Ductwork That Delivers

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Are your HVAC ducts going in according to a specified plan? Are they being tested?

December 01, 2002

 

Manual D calculations lead to different sizes of ductwork. Shown here are 4-inch, 5-inch, 6-inch and 7-inch supply ducts carefully sealed with mastic.

Are your HVAC ducts going in according to a specified plan? Are they being tested? If not, are you feeling lucky today? Because unsatisfactory ductwork delivery systems are common in most parts of the nation.

Consider this online statement from research scientists with Lawrence Berkeley National Laboratory: "A California duct system wastes about 40% of the power it consumes on a hot day." About two-thirds of those losses slip out duct leaks or poorly insulated ducts.

This is not just a California problem. In fact, California and Florida - which account for about 20% of the nation's housing starts - appear to lead the nation in the quality of ductwork installations. But while building codes have long stipulated "substantially tight ductwork," building industry trainers such as John Tooley, a senior scientist with Advanced Energy in Raleigh, N.C., say the quality of ductwork installations in most states leaves a lot of room for improvement. Regions with ductwork located in basements tend to have the leakiest ducts, while those with ducts in crawl spaces and attics typically pay the largest energy penalties.

Fortunately, a growing number of HVAC contractors and builders are attacking ductwork problems with help from a variety of sources. California's HVAC industry is moving forward faster than most.

Drivers for change

Codes get top billing among drivers for change. Mechanical and energy codes are being pushed by results from expanded field research. Ductwork in Florida might be the best in the country thanks to codes, utility activism and building science training.

 

The EPDM gasket in the left piece of the prototype Snap Duct System is pushed into the right piece (above middle) until it snaps into place, creating an airtight seal.

Consumer complaints are a growing factor. Last year a homeowner survey, "What Consumers Want," by Contracting Business magazine showed that comfort expectations and complaints have increased. Just more than half (55%) of respondents said they were very satisfied with their comfort systems - a number unchanged since 1994. But only 40% of respondents said all their rooms are comfortable, down from 51% in 1994. Once consumers are educated, the survey reported, they are willing to spend significantly more (as much as 30% to 50% of total system cost) to have comfortable, healthy and safe indoor environments.

Training and testing provided by third-party entities such as Building America, Masco Corp.’s Environments for Living, Engineered for Life, E.I.C. Corp.’s Comfort Home and energy utilities are helping accelerate improvements in HVAC ductwork.

Several builders acknowledge that mold is a recent driver for their improved ductwork designs and installations. There is a definite link between mold and ductwork leakage. Pressure problems within spaces (rooms and in-floor cavities) can pull moisture indoors or force moisture into the exterior shell. When that happens, mold can develop in unseen locations.

Just part of the picture

HVAC systems are the heart of a high-performance home. And, yes, they are complex, interactive systems. For that reason, a perfectly designed and installed ductwork system won't prevent all HVAC problems.

First, find an HVAC contractor and installers whose training includes good building science principles. They need to understand both ductwork and related house pressures. At the end of the day, they must be able to connect the dots that make their HVAC systems so crucial to healthy, safe, comfortable, durable and affordable housing.

Then comes properly sized heating and cooling equipment, based on calculations. Identify how much conditioned airflow each room needs. Finally, performance testing and commissioning must be part of the equation. Ductwork design and spec list

The Energy & Environmental Building Association's Builder's Guides - for cold, mixed, hot-humid and hot-dry climates - are widely used by Building America builders and anyone seeking systems-built design. Updated annually by Building Science Corp. (Westford, Mass.), each book lists recommendations for ductwork. Building America consulting teams are just now establishing a uniform set of HVAC performance standards. The following list of key ductwork design and performance specs is taken from these and other sources.

Design: Check all your sets of plans, and most of you won't find a single sheet that lays out the heating system. Nearly all systems are installed based on rules of thumb, current-practice thinking and whatever is on the HVAC truck. If every room in a house has one or two 6-inch supply ducts running to it, no one accurately designed the system.

