The HVAC problems that show up in an IAQ assessment are usually not the ones that prompt a service call. The system is running. The house is cool. No one is complaining.
But the data tells a different story. Here are the problems we see most often across the homes we assess in the western San Fernando Valley and Conejo Valley.
Filter Bypass
This is the most common issue, full stop. We find it in roughly half the homes we assess. The filter is the right rating, installed correctly, and sometimes brand new. But there are gaps around the frame.
These gaps exist because residential filter housings are not precision equipment. They are sheet metal boxes with approximate dimensions. The filter slides in, the door closes, and air does what air does: it takes the path of least resistance. A quarter-inch gap creates a highway for unfiltered air.
The impact is measurable. A home with proper filter fit and MERV 13 media typically shows indoor PM2.5 under 10 micrograms, even when outdoor levels are elevated. The same home with filter bypass can show 20 or higher.
The fix is often simple. Foam tape, cut-to-fit gaskets, or a new filter frame that matches the filter dimensions. Total cost: under $20 in materials.
Duct Leakage in Attic Spaces
Most LA homes in our service area have the air handler and ductwork in the attic. Attic temperatures in the Valley reach 140 to 160 degrees in summer. The ducts are usually flex duct connected with zip ties and duct tape.
Over time, connections loosen. Tape degrades. The duct system, which was maybe 85% efficient when installed, drops to 70% or lower. That means 30% of your conditioned, filtered air goes into the attic. The system compensates by running longer. And it pulls makeup air from wherever it can: attic gaps, wall cavities, the garage if returns are near the garage wall.
This is why a home can have a good filter and still show elevated PM2.5. A portion of the air in the house never passed through the filter. It came in through duct leakage paths.
Oversized Systems
Oversizing is pervasive in residential HVAC. Contractors tend to install systems larger than the load calculation calls for, partly as a safety margin and partly because no one complains about a system that cools too fast.
But an oversized system short-cycles. It reaches the thermostat set point quickly, shuts off, and sits idle. During those idle periods, no air is moving through the filter. Humidity is not being managed consistently. Temperature swings are wider.
In homes with oversized systems, we see more temperature variation between rooms and less consistent PM2.5 control. The fix is not replacing the system. It is changing the fan scheduling to run the fan independently of the cooling cycle, providing continuous filtration even when the compressor is off.
Inadequate Return Air Paths
A bedroom with a closed door and no return air register creates a positive pressure zone. Supply air pushes in, the door blocks the return, and air escapes through gaps under the door, around the door frame, or into the attic through any available opening.
This starves the return side of the system, reduces airflow across the filter, and creates room-by-room imbalances. The master bedroom reads 1,500 ppm CO2 at 6 AM while the living room is at 600.
Solutions range from simple (undercutting the door, adding a transfer grille) to involved (running a dedicated return duct). The right approach depends on the specific room layout.
Condensate Drain Issues
The evaporator coil produces condensation every time it runs. That water drains through a condensate line to the exterior or to a drain. When the line clogs — which happens gradually from algae and debris — water backs up. In some configurations, this triggers a float switch that shuts down the system. In others, water drips into the drain pan and eventually overflows.
Beyond the obvious water damage risk, a backed-up condensate drain creates standing water near the evaporator coil. Standing water plus warm air equals biological growth. This is how a mechanical issue becomes an air quality issue.
A flush of the condensate line during annual maintenance prevents this. It is a five-minute task that avoids a potential problem.
If any of this sounds familiar, an IAQ assessment will tell you exactly which issues are affecting your home and in what order to address them. 90 minutes, $195. Our free IAQ guide covers the connection between HVAC condition and your air quality numbers.