Title 24 HVAC Load Calculations

Under §150.0(h)1, heating and cooling loads are calculated by a method based on ACCA Manual J, the ASHRAE Handbook or the SMACNA residential manual. The indoor design temperatures are fixed at 68°F for heating and 75°F for cooling, the equipment is then selected under ACCA Manual S-2023, and the results go on the Certificate of Compliance.

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Manual J is one of three accepted methods

Section 150.0(h)1 does not name a single procedure. Loads may be determined by a method based on the ASHRAE Handbook (its Equipment, Applications and Fundamentals volumes), the SMACNA Residential Comfort System Installation Standards Manual, or ACCA Manual J, which the code’s definitions identify as ANSI/ACCA 2 Manual J-2016. A calculation to any of the three complies.

The compliance manual states that heating loads must be calculated for new space heating systems and cooling loads for new space cooling systems. That reaches a new home, an addition, and an entirely new or complete replacement system in an existing house, which §150.2(b)1C brings under §150.0(h).

The design conditions are set by the code

Section 150.0(h)2 fixes the indoor design temperatures at 68°F for heating and 75°F for cooling. Outdoor conditions come from Reference Joint Appendix JA2, the ASHRAE Fundamentals volume or Manual J, within two limits: the heating design temperature may be no lower than the 99.0 per cent heating dry bulb or the heating winter median of extremes, and the cooling design temperature no higher than the 1.0 per cent cooling dry bulb and mean coincident wet bulb.

The compliance manual notes that load calculations are sensitive to these choices. If the project’s city is not listed in JA2, or a nearby city matches the site better, it says to ask the local building department.

From load to equipment

Section 150.0(h)5 requires the equipment to meet the calculated loads and to be selected under ACCA Manual S-2023. Three points stand out:

  • No minimum cooling capacity. The manual explains that meeting cooling loads is not a requirement of the building code.
  • Furnaces are sized from Manual S-2023 Table N2.5.
  • Heat pumps must meet the heating load without counting supplementary heat, and have no maximum heating capacity. In a cold climate that can leave the unit oversized for cooling, which the manual accepts as the smaller inefficiency.

Electric resistance backup on a heat pump is capped separately, at 2.7 kW per ton of nominal cooling capacity under §150.0(h)8. The rest of the heat pump rules are on Title 24 heat pump requirements.

Additions add three rules

  • Block loads. Exception 1 to §150.0(h)1 lets an addition be sized from the total load of all rooms served by the central equipment.
  • Infiltration. Under §150.2(a)1E the envelope leakage assumed may not exceed Table 150.2-C, which sets air changes per hour by floor area and by single-story, two-story or townhouse. A tested result under RA3.8 may be used instead.
  • Capacity caps. Unless airflow is field verified at 350 cfm per ton or more, Tables 150.2-A and 150.2-B cap capacity; a single-speed cooling-only system, for example, at the design cooling load plus 6 kBtu/h. Ductless systems are exempt.

Where the existing system is extended to the addition, the existing equipment need not meet Part 6, but its heating capacity must satisfy CRC 303.10, and the manual says the calculation should cover the whole area the system serves. When an addition needs a report at all is on does a room addition need Title 24.

Who prepares it, and where it goes

The compliance manual names three people who may prepare the calculation: a mechanical engineer, the mechanical contractor installing the equipment, or someone qualified under Division 3 of the Business and Professions Code, such as an energy consultant. The CF1R must include the results of the heating and cooling load calculations, so the sizing on the permit set and the equipment on site have to agree. The energy model itself is a separate exercise, described on Title 24 calculations.

Common Title 24 Questions

  1. When is Title 24 required for a home?
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  3. Does a room addition need Title 24?
  4. What plans are needed for a residential Title 24 report?
  5. How much does a residential Title 24 report cost?
  6. How long does a residential Title 24 report take?

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