If your San Diego AC runs constantly and never shuts off, one of two things is happening. Either the system can’t actually reach the set point (too small, too restricted, or losing capacity), or the thermostat can’t tell that the room is cool enough to stop. Both waste power, both wear the compressor, and both have clear diagnoses. The fix matters a lot in San Diego specifically because of how our climate is split: a system sized for our long mild coastal season often runs flat-out during inland heat events and never catches up.
This guide ranks the causes by how often we see them, gives you a quick way to tell them apart, and lays out the decision framework for the one that costs the most to fix.
The five real causes, ranked by likelihood
1. System is undersized for the actual heat load (most common inland)
This is the single biggest cause we see in inland San Diego (Escondido, San Marcos, Poway, Santee, El Cajon, Lakeside, Ramona). Many of these homes have AC systems that were sized for coastal-equivalent conditions or sized off a square-foot rule of thumb instead of a real Manual J calculation. On a 76-degree June day in Carlsbad the system is fine. On a 102-degree September Santa Ana in San Marcos, the system runs from 11 a.m. to midnight and the house never gets below 78.
How to tell: the unit runs continuously on hot afternoons but cycles normally in the morning and evening. Indoor temperature stays 4 to 8 degrees above set point during peak heat. The unit isn’t broken. It’s just maxed out.
Honest tradeoff: this is the expensive one to truly fix. Options ranked cheapest to most expensive:
- Shade work, attic insulation, attic ventilation, and duct sealing. $400 to $4,000. Often gets you 2 to 4 degrees back without touching the AC. Best first step.
- Adding a window unit or mini-split to one problem room (typically the west-facing bedroom). $1,800 to $4,500 installed.
- Upsizing the full system from, say, 3 tons to 4 tons. $9,500 to $15,500. Only makes sense if a Manual J actually confirms undersizing, not just hot afternoons. See AC sizing and Manual J in San Diego.
The mistake: upsizing without doing the load calc first. An oversized system short-cycles, which is its own problem.
2. Low refrigerant from a leak
When refrigerant drops below spec, the system can still blow cool air, just not cool enough to satisfy the thermostat. So it runs and runs. On older systems still on R-22, this is extremely common. On newer R-410A or R-454B systems, it points to a real leak that needs finding and sealing, not just a recharge.
How to tell: supply air is cooler than return air, but only by 12 to 14 degrees instead of the normal 18 to 22. Ice may build on the indoor coil or copper suction line. Run times keep getting longer over a season. Energy bill climbs without weather explaining it.
Fix cost: leak search plus seal plus recharge runs $475 to $1,400 depending on where the leak is. R-22 recharges alone now exceed $150 per pound and a system holds 4 to 8 pounds. On R-22 systems with a real leak, replacement is almost always the better call. We broke down the math in AC refrigerant leak signs and cost.
3. Dirty filter or blocked return
The cheapest, easiest cause. A clogged filter chokes airflow across the evaporator coil. Less air across the coil means less heat transfer, which means longer run times to remove the same heat from your house. In dusty inland zones with construction nearby (Otay Mesa, parts of Chula Vista, new builds in San Marcos) filters can clog in 6 weeks.
How to tell: pull the filter. Hold it up to a light bulb. If light barely passes through, it’s done. Bonus: airflow at supply vents feels weak.
Fix cost: $8 to $35 for a replacement filter. Five minutes. If a fresh filter doesn’t change anything within 24 hours, move on to causes 1, 2, or 4.
4. Thermostat problem (placement, calibration, or wiring)
A thermostat in a hot spot (direct afternoon sun, near a kitchen, on an exterior wall) reads warmer than the rest of the house and keeps the system running. A miscalibrated or failing thermostat does the same. A loose C-wire on a smart thermostat can cause sensor errors that show as nonstop running.
How to tell: walk around with a separate room thermometer. If most rooms are at or below set point but the thermostat still reads high, the thermostat is the issue, not the AC. Also check that “fan” is set to AUTO, not ON. Fan ON keeps the blower running constantly even when cooling is satisfied, which feels like the same problem but isn’t.
Fix cost: thermostat relocation $185 to $385. Replacement with a new smart thermostat $250 to $525 installed. Calibration on a working unit is usually free during a tune-up.
