Engineering brief by Marcus Halverson, P.E., LEED AP
Wildfire Smoke Indoor Air Quality for Altadena, La Canada and Pasadena HVAC Systems
This engineering brief is about smoke-season filtration and return leakage control. The practical lens is Altadena, but the same decision logic applies across Los Angeles because the basin is a patchwork of coastal air, valley heat, hillside access, older ductwork and premium remodel expectations. A good HVAC plan is not just equipment selection. It is a sequence of load, airflow, electrical, access, controls, permits, maintenance and documentation decisions — and each step has to be done in the right order or the next one becomes more expensive.
For context, Altadena brings foothill heat, wildfire smoke exposure and rebuilt-home HVAC planning. The related service is Indoor Air Quality, where the normal intent is filtration, ventilation, humidity, wildfire smoke and dust control. That combination is exactly where thin advice fails: a rebate chart, a brand ranking or a single SEER2 number cannot tell you whether your home has the return capacity, drain route, line-set path or service clearance to make the upgrade work. The data points below come from 19 years of LA mechanical practice, ACCA Manual J/D/S, ASHRAE 62.2, the U.S. Department of Energy heat pump program documentation and current 2025–2026 LADWP and TECH Clean California program language. Where I cite a source, the link goes to the original — not a marketing summary.
Read this once before you sign anything. The decisions you lock into the proposal are very hard to undo six months later when the system has been operating outside its design window.
1. Why HVAC is your first line of defense during smoke events
Wildfire smoke episodes in Los Angeles — Eaton Fire in January 2025, Palisades Fire in January 2025, recurring Santa Ana driven brush fires — produce indoor air that can exceed AQI 200 within hours when outdoor smoke pushes through normal building leakage. The HVAC system is the largest single tool you have for filtering indoor air, but only if the cabinet, filter and return paths are configured correctly. EPA wildfire smoke guidance is unambiguous: a properly-fit MERV 13 filter run continuously through a residential HVAC system reduces fine particulate by 60–90% in occupied rooms.
For Altadena homeowners — particularly in Altadena, La Canada Flintridge and Pasadena where the 2025 Eaton Fire impact was severe — the IAQ conversation is no longer optional. It is part of standard residential HVAC maintenance. The good news: the upgrade is affordable when planned, and the same upgrades improve comfort and reduce dust during normal operation.
2. The MERV 13 question — when it works and when it does not
MERV 13 filters capture more than 90% of particles in the 1–3 micron range, which includes the PM2.5 fraction that dominates wildfire smoke. The filter rating is necessary but not sufficient. The filter must (1) actually fit the cabinet without bypass air, (2) have enough media surface area to keep face velocity under 300 FPM, and (3) operate within the blower’s pressure-rise capacity.
One-inch fiberglass slot grilles typically cannot do MERV 13 cleanly — face velocity is too high, media area is too small, and pressure drop is too steep. Upgrading the filter rating in a 1-inch slot grille without changing the cabinet creates bypass leakage around the filter, which negates the rating. The actual fix is a 4-inch or 5-inch media cabinet (Honeywell F100, Aprilaire 213, Lennox HCC, Carrier EZ Flex) sized for the system airflow. Cost: $400–$900 installed. Lifetime: 15+ years on the cabinet, with filter replacement every 6–12 months.
3. Return leakage — the quiet IAQ failure
If the return-air pathway draws air from the attic, the basement or unconditioned spaces, the filter is filtering the wrong air. During wildfire smoke episodes, attics and unconditioned spaces typically carry higher particulate concentrations than the conditioned interior because outdoor air infiltrates through soffit vents, ridge vents and envelope cracks.
We measure return leakage with smoke pencil tests, photo documentation and pressure mapping during IAQ diagnostics. Common findings: returns drawing through unsealed platform-return chases (10–25% leakage), filter cabinets where the door does not seal (5–15% bypass), return ducts in attics with leaking joints (5–20% leakage). Sealing those paths is part of the IAQ scope, not an extra.
4. Fan-only mode — the smoke event protocol
During a wildfire smoke event, the recommended HVAC protocol is: switch the thermostat fan setting to "On" (continuous fan), close any outdoor-air ventilation damper if your system has one, run portable HEPA air purifiers in occupied bedrooms (CADR-rated for the room volume), and keep windows closed. The HVAC system continuously circulates indoor air through the MERV 13 filter, which traps particulate that infiltrated through normal envelope leakage.
What not to do: turn the system off "to save energy." With the fan off, particulate accumulates in occupied rooms. The energy cost of continuous fan operation during a 2–3 day smoke event is typically $5–$15 — far less than the health cost of unfiltered indoor air. Variable-speed ECM blowers (standard on inverter systems) run continuous fan at low speed efficiently.
5. Air purifier integration and ERV considerations
Whole-home air purifiers — UV light, bipolar ionization, advanced filtration — are popular add-ons but require careful selection. UV light is effective on biological growth on coil surfaces but does little for particulate. Bipolar ionization claims are mixed; some independent testing shows limited benefit and possible byproduct generation. We are conservative on bipolar ionization without published independent validation.
The most reliable IAQ stack is: properly-fit MERV 13 media filter, sealed return paths, continuous-fan capability, and a portable HEPA in primary occupied spaces. For homes with heat-recovery ventilators (ERV/HRV), close the outdoor-air damper or switch to recirculation mode during smoke events. Bringing in fresh outdoor air during a wildfire is counterproductive regardless of how good your filtration is.
