Roughly 20% of your home’s air conditioning (AC) never arrives in the room it is meant to cool [1]. A new unit is sold on a factory rating, but the hidden air ducts actually decide the result. Field tests cited in the InterNACHI inspection standard show duct leaks waste 15% to 20% of heating and cooling energy on average. This happens even in new homes [1].

The big picture:

The standard math used to size a new unit assumes your ducts are perfect. The Manual J method estimates heating and cooling needs on two assumptions. It assumes zero duct leaks, and that the heat lost through the ducts does not change with how long the run is [1].

The standard openly states this limitation, but real-world tests prove the opposite. Leaks average 15% to 20% across California field tests [1]. Heat loss also grows with the length of the duct run.

One Sacramento attic lost 12% of its cooling at the air vent nearest the furnace. It lost more than 40% at the farthest bedroom [1]. The standard notes that a far bedroom duct is likely too small by design [1].

What strikes me is how cleanly this splits the money. Heating and cooling takes up 52% of a home’s yearly energy use, according to a 2020 survey from the U.S. Energy Information Administration (EIA) [2].

Apply the leak rate to that large slice. Roughly 8% to 10% of the energy a home consumes vanishes through a gap the quote never mentions. That is my arithmetic on two cited figures, not a measured saving.

Federal rules also set new efficiency floors for units built on or after January 1, 2023 [4]. Split-system air conditioners under 45,000 British thermal units per hour (Btu/hr) need a 13.4 Seasonal Energy Efficiency Ratio 2 (SEER2). Heat pumps need a 14.3 SEER2 and 7.5 Heating Seasonal Performance Factor 2 (HSPF2) [4].

The floor tells you what a new unit must be. It does not say how much of that rating reaches the room.

By the numbers

  • 20% — Duct leak penalty: Leaks raise heating and cooling energy use by 15% to 20% on average across homes with ducts in attics and crawlspaces [1].
  • More than 40% — Far vent loss: A Sacramento attic lost 12% of duct energy at the near vent and over 40% at the far bedroom [1].
  • 52% — Heating and cooling share: Space heating and AC make up 52% of yearly home energy use [2]. The average home electricity price sat at 17.30 cents per kilowatthour (kWh) in 2025 [3].
  • 350 cfm/ton — Airflow floor: The standard check requires indoor coil airflow above 350 cubic feet per minute (cfm) per ton [1].

What I’d watch:

The people closest to this problem treat the ducts as a core part of the machine. Three details show up in how they write the job scope.

  • Load math comes first: Sizing follows a room-by-room estimate of cooling needs. The duct design uses a matching step, and the unit is picked from the final result [1].
  • Airflow gets measured: The final check includes a physical test of indoor coil airflow, requiring above 350 cfm per ton [1]. This reading at the unit is the first proof a homeowner can ask for.
  • Leaks are tested twice: The standard directs that leaks be measured as part of the final check using an air pressure test [1]. A before-and-after number is the only proof that a duct repair actually worked.

I keep coming back to the comparison problem. Two shops can quote the exact same box at different prices. Neither quote can be weighed if the duct health is missing.

The leak number is the one detail that makes the quotes comparable.

The catch

The leak finding measures a known cost, not a promised savings figure. The standard notes that a duct fix cut run times in the field, but it does not say exactly what a fix recovers [1]. Anyone promising a specific percentage saved is guessing past the source data.

The sharpest tests are also old and highly local. The 12% and 40% loss readings come from a single 1995 study in one Sacramento attic [1].

A tight system with ducts inside the living space faces entirely different math. My read is that the finding is a reason to test the system, not a final verdict on your house.

Testing also carries its own cost. An air pressure test and a room-by-room load check take real work right when a broken unit already strains the budget. But this test buys vital facts before you cut the check.

The federal floor does not close the gap either. The 13.4 SEER2 air conditioner minimum and the 14.3 SEER2 heat pump minimum dictate what a unit must be. They do not tell you what it actually delivers [4]. A highly rated box on a leaky duct system still runs against a leak penalty of 15% to 20% in the attic [1].

At a glance

  • The Big Shift: A new AC unit is sold on a factory rating, but field tests show duct leaks waste 15% to 20% of heating and cooling energy [1]. The standard sizing math assumes zero leaks [1].
  • Why It Matters: Heating and cooling eats up 52% of home energy, meaning the leak penalty hits the largest line item on the power bill [2]. Cooling loss is also wildly uneven across the house [1].
  • What I’d Watch: I am watching whether the project scope carries physical tests rather than verbal promises.
  • Load calculation: A room-by-room estimate of cooling needs, finished before a model number is picked [1].
  • Airflow check: A physical reading of indoor coil airflow above 350 cfm per ton at final sign-off [1].
  • Leakage test: A before-and-after duct leak test to prove the repair worked [1].
  • The Catch: The 15% to 20% figure measures what leaks cost, not what a fix guarantees to recover [1]. The SEER2 efficiency floors set a factory minimum, not a delivered result [4].

Related reading

Sources

[1] InterNACHI, “HVAC Inspection” — field research on duct leakage and conduction loss, the Manual J assumptions, and the acceptance criteria for airflow and sizing. https://www.nachi.org/hvac.htm [2] U.S. Energy Information Administration, “Use of energy in homes” — 52% of household energy consumption for space heating and air conditioning (2020 data). https://www.eia.gov/energyexplained/use-of-energy/homes.php [3] U.S. Energy Information Administration, “Prices and factors affecting electricity prices” — 2025 annual average retail price, residential 17.30¢ per kWh. https://www.eia.gov/energyexplained/electricity/prices-and-factors-affecting-prices.php [4] 10 CFR 430.32, Energy and water conservation standards and their compliance dates — SEER2 13.4 for split-system air conditioners, 14.3 SEER2 / 7.5 HSPF2 for split-system heat pumps, manufactured on or after January 1, 2023. https://www.ecfr.gov/current/title-10/chapter-II/subchapter-D/part-430/subpart-C/section-430.32