What HVAC system is best for an ADU?

For the overwhelming majority of California ADUs, the best HVAC system is a ductless mini-split heat pump — it provides both heating and cooling in one all-electric unit, needs no ductwork, and is the most direct route to Title 24 compliance. Larger multi-bedroom units sometimes favor a ducted heat pump, but the underlying technology is the same: a heat pump.

Why heat pumps win for ADUs

California's mild, Mediterranean climate is nearly ideal for heat-pump operation, and the 2025 energy code treats heat pumps as the prescriptive baseline for new-construction space conditioning. A heat pump moves heat rather than burning fuel, delivering roughly two to four units of heating for every unit of electricity it consumes — far more efficient than electric-resistance baseboards. Because it contains no combustion, it also satisfies the all-electric reach codes adopted by Los Angeles, San Jose, San Francisco, Oakland, and dozens of other California cities.

Matching the system to the ADU

Climate-zone considerations

• Coastal and inland zones (1, 3, 5–13): standard high-efficiency mini-splits perform excellently.
• Desert zones (14, 15): prioritize cooling capacity and size carefully to the zone's high summer design temperatures.
• Mountain zone (16) and cold edges of zones 1 and 11: specify a cold-climate-rated heat pump that maintains capacity below freezing.

How to decide — checklist

1. Confirm your climate zone by parcel address.
2. Run a Manual J to establish per-room heating and cooling loads.
3. Check whether your city's reach code prohibits gas (it likely points you to all-electric anyway).
4. Choose ductless for compact or low-ceiling plans; consider ducted only when there is room to route ducts and you want a single thermostat.
5. Verify the chosen equipment meets current Title 24 efficiency minimums.
6. Add ASHRAE 62.2 whole-building ventilation as a separate documented system.

What about gas furnaces and other options?

A gas furnace heats but doesn't cool, requires a separate AC system to handle California summers, and is prohibited outright in cities with all-electric reach codes — so it rarely makes sense for a new ADU. Electric-resistance baseboards are cheap to install but expensive to run and carry a poor Title 24 efficiency margin. A heat pump avoids both problems by combining efficient heating and cooling in one unit, which is why nearly every modern ADU design centers on one.

Code thresholds and local mandates vary by jurisdiction — confirm current details with your local building department. Our mechanical plans specify the exact system, sized to your ADU and climate zone, with the Manual J and equipment schedule included. See our mechanical guide for deeper detail, or start an order.