The bill arrives and the number doesn’t make sense. The thermostat is set exactly where it was last summer. Nobody left windows open. The AC has been running steadily, maybe more than it should, but it’s running. So why is the electricity bill higher than it’s ever been?
For a lot of Riverside homeowners, the answer isn’t the AC unit itself. It’s the ductwork. Ducts that leak, even modestly, force the system to run far longer to hit the same thermostat setpoint, burning electricity while delivering less comfort. PG&E estimates the average California single-family home loses 30 to 40% of conditioned air to duct leakage, meaning a system that seems to be working fine may be losing a third of its output before air ever reaches a vent. ServLegacy Air Conditioning and Heating works with homeowners across Riverside County who are dealing with exactly this problem.
Why the AC Works Harder Without Cooling Better
When conditioned air leaks out of the duct system before reaching the living areas of the home, the AC doesn’t know. The thermostat is still calling for cooler air, so the system keeps running. University of Florida IFAS extension research found that ducts leaking just 20% of conditioned airflow can cause an HVAC system to work approximately 50% harder to meet demand. The U.S. Department of Energy estimates up to 30% of conditioned air can be lost through duct leaks in a typical home. That gap between effort and result is what drives the bill up without any change in thermostat habits. The system isn’t broken the way a failed compressor is broken. It’s losing the air it already paid to cool and running extra cycles to make up the difference.
Two Types of Leaks, Two Ways They Drain Money
Supply Duct Leakage
Supply ducts carry cooled air from the air handler out to each room register. When a supply duct leaks, that cooled air dumps directly into an attic, wall cavity, or crawl space before reaching the room it was meant to cool. It’s a direct loss of purchased cooling. The room stays warm, the thermostat keeps calling for more, and the cycle repeats.
Return Duct Leakage
Return ducts pull air from the living space back to the air handler to be cooled again. When the return side leaks, it draws in unconditioned air from wherever the duct runs, often a scorching attic. The AC then has to condition that far hotter air on every cycle. Research from The Energy Conservatory indicates that if just 10% of return air comes from a hot attic, system efficiency and capacity can drop by as much as 30%.
Both leak types can exist in the same duct system simultaneously, and neither is visible without testing. The ducts themselves are hidden in walls, ceilings, and attic spaces.
Why Riverside Summers Turn a Nuisance Into a Real Cost Problem
Duct leakage is a problem everywhere, but the Riverside climate turns a manageable inefficiency into a serious cooling cost driver. Riverside County averages 140 days per year where temperatures exceed 90 degrees Fahrenheit, based on NOAA 30-year climate normals. That means a leaking duct system operates under peak-penalty conditions for nearly half the calendar year.
The attic factor makes it worse. During summer months, uninsulated or poorly insulated attics in Inland Empire homes can reach temperatures well above 140 degrees Fahrenheit. Cooled air that leaks from supply ducts into that space is absorbed instantly, delivering zero benefit to the home. Meanwhile, return leaks in those same spaces are pulling 140-degree air straight into the system, forcing the AC to start each cooling cycle from a far worse position than the thermostat reading suggests.
Riverside falls within California Title 24 Climate Zones 10 through 14 depending on elevation and proximity to the desert or foothills. These are among the highest-demand cooling zones in the state, which means the dollar cost of every percentage point of duct leakage is higher here than in coastal cities that rarely see triple-digit days.
Warning Signs Homeowners Can Spot Without a Ladder
Most duct problems aren’t visible, but their effects show up in predictable patterns. These are worth paying attention to before another high-bill month arrives:
- Rooms that stay warm even when the AC is running. If one bedroom or the back of the house never reaches the set temperature, the supply duct serving that zone may be leaking conditioned air into an unconditioned space before it arrives at the register.
- Bills that climb season over season without any change in habits. When usage patterns and rate structures stay roughly the same but costs keep rising, a system running longer to compensate for conditioned air loss is a likely explanation.
- Dust accumulating faster than expected on vents or surfaces. Return duct leaks pull attic air, which carries dust, fibers, and particulates, into the system, and that material gets distributed through every room the system serves.
What California’s Energy Code Actually Requires
California Title 24 sets a maximum duct leakage threshold of 15% for HVAC work that doesn’t involve full duct replacement, and 5% or less for systems with new ductwork under the current 2025 energy code standards. Both thresholds must be verified by a certified HERS rater, a Home Energy Rating System professional who uses a duct blaster pressure test to measure actual leakage under controlled conditions.
Much of Riverside’s housing stock was built between the 1960s and early 2000s, and homes in that range commonly have original ductwork that was never tested and has been deteriorating for decades, with many operating at leakage rates well above the 15% threshold the energy code allows even under its most lenient compliance scenario. One thing worth knowing before attempting a DIY fix: standard duct tape isn’t an approved sealant under California’s energy code or the California Mechanical Code. Approved materials include UL-listed mastic compounds and foil-backed tape, both of which bond to duct surfaces in ways that hold up under the thermal cycling of a hot attic. Standard duct tape degrades quickly in those conditions and tends to fail within a season or two.
Diagnosable & Fixable Before Next Summer
Leaky ducts aren’t a reason to replace the whole HVAC system. In most cases, the equipment is performing exactly as it should. The duct system is simply failing to deliver what the equipment produces. A professional assessment can measure actual leakage rates, identify where the losses are concentrated, and outline what sealing or repair work could realistically change the numbers.
ServLegacy Air Conditioning and Heating offers free estimates on duct assessments and repairs, with flexible financing through Optimus for homeowners who want to address the problem without a large upfront cost. The team is bilingual, serving both English- and Spanish-speaking homeowners across Riverside County, and available for Saturday appointments when weekday scheduling doesn’t work. Call (951) 291-9954 before peak summer to get a clear picture of what’s driving those bills.