What a HERS rater is and why they're separate from the installer
A HERS rater is a certified third-party verifier (certified through a California-recognized provider like CalCERTS or CHEERS) whose job is to independently confirm that specific Title 24 performance requirements are met in the field. HERS raters are not employed by the HVAC contractor — the separation is deliberate. The installer does the work; the rater confirms the work meets code.
On an HVAC changeout, the rater typically runs two tests: duct-leakage testing (always) and refrigerant-charge or airflow verification (depending on the compliance path). Results are filed with the local building department as part of permit closeout.
How the duct-leakage test works mechanically
The rater seals all the supply and return registers in the home. A calibrated fan — a 'duct blaster' — is attached at the air handler or at a return grille and pressurizes the duct system to a reference pressure (25 Pascals, roughly the pressure the system runs at in normal operation).
The flow required to maintain that pressure is the leakage rate — measured in cubic feet per minute at 25 Pa (CFM25). Lower is better. The test takes about 20 minutes once setup is done.
Pass thresholds and what fails
Current Title 24 thresholds for altered or new duct systems are expressed as a percentage of the system's nominal airflow — typically allowing a maximum of 10% leakage, with lower limits for some paths. For a typical 3-ton system at 400 CFM/ton (1200 nominal CFM), the duct system must test at or below 120 CFM25 leakage to pass.
What fails: unsealed takeoffs at the plenum, loose boot-to-drywall connections at registers, torn flex-duct insulation jackets that expose leaky connections, and (very commonly) return-air plenums that were never properly sealed to the air handler. A failed test means the installer re-seals and the rater retests; a second failure can delay permit sign-off.
Why this matters beyond the paperwork
A leaking duct system wastes a significant fraction of the conditioned air it's supposed to deliver — straight into attics and crawlspaces where no one benefits. Independent studies in California housing stock have found pre-sealing leakage averaging 20–30%. The sealing-and-testing process pays back in energy savings and comfort improvement that the homeowner notices.
There's also a safety angle: a severely leaky return system can pull air from the attic or crawlspace back into the house — carrying attic dust, insulation fibers, or in worst cases combustion byproducts from a nearby appliance. Sealed, tested ductwork avoids that failure mode.
Common questions
Is HERS testing always required on a changeout?
On most residential HVAC changeouts in California, yes. There are specific exemptions (very small alterations, certain system types) and some compliance paths have different triggers, but the default assumption on a standard AC or furnace replacement is that HERS testing is part of the project. The installer confirms the exact requirement for your project at quote.
Does the homeowner pay the HERS rater separately?
Usually the installer coordinates and includes the HERS fee in the project. The rater's test fee is a separate line item on our quotes for transparency — we show what the test costs rather than hiding it.
What if my ducts fail the test?
The installer seals the leaks and the rater retests. On older or poorly-installed duct systems, a first-pass fail is common. A second fail usually points to more systematic issues — torn flex-duct runs, an undersized return plenum — and may require more extensive work. We assess duct condition before quoting changeouts so we're not surprised at the HERS visit.
Can my ducts be left as-is if they're already working?
If the test shows leakage above the Title 24 threshold, no — sealing is required. If the ducts pass as-is, they're fine. Many homes with 1990s-forward construction and reasonable sealing pass the first test. Older systems with cloth-tape joints usually need work.