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. According to Wendy Hawthorne and Susan Reilly, "Our testing and analysis has shown that perimeter duct distribution is unnecessary when the building thermal envelope, particularly the windows, meet specified levels of energy efficiency for a given climate."
Furthermore, "The additional cost of the windows will, in many cases, be less than the savings incurred by reducing the duct system size and complexity. Energy savings resulting from reduced duct heat loss, reduced fan power, and the improved thermal envelope can be significant."
Changing to a minimized duct design concept has the potential to save energy in the following ways, according to the authors:
- Reducing ductwork outside the conditioned space results in less conductive heat loss and improved delivery effectiveness. It also potentially results in less duct leakage to outside the conditioned space.
- Simplifying and minimizing the duct design can result in lower external static pressure loss, which can convert into fan energy savings.
- Minimizing the duct system often requires selection of glazing that is more efficient than what is typically used by production builders. For example, in order to achieve comfort in Denver, a builder would have to upgrade from a clear IG unit to a low-E IG unit.
- The cost savings from the reduced ductwork can be used to improve the duct system by duct sealing or insulating.
According to the authors, a prototype installation in a one-story new home in Colorado resulted in approximately a 20% cost reduction in the duct system by moving ducts from the crawl space to the attic. Uninsulated base case ducts were extended to the perimeter under each window. Supply run joints were sealed with duct tape, while returns were sealed with mastic. The attic ducts terminated at ceiling diffusers located near the interior walls. Blow insulation and batts brought the insulation value to about R-19.
Installed cost of the base system including ducts, plenums, insulation and diffusers was $1,845; installed cost of the alternative system was $1,359 - a savings of almost $500, with most of the savings from labor and duct/fitting reduction.
The authors conclude that further work is necessary to broaden and refine guidelines to include more glazing options and the combined effect of floor temperature and glazing size and configuration. More work is also required to more accurately determine convection current flow rates and turbulence from a cold window surface.
"These initial results, however," they write, "suggest that builders and hvac contractors will be able to trade ductwork for high-performance windows to deliver the same or better comfort at no additional capital cost while achieving significant annual energy savings."