Duct Doctors Write Prescription for Efficiency

Various research projects over the last ten years have shown that air escapes from ducts in virtually every home with forced air heat. On average, duct leakage cuts heating system efficiency by 30 percent. Plus occupants find it more difficult to stay comfortable.

While no one disputes the problem, little has been done to correct duct leakage in residential buildings. Houses built today are just as leaky as ever. To address the problem, the Bonneville Power Administration sponsored a project to provide hands-on training to a small group of heating contractors. Their goal was to build homes with exceptionally tight duct systems. They solicited heating contractors in Oregon, Washington, Idaho and Montana. Contractors received training in the classroom and on site.

The garage is a poor location for the air handler, because return side leaks can pull auto exhaust and fumes from stored chemicals into the house. Typically, more air leakage occurs near the air handler than in the ducts.

"Average duct leakage was reduced to 78 cfm from the typical 300 to 400 cfm," said Ted Haskell, energy agent with the Oregon State University Extension Service. "The project showed that significant improvements are possible," he said. "The contractors were able to reduce air leaks by 80 percent compared to typical houses."

The cost to achieve these savings averaged $200 to $300. "While some contractors may not want to invest $300 to reduce leakage to 50 cfm," said Haskell, "it's possible to spend half that amount and still get most of the savings."

After all the training and testing, Haskell and the other duct doctors made of list of remedies for contractors.

System Design

Never use building cavities as ducts.

No matter how hard you try to seal a panned joist or wall cavity, you'll never make it tight enough. In fact, one contractor convinced the project's organizers that he could seal a panned joist. He failed even after concerted effort. The three houses with panned joists were all among the top five leakers. One house had 460 cfm of duct leakage.

Building cavities make poor ducts because:

Locate as much of the air distribution system as possible inside the conditioned space.

Leaks that occur into the conditioned space are much less serious than leaks to the outside. During the design phase, find ways to route ducts within the building envelope. The air handler is most important because it is the source of most leaks. The garage is a bad location for air handlers because return side leaks can suck gases from the garage into the house.

Seal the electronic air cleaner.

Since this device always goes on the return side of the air handler, leaks here draw air into the system. This is another good reason to locate the air handler away from any potential source of indoor air pollution. Basements, crawlspaces, attics and garages all contain potentially harmful airborne particles and gases. Wherever it's located, carefully seal all joints and openings. During this project, contractors encountered a brand of air cleaner that could not be sealed because it was designed to draw air from the surrounding space.

Allow enough clearance so that all joints can be accessed for sealing.

To apply mastic, the installer needs enough space to handle a small paint brush. The biggest air leaks are usually at the air handler. Because it tends to be pushed into a corner, many of these serious leaks aren't accessible for sealing.

Plan ahead for thick duct insulation.

Wherever ducts run through unconditioned areas, allow enough space for the specified insulation. Experts recommend insulation values up to R-11 (3-1/2 inches). R-8 insulation (2 inches) is common for many energy codes. Where ducts pass through concrete slabs or foundation walls, allow room for rigid insulation. Placing insulation before the pour makes a clean detail.

Plan a pressure balanced system.

Every room with a supply register and a door must have an adequate return flow that is not blocked when the door is closed. One option is a separate return duct to each room. Another option is passive pressure relief, which allows air to flow freely between rooms via undercut doors, transoms, transfer grilles and crossover ducts. Provide one square inch of free vent area for each CFM of supply air delivered to the room.

Duct Sealing

Use duct mastic.

It may be messy but duct mastic works. Here are a few tips for using mastic:

Use mechanical fasteners.

Three sheet metal screws should secure each metal-to-metal connection. Use two compression straps to join flex duct to metal duct collars, boots or splices. First, butter the collar with mastic. Then fasten the inner liner to the collar with the strap. Use a second strap to fasten the outer layer of the flex duct.

Use compatible products.

When using mastic with flex duct, use products that are approved by the manufacturers of each product.

Pay attention to the air handler.

Half or more of the duct leakage occurs at the air handler. The biggest leak occurs where the main trunk or plenum joins the cabinet. Because the cabinet is often tucked into a corner or nook, access can be difficult. Seal cabinet seams and duct joints with mastic. For a better appearance, try "tape applied mastic." This has a foil backing with a layer of butyl sealant at least 15 mils thick. Cover the air handler's access panels with gray cloth tape, so it's easier to change the filters.

Use a duct tester.

To test duct tightness and find leaks, you need a Duct Blaster. Using this device you can test duct sections during installation, while the joints are still accessible for sealing. For more information on the Minneapolis Duct Blaster call The Energy Conservatory.

House Pressure and Flow Balancing

Check house pressures near all combustion appliances.

Perform the test with bedroom and bathroom doors closed. Flue gases from standard water heaters, wood stoves, fireplaces and other combustion devices can't exhaust properly under strong negative pressures. For example, a gas water heater should not be exposed to negative pressures in excess of -5 pascals and the limit for a fireplace is -3 pascals. A power vented gas water heater tolerates greater negative pressures, up to -15 pascals. Armed with a manometer, you can compare the air pressure near these devices with the pressure outside. If pressure conditions pose a risk, make adjustments before the home owner complains. Better yet select sealed-combustion equipment that doesn't require indoor air for venting.

Measure and balance air flow for each room with a closeable door.

Balance flows with dampers at the takeoff end of each branch duct. Restricting flow at the registers increases pressure-and air leakage-in the duct.

Getting the Business

"Heating contractors find themselves in a very competitive situation," said Haskell. "When $50 can lose a bid, you have to watch costs carefully." However, to solve the duct leakage problem, heating contractors must put extra time into system design and air sealing. The key is a duct tester, such as the Duct Blaster. It allows workers to check their work, learn where leaks occur and evaluate the most effective sealing methods. A duct tester also helps sell customers on the extra costs of duct sealing. Some heating contractors have enough interest in stopping duct leakage to justify the expense. Utilities and energy agencies may offer duct testing services.

"If a heat pump malfunction caused a 30 percent reduction in efficiency," said Haskell, "contractors wouldn't hesitate to spend the money to fix the problem. The 30 percent penalty from duct leakage should be no different."

Details, Details

All references to leakage are given under test conditions at 50 pascals air pressure. The actual pressure in ducts ranges from as little as 10 pascals at the registers to over 100 pascals near the air handler. The leakage numbers may or may not apply to actual conditions. All measurements reflect leakage from ducts to the outside of the building.

The "Improved Air Distribution Systems for Forced Air Heating" project was funded by the Bonneville Power Administration and operated by the state energy offices in Oregon, Idaho, Washington and Montana. For a copy of the report call the BPA Public Information office 503-230-7334 or fax to 503-230-4470.