Ductwork that Doesn't Deliver

The discomfort experienced by the owners of one Fort Collins study home was caused by a combination of uneven heating by the sun and ductwork problems.

The rear elevation of this 4,700 square foot, two-story home faced south. Most of the glass on that elevation was on the main level, delivering a significant amount of uncontrolled solar gain and keeping that zone overly warm on sunny winter days. The main-level thermostat rarely called for heat in that situation.

The insulated, garden-level basement had a moderate amount of south-facing glass. It stayed warm; in fact, a complaint was that it was also too warm in the winter. In addition to solar gain, it received a lot of unintentional heat from duct leakage. This home’s ductwork was the second leakiest of the 40-home testing subsample (3,541 CFM25). Duct leakage is often greatest in the basement because duct pressures are highest there.

Meanwhile, the upper level stayed cool; the homeowners complained in particular that the master bedroom in the northwest corner ran cold. To compensate for this, the homeowner had closed all the basement and main-level supply registers—at the advice of the builder—in an attempt to drive more conditioned air to the upper level. That strategy wasn’t working. In the home’s upstairs master bedroom, located farthest from the furnace, supply ducts were providing reasonable flow, but the upstairs return ducts weren’t drawing measurable air flow.

The testing contractor for the Fort Collins housing performance study coined a phrase to describe similar situations he had observed in other Colorado homes built in the early 1990s. “Random conditioning” indicates that the occupants have poor control over heating and cooling, and conditioned air is not delivered where it’s needed.

Solutions would need to involve more thought toward the sun and window specifications, more careful design and installation of the duct system, and duct sealing. A controller that intermittently cycles the air handler blower, to help circulate air between zones, might also be a good choice to help overcome the limitations of single-zone control in this large, solar-driven home.

Last Updated: 08/21/2003