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Q.What factors must be considered when deciding whether to use a large hot-air furnace with two zones or two smaller furnaces?

A.Jeri Donadee, vice president of H.B. McClure Co., a heating and cooling contractor in Harrisburg, Pa., responds: A single unit is almost always more cost effective than multiple smaller units. Multiple units should be considered only if the house design will not permit adequate means of ducting the various zones from a single heating plant.

With a single unit, the installed cost, operating costs, and life-cycle maintenance costs are almost always lower. If the customer wants options like air conditioning, a humidifier, or an air cleaner, the installed cost advantage in favor of a single unit is even greater. Zone system manufacturers claim that the operating cost of a single unit can be 30% lower than the cost of two smaller units.