Do I need a new air conditioner for the addition?

Not necessarily. Existing equipment can serve it, but the extended duct system still gets tested. Installing a new system for the addition brings new-home rules instead.

Which is easier to pass, 10 percent total or 7 percent to outside?

It depends on where the leaks are. Leaks into conditioned space count in the first test and not in the second, so either measure can be used, and the installer picks the one the system can meet.

Extending Existing Ducts to an Addition: Title 24 Rules

Extending existing ducts into an addition, of any length, puts the existing system and the extension under Section 150.2(b)1Di and 1Dii. The combined duct system must leak 10 percent or less in total or 7 percent or less to outside, or be sealed and smoke-tested by an ECC-Rater. Ducts or equipment in a garage face 6 percent.

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Any length counts

Exception 4 to Section 150.2(a) applies "when any length of duct is extended" from an existing system to serve an addition. Alterations elsewhere get a 25-foot threshold; this exception has none. New duct in unconditioned space takes R-6 in zones 3 and 5 to 7, or R-8 elsewhere.

One system, one number

The old ducts and the extension are tested together, as a share of the airflow of the equipment serving them. For a 3-ton single indoor unit that airflow is taken as 1,200 cfm, so the limit is 120 cfm in total or 84 cfm to outside. Exception 4 pulls in all of Section 150.2(b)1Dii, which includes a 6 percent clause for new or replacement ducts, air handlers, coils or plenums in a garage. An existing duct built, insulated or sealed with asbestos is excepted.

Getting the test ready

Every boot, plenum, connector and transition is installed and sealed, with no cloth-backed rubber adhesive tape on new or altered duct.
All registers and grilles are sealed except one large return, and the filter is out.
A fan flowmeter holds the ducts at 25 Pa and its reading is divided by the airflow figure.
The leakage-to-outside version adds a blower door holding the house at 25 Pa, with the duct fan keeping zero pressure between ducts and house.

If the old ducts miss both numbers

The installer measures, seals every accessible leak and measures again. The ECC-Rater then runs a smoke test and inspects plenum and coil connections, refrigerant line and condensate trap penetrations, the air handler door panel (metal tape is acceptable), register boots and joints at takeoffs and tees. Smoke only from a gasketed furnace cabinet does not fail it, and sampling is not allowed.

Whether the old equipment may serve the addition is on existing HVAC for an addition. The 25-foot trigger is on duct replacement, all test triggers on duct leakage testing, and the general rule on duct leakage. Start a report at residential.

Common Title 24 Questions

Frequently Asked Questions

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