Right-Sizing Your Furnace and AC

Why size matters

A furnace or air conditioner that is too big turns on and off more often. That wears parts faster, wastes energy, and leaves rooms uneven. An oversized AC also removes less humidity.

One that is too small cannot keep up on the coldest and hottest days.

How the size is figured

We size every system with a Manual J load calculation, the industry standard from the Air Conditioning Contractors of America (ACCA). It accounts for your climate, the size and layout of your home, insulation, and windows.

That is how we match the furnace or AC we quote to your home, not just its square footage.

Too big, right size, too small

Oversized

Too big

  • Turns on and off in short bursts, never reaching full efficiency1
  • Shuts off before it pulls the moisture out, so the air stays damp and can grow mold and mildew1
  • More starts and stops mean more wear and an earlier failure2
  • Units oversized by 50% or more used about 9% more cooling electricity in a Florida study2
Right-sized

Right size

  • Longer, steadier runs that keep every room even with fewer drafts1
  • Pulls more humidity out of the air1
  • Quieter, with lower utility bills1
  • Manual J has safety factors built in, so it still keeps up on extreme days2
Undersized

Too small

  • Runs nonstop on the hottest and coldest days and still cannot hold the temperature
  • The fix is the same: size it to the home with a load calculation, not a guess2

Why short runs waste energy

Minutes the AC has been running Efficiency (EER) 05910 067 Oversized: shuts off here Right-sized: keeps going

An air conditioner is least efficient when it first starts and reaches peak efficiency after about 10 minutes. Running 9 minutes instead of 5 is about 17% more efficient.1

Square feet alone gets it wrong

2,000 sq ft home, standard build4 to 4.5 tonsManual J cooling load 52,100 BTU/hr
Same 2,000 sq ft, better insulated2 tonsManual J cooling load 23,300 BTU/hr

Department of Energy example: a 2,000 sq ft home in Raleigh, North Carolina, before and after insulation, window and duct upgrades.2

Same size house, half the system. Insulation, windows, air leaks and where the ducts run all change the answer, which is why the Department of Energy warns against sizing by floor area alone.2

~40%of contractors in one large survey said they oversize equipment on purpose.2
Manual Jis the sizing method the Department of Energy recommends. It is how we size every system we quote.2

The ducts count too

Manual J sizes the furnace or AC. Manual D is the Air Conditioning Contractors of America standard for designing and sizing the ductwork that carries the air.5

Up to 30%of a cooling system's peak capacity can be lost through ducts running in a hot attic.2
Leaksin supply ducts lose conditioned air, and leaks in return ducts add load and pull air from places it shouldn't come from.2

Sources

  1. ENERGY STAR, U.S. Environmental Protection Agency. Right-Sized Air Conditioners (fact sheet).
  2. U.S. Department of Energy. Right-Size Heating and Cooling Equipment, Technology Fact Sheet DOE/GO-102002-1490, January 2002.
  3. Consumer Reports. Gas furnace buying guide.
  4. Consumer Reports. Central air conditioning buying guide.
  5. Air Conditioning Contractors of America (ACCA). Manual D, Residential Duct Systems.