What are common HVAC mistakes on ADU projects that cause permit delays?

Most ADU mechanical plan-check corrections come from a short, predictable list of omissions — a missing load calculation, an incomplete equipment schedule, ventilation that wasn't designed, or an outdoor unit shown without setbacks. Each of these turns a clean approval into a correction cycle that adds weeks to the permit timeline.

The mistakes that delay permits

1. Missing Manual J heat load calculation. Plan reviewers increasingly require the actual load calculation, not just an equipment schedule. Without it, the reviewer cannot confirm the system is correctly sized, and the plan is kicked back.
2. No equipment schedule. Showing HVAC on the floor plan without a table listing manufacturer, model, capacity (BTU/hr), and efficiency ratings leaves the reviewer unable to verify Title 24 compliance.
3. Equipment below Title 24 minimums. Specifying a unit that doesn't meet current minimum efficiency — uncommon with modern mini-splits but possible with older or commercial models — triggers a correction to re-select equipment.
4. Whole-building ventilation not designed. A frequent and avoidable miss: the plan handles heating and cooling but ignores the mandatory ASHRAE 62.2 whole-building ventilation system, which is separate from bath and kitchen fans.
5. Kitchen exhaust not ducted to exterior. Recirculating range hoods shown where the California Mechanical Code requires exhaust ducted to the outside.
6. Outdoor unit (condenser) location not shown — or shown without setbacks. The condenser must sit at code-required distances from property lines and from neighboring windows, and noise and clearance requirements must be respected.
7. Refrigerant line-set routing not shown. The path from each indoor head to the outdoor unit must be drawn so the reviewer can confirm it is physically feasible and code-compliant.

Why these particular items get flagged

Reviewers are checking two things: that the system is safe and correctly sized, and that it matches the energy-compliance documents. The load calculation and equipment schedule together prove sizing and efficiency. Ventilation, ducted exhaust, and condenser placement prove the design meets the Mechanical Code and won't create a nuisance for neighbors. When any of these is absent, the reviewer cannot sign off — so the omission, not a design flaw, is what causes the delay.

A second, costlier failure mode: building differently than the approved plans

Even after approval, installing HVAC differently from the stamped drawings causes problems at inspection. Minor field adjustments can be handled with a field-change note, but switching system type, relocating the condenser, or substituting a different equipment model requires a formal plan revision approved before the work is done. Getting the design right up front avoids this entirely.

Pre-submittal HVAC checklist

1. Include a Manual J load calculation in the plan set.
2. Provide a complete equipment schedule (make, model, BTU/hr, efficiency).
3. Confirm all equipment meets current Title 24 minimums.
4. Design and document ASHRAE 62.2 whole-building ventilation.
5. Show a kitchen hood ducted to the exterior.
6. Locate the outdoor unit with property-line and window setbacks shown.
7. Draw the refrigerant line-set routing.

Code thresholds and local amendments vary by jurisdiction and change between code cycles — confirm current details. Our mechanical plans include every item on this checklist, and city corrections are handled at no extra charge until your permit issues. Order here.