When it's broke, fix it!

"It's a great horse to ride, but look out you don't ride it in the wrong direction."
- Will Rogers

In about 95 percent of new Colorado homes with forced-air HVAC systems, air is returned to the blower through building cavities. Gypsum-covered walls and panned floor joists carry air down to a return-air plenum in the basement ceiling and hack to the furnace.

At least that's how the theory goes. Too had most building cavity returns really don't work the way everyone assumes.

By year's end, the 5 percent who design their return-air systems differently will probably have increased to a 10 percent share. Right now, the short list of production builders with "happy returns" includes Engle Homes, Aspen Homes of Colorado (Loveland). Centex Homes (Fort Collins), Keller Homes (Colorado Springs) and Domega Homes (Pueblo). Their strategy: tweak their designs to allow reliance on simplified, centralized tin or flex-ducted returns. Building cavities are strictly off limits.

Why the switch? Do those builders know something that you don't? Did the switch add cost or save money?

Testing shows 'it's broke'

The Fort Collins housing study, published last year and now on line at www.fcgov.com/utilities, contains data on results of duct leakage testing in 40 randomly selected homes. E-Star's testing elsewhere indicates that the utility's findings apply to ductwork throughout Colorado.

Our ductwork is incredibly leaky. Leakage is reported as a percentage of the rated airflow that can be moved by the furnace box's air handler. Performance standards required by Building America and various building programs limit leakage to between 5 percent and 10 percent of the rated airflow. In the Fort Collins random sample, total leakage averaged 130 percent - 13 to 26 times leakier than performance-based standards.

Most importantly, return-air ducts were nearly twice as leaky as supplies - another fact confirmed by E-Star testing. Another way to look at it: testing showed that on average only 36 percent of the air moved by the HVAC blower entered the system through a return-air grille on the wall. That means nearly two-thirds of the return air is drawn in through leaks.

Truth and consequences

Well, because the return ducts are usually located inside the building shell, what does it matter if they leak? It matters. Sometimes it matters a lot.

Since most of the return-side leaks are typically in basements, the blower draws more air from the basement than is supplied back to it. This creates negative pressure in the basement and positive pressure upstairs. In one-third of the sample homes, that negative pressure was enough to risk back-drafting of exhaust gases down the flues of atmospherically vented water heaters a serious health-and-safety liability.

Additionally, when much or most return-air is drawn from the basement, second-floor bedrooms end up returning little or no air. So when those bedroom doors are closed, the supplies often pressurize those spaces. That can both reduce airflows to those rooms and increase leakage of conditioned air to the outdoors.

Is this data already outdated? Not based on this writer's recent field experience, except for the 5 percent of builders who eliminated building cavities as returns.

The role of codes

Using building cavities to return air to the furnace is specifically allowed in the International Mechanical Code. But test\ing nationwide, not just in Colorado, shows it's a really bad idea. That's why California prohibited use of building cavities for return-air ducting back in mid-2001. In April this year, Oregon starts enforcing a similar regulation. Oregon's case warrants repeating.

"Back in 1998. we started offering a tax credit for sealing ductwork," said Tom Hewes with Oregon's Department of Energy. The credits included $300 for high-efficiency equipment, $250 for ductwork sealed such that it leaked no more than 6 percent of an HVAC blower's rated airflow, and another $150 if you do both. The tax credit, backed by incentives from utilities, created demand for training. Within months, dozens of technicians with HVAC firms and weatherization agencies were trained to seal ductwork. The $450 training required a half-day in the classroom, two days in the field, then a field-test and written test.

From their baseline testing on 1990-97 homes. Hewes said, "the newer the system tested, the worse the duct leakage." Evaluation of test results lead to the recent regulation excluding use of building cavities for return air.

John Tooley, trainer/researcher with Advanced Energy ( Raleigh, N.C.), did the early research and training in Oregon. When told that Oregon was soon outlawing building cavities for return-air ducting, Tooley said, "I love it. Any time you use building cavities as ducts, they leak. You end up with huge holes."

