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Choosing the right mechanical equipment for an airtight, energyefficient house presents a basic problem: In a tight house, so little air leaks in from outdoors that it can be difficult for standard, chimney-vented boilers and furnaces to get enough combustion air. More important, powerful exhaust fans in the kitchen and bathroom may depressurize the house, overcoming the ability of the chimney to vent toxic combustion gases to the outdoors (see "Are Your Houses Too Tight?" 8/94). The result can be spillage of these gases back into the house, with potentially dire consequences for the occupants. There are several solutions to the problem of depressurization in tight houses. A whole-house ventilation system, for example, supplies makeup air and keeps indoor and