Manual J basics
ACCA Manual J (8th edition) is the residential load-calculation standard. It produces two numbers per zone: a sensible heating load at the 99% design day temperature, and a sensible + latent cooling load at the 1% design day temperature. Both are expressed in BTU/hr. Manual J is what an HVAC contractor uses to size the actual equipment — it accounts for building geometry, orientation, envelope assemblies, window U-factor and SHGC, occupancy, internal gains, and infiltration.
A Manual J calculation is required for Title 24 compliance verification at the CF2R stage (the contractor's certificate of installation references the load calc). For design purposes, the loads tell the designer what tonnage to spec. The compliance calculation that drives the Title 24 report (CF1R) uses a separate building-energy model — it doesn't take the Manual J output directly, but the two should agree at the equipment-sizing level.
Manual J's design conditions come from the ACCA tables and ASHRAE-published climate data. For California, the design temperatures vary dramatically across the 16 climate zones — Palm Springs (CZ15) hits 110°F at the 1% cooling design hour, while Truckee (CZ16) sees 5°F at the 99% heating design hour. Equipment sized to handle one extreme will be wildly oversized for the other in mixed climates like CZ12.
Prescriptive efficiency tiers in the 2025 code
Title 24's residential prescriptive path (Table 150.1-A, 2025) specifies a ducted split-system heat pump as the baseline space-conditioning equipment. The required efficiency ratings vary by climate zone, with the hottest cooling zones and the coldest heating zones tightened in the 2025 update:
| Climate zones | HSPF2 | SEER2 | EER2 | Driver |
|---|---|---|---|---|
| CZ1, CZ2, CZ4, CZ6, CZ7, CZ11, CZ13, CZ14, CZ15 | 9.0 | 17.0 | 13.0 | Baseline. |
| CZ3, CZ5 | 9.5 | 17.0 | 13.0 | Cool marine — heating bump. |
| CZ8, CZ9 | 9.0 | 20.0 | 15.0 | Hot inland — cooling-peak tightening. |
| CZ10 | 9.0 | 18.5 | 14.5 | Inland — cooling-peak tightening. |
| CZ12 | 9.0 | 22.0 | 16.0 | Sacramento Valley — most aggressive cooling tier. |
| CZ16 | 10.0 | 17.0 | 13.0 | Alpine — heating-dominated. |
Source: Title 24 Part 6 Table 150.1-A (2025), as implemented in our compliance engine. All numbers are the AHRI-certified ratings the CF1R references.
Two notes on these tiers. First: HSPF2 / SEER2 / EER2 are the M1 test-procedure ratings effective 2023 — distinct from the pre-2023 HSPF / SEER / EER ratings, which run about 10% higher numerically. A heat pump rated SEER 19 under the old test will be roughly SEER2 17–18 under the new one. Make sure the rating on the equipment spec sheet is the "2" variant before comparing to the table above.
Second: these are prescriptive tiers. A performance-path compliance run can sometimes pass with a lower-tier system if the envelope is over-built or the PV is over-sized — and that's how the 2x4 Performance and 2x6 Enhanced design options work on this site. But the AHRI rating still has to be reported on the CF1R, and the contractor's equipment selection has to match.
How climate zone drives equipment selection
In the mild coastal zones (CZ3, CZ4, CZ6, CZ7), almost any current-generation variable-speed ducted heat pump will clear the prescriptive tier. The selection is driven more by physical fit (closet vs attic air handler, vertical vs horizontal duct run) than by efficiency. A baseline Bosch IDS, Mitsubishi M-Series, or Carrier Greenspeed will all qualify.
In the hot inland zones (CZ8, CZ9, CZ10, CZ11, CZ12, CZ13, CZ14), the cooling tier is the binding constraint. SEER2 17 won't clear CZ12; SEER2 18.5 is the floor in CZ10; SEER2 20 in CZ8 and CZ9. The practical equipment universe for SEER2 22 in CZ12 narrows to high-end variable-speed inverter systems — Carrier Infinity 26 (SEER2 26 / EER2 17.0), Lennox SL25XPV (SEER2 25 / EER2 16.0), Daikin Fit (SEER2 22 / EER2 14.5, tight on EER2), Trane XV20i (SEER2 22 / EER2 16). Single-stage equipment is essentially off the table in CZ12.
