Choosing Your Filter Stack
The pre-filter and carbon stages are the same in every tier. The final particle stage is where cost and sourcing pain live — so pick your tier honestly.
Filtration Tiers: Best, Better, Budget
| Tier | Final stage | Single-pass capture | Cost | Sourcing |
|---|---|---|---|---|
| 🥇 Best | MERV-16 or HEPA | High-efficiency particle filtration | $150–300 | Hard — residential sizes are scarce |
| 🥈 Better | Two MERV-13 in series | ~75% if each stage is 50% efficient | $30–40 | Any hardware store — recommended |
| 🥉 Budget | Single MERV-13 | ≥50% in the MERV 13 test range* | $15 | Any hardware store |
🥇 Best: MERV-16 or HEPA final stage
MERV-16 has a minimum composite efficiency of 95% in the 0.3–1.0 µm test range; a HEPA filter is a different, higher-efficiency classification. Those ratings describe standardized test performance, not a universal percentage of all wildfire-smoke particles. The catch: residential-size MERV-16 and HEPA panels are hard to find and often cost $150–300. If you can find one in your size at a sane price, this is the gold standard.
🥈 Better: Two MERV-13 filters in series (recommended for most people)
Filter efficiency compounds. If two stages were each 50% efficient at a particular particle size, the combined penetration would be 25%, or 75% removal. That does not make the assembly a MERV-14 filter: MERV ratings apply to standardized filter tests, not arbitrary combinations. Actual performance depends on the specific filters and airflow. At the low airflows used here, the added pressure drop may be modest, but verify it for your equipment.
🥉 Budget: Single MERV-13
MERV-13 is specified to achieve at least 50% composite efficiency in the 0.3–1.0 µm test range. Actual filter efficiency can be higher, and wildfire smoke spans a range of particle sizes, so don't interpret the rating as "50% of all smoke." That sounds mediocre, but remember: the intake filter isn’t working alone. Combined with indoor recirculation filtration and a reasonably tight envelope, a single MERV-13 intake stage still produces meaningfully better indoor air than no filtration. Be honest with yourself about what it catches, and lean harder on the indoor recirculation stage.
The Carbon Stage (All Tiers)
Thin carbon-dusted pads can provide only limited gas-phase removal because they contain little sorbent and offer very little bed depth. For this design, a 2-inch granular bed is a practical starting point, not a universal minimum; the required depth depends on the media, contaminant, airflow, humidity and target removal.
- An off-the-shelf carbon panel (e.g., Camfil CamCarb PC line, or the V-cell carbon filters and refillable carbon trays at Air Filter USA).
- A DIY tray: perforated sheet metal top and bottom, filled with bulk virgin coconut shell activated carbon. Coconut-shell carbon is a reasonable general-purpose choice for many VOCs, but "best" depends on the target contaminants and the media specification.
For a low-pressure-drop DIY bed, coarse granules such as 4×8 mesh are a sensible starting point. Finer carbon can pack more tightly and increase pressure drop; the actual tradeoff between pressure drop and adsorption depends on the specific media. As you’ll see in Part 3, the carbon bed can be a major part of the pressure budget — mesh size and face area are important design choices.
A plain activated-carbon bed can adsorb many VOCs and other gas-phase contaminants, but performance varies substantially by contaminant and media. Small, polar compounds such as formaldehyde are generally more difficult for unmodified carbon to capture than many larger, less-polar VOCs. If aldehydes are an explicit design target, see the optional chemisorptive-media upgrade below.
Optional upgrade: aldehyde-specific media
If you want to close the formaldehyde gap, the fix isn’t more plain carbon — it’s chemisorptive (impregnated) media that oxidizes or reacts with the gas instead of just adsorbing it. Three routes:
- Potassium permanganate–impregnated media (on alumina or carbon) are one established option for aldehydes and other reactive gases. Purafil and Camfil are examples of HVAC media suppliers; select a formulation using the supplier’s performance data for the contaminants you care about.
- Potassium iodide–impregnated carbon — the media Austin Air uses in its formaldehyde/ammonia filters.
- Amine- or sulfamic-acid–impregnated carbon — reacts with formaldehyde directly; some formulations beat permanganate on HCHO specifically.
You don’t need a fourth housing. Blend the chemisorptive media into your existing carbon tray, or add one thin dedicated panel. You can even source KMnO₄-modified coconut-shell carbon — the same base as your main carbon stage, just impregnated — so it drops straight into the DIY tray.
Two things make this work especially well in this design:
- Low face velocity is generally helpful because it increases nominal contact time and reduces pressure drop; the appropriate operating range still depends on the specific media and contaminant.
- Some permanganate media changes appearance as it is consumed, but color should be treated as a supplemental indicator rather than a precise endpoint unless the manufacturer specifies it that way.
Caveats: impregnated media has finite reactive capacity, so use the media manufacturer’s replacement guidance. Be cautious about consumer products that make specific formaldehyde-removal claims without independent performance data. For a single-pass ventilation application, favor media with published performance information for the contaminants and airflow you actually expect.
Technical references
- U.S. EPA — Indoor Air Filtration: wildfire-smoke filtration guidance, including HVAC filters and portable air cleaners.
- ASHRAE Handbook — Air-Cleaning Systems: gas-phase filtration, adsorption and packed-bed design considerations.
- ASHRAE Journal — Filtration and Air Cleaning: MERV classification and filter-efficiency context.