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In the early days of central heating, a large furnace was typically located in a basement near the center of the building. Hot air rose naturally into each room. As the room air-cooled, it fell naturally into large ducts that returned it to the furnace. When blowers were added to furnaces it increased the flexibility of forced air systems. These blowers had two unfortunate side effects. First, they pressurized the ductwork leading to a significant potential for conditioned air to leak out. Second, it encouraged duct locations outside the house. Today, a typical house has a furnace located in the garage with ducts running through attics and crawlspaces. This configuration can lose between 10 and 15% percent of the heat in the ducts as conditioned air flows from the furnace (or heat pump) to the registers. This can typically transform a 90% efficient gas furnace into a 75% efficient one.
Air leakage from ducts is only half the problem, since ducts often run through unheated attics and crawlspaces, exposing them to extreme temperatures. These unconditioned spaces exist outside the “thermal boundary” of the house. To make matters worse, ducts typically have only R8 insulation, giving them about twice the heat loss rate of a typical exterior wall. Since it requires a considerable amount of energy to heat or cool air inside ducts, it seems unwise to circulate this air through areas that quickly steal that energy.
Designing and building homes where ducts are located inside the thermal boundary can reduce heating and cooling costs, increase energy efficiency and improve occupant comfort. If skillfully done, it can also reduce construction cost.
The vast majority of new homes built today are still constructed with duct-work and or air handlers in unconditioned spaces. That is, they are located in ventilated attics and crawl spaces or sometimes an unheated garage. What this means is that your heating and cooling equipment must be sized larger and work extra hard because the conditioned air is moving through attic and crawl spaces that in the summer can easily exceed 130 degrees and below 40 degrees in the winter. Additionally any air leakage from the duct-work is lost to the outside of your house.
• Why connect a highly efficient heating system to a poorly designed and installed duct system that immediately cuts overall system efficiency by 15 to 25 percent? The impact is even greater during extreme weather, when heating systems strain to maintain comfort. The air inside your ducts is conditioned indoor air. Why circulate this air — that has been heated or cooled at considerable cost — through the outdoors? Running a duct through the attic is like running it out the window and into the next room.
• Ducts in attics and crawlspaces are “out of sight, out of mind” and are the biggest source of serious problems. They can become disconnected. They can get in the way of workers who crush or puncture them. They also provide inviting nests for insects, rodents, and other critters.
• Return duct leaks are under negative pressure (suction) and pull air into them. So furnace cabinets in garages and return ducts that pass through attic spaces draw pollutants from these areas into the house.
By building your new home following these relatively simple procedures you can easily save 20 to 25% on your heating and cooling costs for the lifetime of your home. Not only is this “The Green” way all new homes should be built to, but the result is not only less energy consumption but a more thermally comfortable home.
Todd Allen Miller, Architect
QMA Design+Build, LLC
5000 Boardwalk, Suite 2
Ventnor, NJ 08406
(609) 822-4949 – phone
(609) 822-4429 – fax
NJ New Home Builder License #037561
NJ Home Improvement Contractor #13VH01107300