Heating ducts in a residential home typically are installed the length of the room and come up through the floor beneath the windows. This is known as perimeter heating, since the heat is released around the perimeter of the house, and dates back at least to the 1940s and 1950s research. But why is this done?

The reasoning for this design goes back to when homes typically had single-glazed windows and little or no wall insulation. Warm air had to be introduced into a room at its coldest point, to counteract the effect of cold air making its way in through the glass and window openings, and to keep the windows from icing over in the wintertime.

Recent research by a team of Colorado researchers now may counteract that design strategy. In fact, the Air Conditioning Contractors of America (ACCA) Manual D already allows for some changes in homes with a good thermal envelope and a either a heated basement or an enclosed, nonvented crawlspace.

Windows (fenestrations) have improved, and walls are now almost always insulated (R-11 or better). Engineers Wendy Hawthorne and Susan Reilly of Enermodal Engineering (Denver) and Ren Anderson and Edward Hancock of the National Renewable Energy Laboratory (Golden, Colo.) have authored a paper backing up the argument that perimeter heating is no longer required in many homes.

Why change accepted design practice? It's cheaper. Not extending the duct system can save 20% to 25% over the cost of standard hvac practice, according to the researchers. Or $260 in net savings in a 1,200-sq.-ft. test home, even taking into account the cost of the upgraded windows.

Less ductwork also means less heat lost along the way, and more heat at the delivery point into the rooms. The fan has to work less hard, since the heated air doesn't have to travel as far, so a smaller fan can be used for both initial cost and operating savings. It may even be possible to downsize the heating and cooling plant.

Their research showed little difference in comfort levels between a home with standard perimeter duct heating and unglazed windows, and a home with upgraded windows but without perimeter heating. There was some difference noted, however, at extremely low temperatures of around -10°F, where the perimeter heating system had a slight advantage.