Quality performance starts with good design. All duct sizing should be based on hard calculations from design manuals published by the Air Conditioning Contractors of America. The ACCA's Manual J identifies a home's heating and cooling loads on a room-by-room basis. Each room's target airflow (in cubic feet per minute, or cfm) at a given supply temperature should be specified on the plan. Manuals D, S and T provide material selection answers based on the length and type of ductwork involved, the selected supply takeoffs and boots, the number of elbows needed, plus types and designs of supply registers and grilles. Once in place, a well-designed set of ductwork likely will include supply duct sizes varying from 4 to 7 inches.

One final key design item: Don’t let your contractors use building cavities (e.g., panned floor joists) as return air cavities. "Any time you use building cavities as ducts, they leak," says Tooley. "You can end up with huge holes and pressure problems in the building." A law taking effect April 1 in Oregon will prohibit use of building cavities for return-air ductwork. "I love it!" Tooley exclaims.

Location, location, location: After nearly 20 years of studying building performance problems, Tooley says "the one ductwork-related change I would choose above everything else is getting the ductwork inside the conditioned space." The simple solution for homes with crawl spaces is to seal off those volumes from the outdoors and condition them (per the International Residential Code). That automatically brings all ductwork into the conditioned space.

For slab-on-grade homes, the "indoor ducts" rule involves redesign. Albuquerque, N.M.-based Artistic Homes builds a dropped, drywalled box in its hallways that holds the supply plenum. Some Florida builders working with Building America consultant Steven Winter Associates have used a specially designed roof truss that provides space for the ductwork in a built-in cavity within the bottom chord of roof trusses.

Air sealing: Tooley is particularly concerned with duct tightness. "If a rowboat was 'substantially watertight,' would you be comfortable without a bucket on board?" he asks. "Ducts should be frog-butt tight" - a standard not yet in the ACCA manuals.

The IRC prohibits use of standard gray duct tape because of its short life expectancy. Most plain aluminum tapes won't do the job either because of unrealistic application requirements. Tooley strongly recommends mastic sealant; Masco's Engineered for Life program requires it. Yet because mastic can freeze and fail when applied during cold weather, butyl tapes or mastic tapes are often accepted as expensive but suitable substitutes. Thanks to contractor training programs, Tooley reports, mastic sealant is used by a rapidly growing number of contractors.

Duct leakage performance testing: The expression "you get what you inspect, not what you expect" is possibly more applicable to duct tightness than to any other element of residential construction. The only foolproof way to inspect ductwork tightness is to test it with a pressurization device, typically a "duct blaster." This involves sealing off all intentional openings and then using a calibrated fan to pressurize the ductwork to a given air pressure (25 pascals) and measuring the flow of leaking air.

EEBA and Building America recommend that duct leakage to the exterior be limited to 5% of the total air-handling system's rated airflow at high speed (nominal 400 cfm per ton). They also cap total duct leakage at 10% of the air handler's rated flow at high speed.

Room-to-room pressures: A good guideline to follow is Engineered for Life's standard of no more than 3 pascals of air-pressure difference between any room and the outdoors when interior doors are closed. This difference is measured with a digital monometer when the HVAC system is operating.

Temperature testing: When the HVAC shuts off, there should be no more than a 3-degree temperature difference between the hottest and coldest rooms in the house. That’s the standard Artistic Homes uses for its comfort guarantee. Upcoming ductwork development

Proctor Engineering Group in San Rafael, Calif., likely has done more HVAC systems research and field testing than anyone else in California. Who better to develop the best new ductwork idea in decades? Called the SDS - Snap Duct System - the ducts include EPDM gaskets at key joints. You just shove two pieces together until they snap, and the airtight seal is achieved.

Yes, it will cost more, but it should reduce several headaches. While mastic is a great sealant, contractors don't like it, and it's tough to work with in near-freezing weather. And you won't know if it's being installed everywhere it's needed if you don't test the ducts for tightness. With SDS, contractors could reduce performance testing. It should be ready for manufacturing by the middle of next year. Keep an eye out for this one.

Steve Andrews is a Denver-based energy consultant and freelance writer and EEBA board member. EEBA Builder's Guides can be obtained at www.eeba.org

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