6. Maintenance after a smoke event
After a major smoke event (AQI > 150 for 24+ hours in your area), schedule HVAC maintenance within 30 days. Replace the MERV 13 filter immediately — even a fresh filter loads quickly with smoke particulate. Clean the indoor coil if visible smoke residue is present. Inspect the outdoor coil for ash and char (particularly in fire-adjacent neighborhoods). Check condensate drain for ash blockage. Verify thermostat sensors are reading cleanly — some sensors drift with high particulate exposure.
For Altadena homes that were within 5 miles of an active fire, consider a duct cleaning if visible residue is present in supply registers. NADCA-certified duct cleaning is the only cleaning method we recommend; uncertified "$99 duct cleaning" services typically do more harm than good.
7. The IAQ project that pays for itself
A properly-designed IAQ upgrade for a typical Pasadena, Altadena or La Canada home includes: 4-inch MERV 13 media cabinet with 1-year filter ($400–$900 installed), return path sealing ($200–$600), continuous-fan ECM blower if not present ($600–$1,400 if retrofitting older equipment), and a single portable HEPA for the primary bedroom ($300–$600).
Total cost: $1,500–$3,500. Comfort and IAQ benefit: measurable particulate reduction during smoke events, lower dust accumulation during normal operation, longer equipment life from cleaner coils, fewer allergy and respiratory complaints. The cost is recovered over 5–10 years through reduced equipment service, longer filter intervals on the existing 1-inch system, and better health outcomes that are hard to quantify but easy to feel.
Authoritative references used in this brief
The technical claims above are sourced from published U.S. Department of Energy, ENERGY STAR, ACCA, ASHRAE, EPA, LADWP, TECH Clean California, IRS and manufacturer engineering documentation. Direct links are listed below for verification. Where regulations or rebate programs may shift between writing and reading, treat the program page as the source of truth and use this article as a decision framework.
Reference: U.S. Department of Energy — Heat Pump Systems: https://www.energy.gov/energysaver/heat-pump-systems
Reference: ENERGY STAR — Central Air Conditioner & Heat Pump Buying Guide: https://www.energystar.gov/products/heating_cooling
Reference: ACCA — Manual J, D and S Standards: https://www.acca.org/standards
Reference: ASHRAE 62.2 — Ventilation and Acceptable Indoor Air Quality in Residential Buildings: https://www.ashrae.org/technical-resources/standards-and-guidelines
Reference: EPA — Indoor Air Quality (IAQ) and Wildfire Smoke Guidance: https://www.epa.gov/indoor-air-quality-iaq
Reference: LADWP — Consumer Rebate Program: https://www.ladwp.com/account/customer-service/rebates-and-programs
Reference: TECH Clean California — Heat Pump Incentives: https://techcleanca.com/
Reference: AIM Act / EPA SNAP — Refrigerant Phase-Down: https://www.epa.gov/climate-hfcs-reduction
Reference: IRS — Energy Efficient Home Improvement Credit (Form 5695): https://www.irs.gov/forms-pubs/about-form-5695
About the author
Marcus Halverson, P.E., LEED AP, is the Principal HVAC Engineer & Founder of Skyline Thermal Labs. Marcus Halverson is a licensed mechanical engineer with 19 years of building-systems experience across Los Angeles, including coastal corrosion-zone work, hillside heat pump retrofits and historic-home airflow redesigns. He leads Skyline Thermal Labs’ diagnostics, commissioning and rebate-documentation standards. Marcus has commissioned more than 1,400 residential systems across Greater Los Angeles, including coastal corrosion-zone work in Santa Monica, Venice and Manhattan Beach; hillside heat pump retrofits in Hollywood Hills, Bel Air and Pacific Palisades; and historic-home airflow redesigns in Pasadena, Los Feliz and South Pasadena. He sits on technical-advisory committees for ASHRAE local chapter education and contributes to ACCA Manual D peer-review work.
For a project consultation in Altadena on indoor air quality or any related work, call +1 (213) 277-7557 or use the external booking form. Direct technical questions about this brief can be sent to [email protected].
Fast answers to the questions readers send most
Is MERV 13 always safe? — MERV 13 is generally safe on residential systems sized for it, but only when the filter cabinet fits a true MERV 13 media filter (1-inch fiberglass slot grilles cannot do MERV 13 properly), the filter face area is large enough to keep face velocity under 300 FPM, and the blower can handle the added pressure drop. We measure static pressure before and after upgrading filtration; if the system drops below the manufacturer’s minimum airflow, we change the cabinet, not the filter.
Can HVAC help during smoke events? — During wildfire smoke events, run the system fan continuously with a properly-fit MERV 13 (or higher) filter, close fresh-air ventilation dampers if your system has them, run a portable HEPA in bedrooms, and avoid kitchen exhaust use unless make-up air is filtered. EPA wildfire smoke guidance has specific recommendations for residential HVAC during smoke episodes.
What should be checked before adding filtration? — The honest answer depends on load, ductwork, access, controls and the installed equipment. A diagnostic visit makes those variables visible before a recommendation is made. Call +1 (213) 277-7557 or use the booking form for a written assessment.