Get a Load Of This: Proper Furnace Sizing Requires Careful Calculations
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Get a Load Of This: Proper Furnace Sizing Requires Careful Calculations


If your HVAC technician wants to size a new furnace for your home by its square footage alone, or using some other discredited “rule of thumb” method, you should change technicians. Furnace sizing involves an engineering methodology developed by the Air Conditioning Contractors of America (ACCA), which has been time-tested and energy-budget approved. Here’s why and how.

Heat Gain, Heat Loss

A home’s heat gain/loss totals the heating load placed on the furnace. A new furnace should be sized to meet the load, or it will be over- or undersized. For instance, take two very similar homes, in terms of floor plan and square footage, located in the same neighborhood; the old rule-of-thumb furnace sizing method would result in the same size furnace installed in each home.

However, what about the heat gain/loss properties? Perhaps each home has a different orientation to the sun. One home has Energy Star-qualified windows, and has undergone air sealing and insulation upgrades. The other home has not. It is likely each home will require a different size furnace. Here’s why:

  • Too big – An oversized furnace increases purchase price, and quick cycles on/off, which is uncomfortable, wastes fuel, and creates moisture issues for components.
  • Too small – An undersized furnace struggles to keep up with the heating load. This creates excessive component wear and occupant discomforts.
  • Just right – A right-sized furnace performs at peak efficiency for maximum fuel efficiency.

Furnace Sizing

These are the basic steps you and your HVAC contractor should take when sizing your new furnace:

  • Home preparation – Sealing air leaks and boosting insulation reduces the heating load, heating bill and furnace size.
  • Manual J – ACCA’s Manual J is used to calculate the heating load room by room.
  • Manual D – This methodology is used to evaluate and size and design of the air ducts.
  • Add-ons – Consider add-on equipment when selecting the right size furnace. Zoning systems, Wi-Fi thermostats and other advanced furnace components may affect the needed size of an installed system.

For complete details about proper furnace sizing for your Portland area home, please contact us at Roth Heating & Cooling.

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