When you hire a heating, ventilation and air conditioning (HVAC) contractor to install a new furnace, your first thought probably is, “What size furnace do I need?”

Heating equipment sizes depend on many factors, so your contractor should not simply look at the old furnace and match its size. This article can serve as a brief furnace buying guide, so you know what to expect from your HVAC contractor.


The Indoor Environment & Energy Efficiency Association, known as ACCA (as it was once the Air Conditioning Contractors of America), publishes the industry standards for measuring furnace sizes.

Any reliable contractor should be using ACCA Manuals J, D and S. These guide the measurement of furnace heat load, running of ductwork, and selecting the right furnace size.  Using the manuals correctly will ensure that you receive the correct furnace size for your property’s needs.

New Beats Old

Suppose your old system used 100,000 BTUs to produce 65,000 BTUs to heat your Phoenix home. Modern, 80 percent efficiency furnaces available today can produce those same 65,000 BTUs from only 82,000 BTU input.
Smaller, more efficient furnaces will save you money in two ways:

  1. Original purchase price
  2. Annual energy savings

Bigger is Not Better

To keep costs lower, do not super-size the heating system. Furnace sizes are chosen by mathematics and measurements, not by whim. Buying a 100,000 BTU system when you only need 65,000 BTUs leads to expensive problems, such as:

  • Short cycling
  • Rust deposits
  • Heat exchanger damage
  • Frequent repair and tune-up service calls
  • Shorter furnace life

Contractor Homework

Your local, reliable HVAC contractor will measure and assess your Phoenix home. Calculations from ACCA Manual J must include many factors, including:

  • Condition of weather-stripping and caulking
  • Condition of chimney and liner
  • Condition of existing ductwork
  • Levels of insulation
  • Number of windows and doors
  • Square footage

These are all entered into data tables with ACCA’s Manual J, giving reliable, scientific measurements of heat loss and heat gain in a home. These measurements lead to the design of optimal ductwork from Manual D.

At this point the HVAC contractor may advise rerouting ductwork for maximum efficiency in running the system and saving fuel costs for years to come.

Smiling Contractor with Level Wearing Hard Hat Standing In Custom Kitchen.

Furnace Sizes

ACCA’s Manual S should be used by the contractor to select the correctly-sized heating plant to efficiently and economically heat the home. A rough estimate is to provide 40 to 50 BTUs per square foot in the home for the Phoenix area.

As a guide, consider a 2,000-square-foot home in the Valley of the Sun. Multiply the BTU range and you get a minimum of 80,000 BTUs and a maximum of 100,000 BTUs.

An 80-percent efficient furnace rated at an input of 100,000 BTUs will produce 80,000 BTUs, matching the lower range of the estimate. Gas models exist that offer over 90 percent efficiency, so you can still get the 80,000 BTUs using only a 90,000 BTU input furnace.

Howard Air

Generally, larger furnaces cost more than smaller units. With unique installation requirements, only your contractor can give you a firm estimate of the final costs.

For accurate, professional advice on the right size furnace for your Phoenix-area home, contact us at Howard Air. We are ready to help you today.