Furnace Replacement and Installation, Sized to the House

Making the decision

Efficiency, staging and venting

AFUE describes how much of the fuel a furnace converts to heat over a season. High-efficiency condensing furnaces extract additional heat from the exhaust, which produces condensate and calls for plastic venting and a drain — usually straightforward in a basement, sometimes more involved where the existing chimney is the only flue. Whether that conversion makes sense depends on the home’s layout and how the venting can be routed.

Staging matters as much as efficiency for comfort. A single-stage furnace runs at full output every time it starts. Two-stage and modulating furnaces spend most winter hours at lower output, producing longer, quieter cycles and more even temperatures between floors. Paired with a variable-speed blower, they can also make filtration and humidity control work better. The right choice balances the home’s load, its ductwork and your priorities.

• Venting through an outside wall

  Condensing furnaces vent through a side wall or roof. Termination locations are planned with the exterior in mind — away from windows, doors, patios and walkways where practical, and consistent with code and manufacturer clearances.

• Ductwork above finished ceilings

  Where ductwork runs above finished drywall or plaster ceilings, changes are planned to work from the mechanical room and existing access points wherever possible, avoiding unnecessary openings in finished surfaces.

• Sound from the mechanical room

  Modulating burners and variable-speed blowers spend much of their time at low output. In homes where the furnace shares a wall with a bedroom, family room or study, that quieter operation is often worth weighing alongside efficiency.