How to Size a Furnace Correctly
Bigger isn't better — an oversized furnace short-cycles and wastes energy. Proper sizing uses a heat-loss calculation.
Correct furnace sizing is one of the most important and most overlooked parts of a good installation. Both oversizing and undersizing cause problems.
Why sizing matters
An oversized furnace heats quickly then shuts off, short-cycling and leaving uneven temperatures and humidity. An undersized one runs constantly and struggles in cold weather.
How it's done right
A proper Manual J / heat-loss calculation accounts for square footage, insulation, windows, orientation and air leakage — not a rule-of-thumb or matching the old unit's size.
Key takeaways
- Insist on a heat-loss calculation, not guesswork
- Don't just match the old furnace size
- Account for insulation upgrades and renovations
- Right-sizing improves comfort and efficiency
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