Upgrading the heating and cooling equipment in your house is a good way to improve interior air quality and conserve energy. Ask a reputable local HVAC contractor about the following:
► Induced-draft gas furnaces. Roughly two-thirds of North American homes have forced-hot-air (FHA) systems, so replacing an existing FHA furnace is a great way to increase efficiency without disturbing existing ducts and registers. Induced-draft furnaces can achieve annual fuel-use efficiencies of 90 percent to 97 percent because they extract heat from combustion gases. (Older well — maintained furnaces might have efficiencies closer to
70 percent, or less.) Because combustion gases are cooler, they’re less buoyant, so the system uses a fan to expel them—hence the name induced draft.
► Heat-recovery ventilators. Tightly insulated houses conserve energy, but they may also recycle stale air endlessly. In response to the dilemma of introducing fresh air without expelling conditioned air, heat-recovery ventilators (HRVs) were developed. Typically, an HRV has two fans: one to bring in fresh air and one to expel stale air. It also has a heat exchanger that recovers 75 percent to 80 percent of the heat in the outgoing air and preheats incoming air. HRVs can filter pollen and dust from incoming air and, by
equalizing air pressure in tight houses, prevent potentially dangerous situations such as back-drafting (furnace combustion air, including carbon monoxide, being sucked back down a vent flue by negative air pressure). Some HRVs also remove excess humidity from incoming air.
► Central air cleaners. Most standard central heating/ air-conditioning filters don’t do a good job. If you’re concerned about interior air quality and removing allergens (such as dust mites, mold, and pet dander), you have a variety of air cleaners to choose from, including pleated media, self-charging electrostatic filters, electronic air cleaners, and by-pass HEPA filters. In general, air cleaners vary by fineness of filtering (dust arrestance), ease of installing into existing ductwork, airflow impedance, and cost. To make sense of this and many other HVAC topics, visit www. dulley. com. On air filters, mechanical engineer Jim Dulley notes, "For do-it-yourself installation, selfcharging electrostatic models are ideal because they require no sheet-metal ductwork to install."
► Upgrading ducts. Sealing ducts can prevent air leaks and, in many cases, reduce excess moisture. But if ducts are rusty and as tired as the one shown on p. 333, or are uninsulated as they traverse unheated areas, maybe it’s time to replace them. Three popular types are shown above.
