Furnace Replacement, Sized to the House

The easiest way to size a replacement furnace is to match the old one's output, and it's also the way most sizing mistakes get carried forward for another twenty years. A furnace that's oversized for the house short-cycles — it satisfies the thermostat fast and shuts off before it's run long enough to heat evenly, which is uncomfortable and hard on the equipment. Undersized, and it runs constantly on the coldest days and still can't quite keep up. Either way, matching the old unit's output alone doesn't answer the question.

A proper sizing calculation accounts for the house itself — square footage, insulation, window area, ceiling height, how much of the envelope has been upgraded since the last furnace went in. That's more work than reading a nameplate, and it's the difference between a furnace that runs comfortably most of the time and one that's fighting the house every winter. We won't promise you a lifespan figure for a new unit; how long any furnace holds up depends on the install and how hard it's run, not a number on a brochure.

What this usually involves

• Load calculation instead of matching the old unit's output
• Replacement quotes from a licensed contractor, not a phone estimate
• Ductwork and airflow reviewed, not assumed adequate
• Old unit removed and disposed of
• No promised lifespan number — that depends on the install and use

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