As home performance contractors, we provide energy audits and a range of such services as air sealing, insulation, hvac work, and the like. When we inspect existing homes, we usually find heating appliances that vary in age and efficiency and were never designed to work as a system. Last summer we replaced the equipment in just such a home in San Francisco; we installed a new energy-efficient system that provides heat, hot water, and air exchange.

The 1,600-square-foot main floor of the house had been remodeled several years before, and at that time the furnace and ductwork were replaced. Despite the new equipment, the owners still had high utility bills, uneven heat distribution, and poor indoor air quality. When they decided to convert an unused ground-level basement into 800 square...

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