energyrater.biz
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