5. Leaking or disconnected duct in the attic
San Diego attics get brutally hot (140°F+ in summer). If a duct is leaking or has disconnected from a register boot, conditioned air dumps into the attic and you cool the attic instead of the house. The system runs constantly trying to satisfy a load that’s bleeding away into the rafters.
How to tell: some rooms barely get any airflow. Energy bills are notably higher than neighbors with similar homes. A duct blaster test (typically $250 to $450) confirms it.
Fix cost: duct seal and repair $650 to $2,800 depending on how much of the system is leaking. Often qualifies for SDG&E rebates. Pairs well with replacing aging flex duct on systems over 15 years old.
Decision framework: which cause is yours?
Use this short checklist in order. Stop at the first one that matches.
- Did you check the filter and it was filthy? Replace it. Wait 24 hours. If still running constantly, move on.
- Does it only run constantly on hot days (90°F+) but cycle normally on mild days? Cause 1 (undersized). Start with shade, insulation, duct sealing before upsizing.
- Is the supply-to-return temperature split under 16°F at the registers? Cause 2 (refrigerant). Leak search needed.
- Does the thermostat read several degrees higher than other rooms? Cause 4 (thermostat). Check placement and wiring.
- Do certain rooms barely have airflow despite the blower running hard? Cause 5 (duct). Schedule a duct inspection.
In San Diego specifically, cause 1 is the one homeowners most often misdiagnose as cause 2. “It used to cool fine, now it can’t keep up, must be low on refrigerant” is the common assumption. Sometimes true, but often what changed isn’t the refrigerant, it’s that this year is hotter, or you added a sunroom, or the attic insulation has compressed over 20 years. A real diagnostic confirms which.
What this is costing you on your SDG&E bill
A 3-ton AC pulling 3,500 watts running 14 hours a day at the SDG&E summer peak rate of about $0.42/kWh (TOU-DR1, 4 to 9 p.m. on-peak block, 2026 rates) costs roughly $20 per day. A system that should be running 7 hours instead of 14 wastes about $10 a day, or $300 over a hot month. Two hot months a year is $600. That’s a real number worth fixing, especially when the fix might be a $25 filter.
Frequently asked questions
Is it bad for an AC to run constantly?
Yes. Compressors and blower motors are rated for a duty cycle. Constant running shortens their life by 20 to 40 percent and dramatically increases electric bills. It’s also usually a sign of a real underlying issue that gets worse if ignored.
Should my AC run all day during a San Diego heat wave?
On 95°F-plus days inland, run times of 10 to 14 hours are normal for a correctly sized system. Continuous running with the house still 4+ degrees above set point is not normal and points to undersizing, low refrigerant, or duct loss.
Will a bigger AC fix the problem?
Sometimes. Only if a Manual J calculation confirms the existing system is undersized for the actual house. Upsizing without that calc often creates a new problem: short cycling, poor humidity control, and reduced comfort. Do the load calc first.
Can my smart thermostat be making the AC run constantly?
Yes, in three ways. Sensor placement (direct sun, near a heat source), a loose C-wire causing erratic readings, or a setting that uses geofencing or remote sensors averaging in a hot room. Check all three before assuming the AC itself is the problem.
How much does it cost to find out why my AC is running constantly?
A diagnostic visit in San Diego runs $89 to $145, usually credited toward the repair. The diagnostic should include measuring supply and return temperatures, checking refrigerant pressure, inspecting filter and ductwork visually, and reviewing thermostat settings. Anyone who quotes a fix before doing those is guessing.
Is constant running worse than short cycling?
They’re different failure modes with different damage patterns. Constant running cooks bearings and shortens compressor life slowly. Short cycling stresses the start winding and capacitor every cycle and tends to fail faster, often within months. Both should be fixed.
What to do next
If your AC has been running constantly through this last hot stretch, start with the filter today and the thermostat placement check tomorrow. If neither of those moves the needle, book a diagnostic before the next heat event arrives. Call (442) 777-6440 for a free phone consult or to schedule a diagnostic with a vetted San Diego HVAC pro from our network.
Related reading: AC short cycling causes and fixes · AC not keeping up troubleshooting · AC sizing and Manual J in San Diego