System solutions

Design a simplified, centralized return-air system that relies on either one or two hard ducts or flex ducts. In a two-story home, the typical approach consists of a single, vertical duct that pulls air from the second-floor ceiling as well as the main floor. Finding the space in today's open floor plans may require minor redesign. The typical approach is to run the duct up through the corner of a closet.

Part of this solution used by participating builders requires a systems approach. First they cut heating and cooling loads in half by upgrading their insulation package. purchasing low-e windows and tightening up their building shell. Then they require their HVAC contractors to perform equipment sizing and ductwork designs, using ACCA manuals J and D, respectively. This allows up to a 50 percent reduction in equipment size. In turn, that reduces the amount of airflow required, which reduces the size of the duct chase.

Air returns from bedrooms to the central return duct through off set transfer-air grilles, through jump ducts, beneath door undercuts or from dedicated ducts. The latter approach must result in a pressure difference no greater than 3 pascals (with door closed). To achieve this spec in larger bedrooms and master suites, it may be easier to hard-duct an additional return to that space.

Participating builders

"We've not seen it possible to sufficiently tighten building cavity return-air systems." said Bob Eikenberg of Engle Homes. "Day 1 we knew this; we didn't even attempt it, If you want to have substantially tighter ductwork, and you're still using framing cavities, you're wasting your time.'

With their hard-ducted and flex-ducted return systems, Eikenberg reports Engle gets the leakage factor down to IO percent and 5 percent, respectively. As for the costs he hasn't done a detailed breakdown.

"My instincts tell me it costs less when you centralize and simplify your return system," he said, while acknowledging that HVAC contractors then have to spend extra time designing, sealing and testing the systems. "If you're interested in tight ductwork as a goal. you have to test to see if you achieved your goal."

Last year, Centex Homes introduced a comprehensive systems-design approach in their new Fort Collins development called Linden Park. Dewing performance testing, one of their homes was the first in the state to show zero leakage from their return-air system. Said Centex's Justin Jones, "We found that hard-ducted returns greatly simplify the sealing effort."

Participating contractors

Contractors fully agree with their builders about the return-air system.

"The secret to a good return-air system is to do away with joist-space returns," says Jim Woods, owner of Woods Heating and Cooling in Larimer County, whose five crews install systems for Aspen Homes. "Bring your returns as close to the home's center as possible. If you want to meet a good performance standard, we need a centralized chase with the floor plan modified to accommodate that. This gives you a lot less joints, bends and connections that could leak and need sealing."

Woods is pleased with this new direction - the biggest change he's seen in his 25 years in the business - while admitting that it's going to he an uphill battle with many other builders. "Most builders are more concerned with cost than with performance. That's not going to get us where we need to go with up-graded HVAC." He states that his labor costs have gone up, given the focus on sealing and careful design plus follow-up testing: but if a builder's design allows him to put in a simplified design, the ductwork package may end up being "somewhat of a wash" in terms of installed cost.

Gustafson Heating does a lot of Engle's HVAC installations. Gustafson's Tom Gardner said new performance objectives entail a steep learning curve by HVAC technicians. "Simplification is of vital importance to achieving tight ductwork," said Gardner. "And I just can't imagine using building cavities for returns and getting duct leakage down to their average of 6 percent to 9 percent of rated airflow."

Some other HVAC contractors helping builders who specify redesigned and upgraded return-air systems are Northern Colorado Air, Mountain Air, Eastside Heating and Cooling, Controlling Systems, and Atlantic Heating (Colorado Springs).

Steve Andrews consults with builders for E-Star Colorado and writes on energy issues (sbandrews@att.net). E-Star (www.e-star.com), is a nonprofit home energy rating system that works with both new and existing homes statewide.

2008 Built Green Colorado

Home Builders Association of Metro Denver, 9033 E. Easter Place, Suite 200, Centennial, CO 80112
(303) 778-1400 fax: (303) 733-9440  info@builtgreen.org

Last Updated: 10/05/2007