In CZ16 alpine (Truckee, Mammoth, Bishop), the HSPF2 10.0 tier and the cold-climate operation at 17°F is what matters. The relevant lookup on AHRI is the integrated HSPF2 plus the maximum heating capacity at 17°F as a fraction of nominal cooling capacity. Mitsubishi's hyper-heat (H2i) line, Fujitsu's AOU-XLTH, and the cold-climate Bosch CL5000i are the equipment most likely to clear CZ16's requirements. Standard heat pumps that lose 60% of their nominal capacity at 17°F will leave a CZ16 home cold on the design day, regardless of what the report says.
Dual-fuel and backup heat strategy
For Add+Alt projects in cold inland zones, dual-fuel (heat pump + gas furnace backup) remains a legitimate strategy when the customer is keeping an existing gas furnace and adding a heat pump for cooling. The report captures the heat pump as the primary and the gas furnace as the backup; the controls are set to crossover at a temperature (typically 25–35°F) below which the gas furnace takes over.
For Newly Constructed projects, Title 24 2025 makes dual-fuel difficult — TDV multipliers penalize gas heating heavily enough that NC compliance fails with a gas furnace in nearly every climate zone (the equipment-preference validator on this site coerces NC gas requests to no-preference for that reason). NC projects typically run all-electric: heat pump for heating + cooling, with electric resistance backup strapped at 5–10 kW for the rare cold snap.
In CZ16, even the resistance backup matters. A 5 kW strip won't recover a setback in a 3,000 sf home on a 5°F morning. Spec backup based on the Manual J heating load minus the heat pump's 17°F capacity — not based on tradition.
Ducted vs ductless
The prescriptive baseline assumes a ducted system with sealed ducts (duct leakage ≤5%, R-8 duct insulation in unconditioned space, ECC-Rater-verified). Mini-split heat pumps are common in California but introduce two compliance complications:
- Per-room sizing. A 4-head multi-split serving four rooms in a 1,800 sf home will not satisfy the prescriptive whole-house assumption if the indoor unit capacities don't cover the loads of the unconditioned spaces (hallways, baths). The Manual J needs to be run room-by-room and the head sizing has to match.
- Refrigerant charge verification. Mini-split systems still need refrigerant-charge verification under the 2025 code in the RCA zones (CZ2 and CZ8–CZ15). The pre-charged line sets on a mini-split sometimes lull installers into skipping the verification — that's a CF3R failure waiting to happen.
For Add+Alt projects with separate-minisplit topology (the addition gets its own mini-split, the existing residence keeps its existing system), the compliance run treats the addition as its own zone and the existing as its own — the mini-split doesn't need to match the existing system's efficiency tier as long as both meet the prescriptive minimum for their respective scopes.
Common sizing mistakes
A few patterns we see often enough they're worth flagging:
- Oversizing cooling. The traditional "ton per 500 sf" rule catastrophically oversizes for well-insulated current-code homes. A 2,500 sf CZ3 home built to current Title 24 prescriptive often has a sensible cooling load under 2 tons (24,000 BTU/hr). Installing 4 tons because "you can't be too big" leaves the compressor short-cycling, the latent removal essentially zero, and the system running below its rated SEER2.
- Undersizing heat pumps in CZ16. A heat pump sized for the cooling load is dramatically undersized for the CZ16 heating load. Equipment that nominally delivers 36,000 BTU/hr at 47°F will deliver maybe 18,000 BTU/hr at 5°F. Size for heating, then verify the cooling tonnage doesn't oversize the summer load.
- Dual-fuel crossover set too high. Some thermostats default to a 35°F crossover regardless of actual equipment performance. Modern variable-speed heat pumps deliver useful heat well below 35°F; locking the gas furnace in prematurely burns gas the project's TDV math didn't account for.
- Ignoring the AHRI cert. The CF1R lists a specific AHRI reference number. If the contractor swaps the air handler for a different model in the field — even a similar one — the AHRI rating changes and the CF3R verifier will catch it. Document the AHRI lookup at submittal, not after.
- Missing the SEER vs SEER2 conversion. A spec sheet that says "SEER 20" without the "2" is using the pre-2023 rating. It is not interchangeable with SEER2 20 on the CF1R. Always confirm the rating is M1 (post-2023) before reporting.
Bringing it together
A well-specced HVAC system is one that (a) has a current Manual J load calc per zone, (b) satisfies the climate zone's prescriptive HSPF2 / SEER2 / EER2 tier with M1 ratings on the AHRI directory, (c) is sized for the binding load (heating in CZ16, cooling in CZ8–CZ15, balanced elsewhere), and (d) has duct leakage and refrigerant charge field-verified by an ECC Rater. The CF1R will surface the tier; the CF2R will capture the model number; the CF3R will close the verification. Mismatches between any of the four show up as plan-check comments or post-occupancy compliance gaps.