Sizing It, and Checking Your Unit Can Take It
In the example filter bank used in this guide, the carbon bed is a major — and often the dominant — contributor to pressure drop. A high-resistance final particle filter can instead become the dominant component, so use actual filter pressure-drop data when available.
Sizing: The Carbon Face Is the Whole Game
Gas-phase filtration guidance is application-specific: face velocity, bed depth, media properties, contaminant concentration and humidity all affect performance. For the granular bed modeled here, pressure drop becomes an important constraint well before very high face velocities. Two practical design rules follow:
Rule 1: Keep carbon face velocity low
Pressure drop through a packed bed has both linear and velocity-squared terms, so face area is a powerful design lever — reducing face velocity can cut resistance substantially. For the example residential airflows in this guide, roughly 50–70 FPM is a useful design range for a large granular-carbon face. A 16×16” face is deliberately larger than many duct connections because the carbon bed is sized to airflow, not duct diameter. Lower velocity also increases nominal empty-bed residence time.
Rule 2: Run reduced airflow in smoke mode
During a smoke event, use the minimum filtered outdoor airflow that meets the home's ventilation needs while keeping the smoke load manageable. 40–60 CFM is a planning range used in this guide, not a universal requirement. Occupancy, house size, ventilation requirements, indoor sources and the unit’s controls all matter. Less flow means less pressure drop and longer nominal carbon residence time. Use CO2 as a ventilation indicator rather than a hard health cutoff.
For the assumptions used here, the modeled values land in the following range. These are planning estimates, not universal filter specifications; actual resistance depends on the exact filters, carbon media, housing and duct layout. With a 16×16 face, 2” of 4×8 mesh carbon, a low-resistance HEPA stage, and an allowance for the bypass ductwork, the whole bank computes to roughly:
| Smoke-mode airflow | HEPA + carbon bank | MERV-13 + carbon bank | Feasible for… |
|---|---|---|---|
| 60 CFM | ~0.27” w.c. | ~0.20” w.c. | often feasible with adequate static headroom |
| 80 CFM | ~0.40” w.c. | ~0.30” w.c. | often near the limit of modest-static units |
| 100 CFM | ~0.54” w.c. | ~0.41” w.c. | typically requires substantial static headroom |
| 130 CFM | ~0.82” w.c. | ~0.61” w.c. | requires substantial static headroom |
Note: the MERV-13 column models a single MERV-13 final stage. A two-MERV-13 build will have additional resistance; use the actual pressure-drop data for both filters when checking the unit.
Shrink that carbon face to 12×12” and the modeled carbon resistance rises sharply — in this example, the 60 CFM total jumps to ~0.44” and 100 CFM to ~0.95”. Use those figures as a sizing illustration, not as a substitute for pressure-drop data or measurement on the actual assembly.
How much should you trust these numbers?
The carbon-bed figures are clean-bed estimates with roughly ±2× uncertainty in this simplified model — granule shape, packing density, media properties and housing effects all matter. If your build is near the edge of your available static-pressure budget, verify the assembled bank with a manometer and, ideally, measure the resulting airflow before you close everything up.
The full derivation, including the Ergun-equation math, real-world validation, and an honest accounting of that uncertainty, is in the pressure-drop appendix.
Dampers: match your duct diameter
The dampers, unlike the filters, do size to your duct:
| Your duct | Typical continuous airflow | Suggested smoke-mode airflow | Suncourt dampers |
|---|---|---|---|
| 4” | 20–60 CFM | 30–50 CFM | ZC204 (NC) + ZO204 (NO) |
| 6” | 40–150 CFM | 40–60 CFM | ZC206 (NC) + ZO206 (NO) |
| 8” | 100–250 CFM | 60–80 CFM | ZC208 (NC) + ZO208 (NO) |
(The links above are the plug-in-transformer versions, ~$78–95 each. Suncourt’s ZoneMaster series also covers 5”, 10”, and 12” round duct — ZC210/ZO210 and ZC212/ZO212 for the larger sizes — if your ducting differs. Avoid the “S” (stud-mount) and “X” (no-transformer) variants unless you specifically want those.)
Will Your HRV Handle It? (Check Before You Build)
First, the useful physics: adding a filter normally moves the operating point toward lower airflow. The exact response depends on the fan and its controls. Constant-CFM ECM units may increase speed and electrical input to hold their setpoint until they reach their control limit; other units simply deliver less air. Because this design adds resistance only to the incoming side, a balanced unit may become supply-shy in smoke mode unless its controls compensate.
The check that actually matters is airflow. A spec like “155 CFM @ 0.2 in. w.g.” describes your unit’s total available static pressure at that flow — not a budget reserved for your filter bank. Your intake hood, duct runs, and elbows are already spending part of it. So:
- Pick your target smoke-mode airflow.
- On your unit’s fan/performance curve, read the static pressure it can still deliver at that flow (available static rises as flow drops — another win for Rule 2).
- Subtract your estimated rest-of-system losses (hood + ducts + elbows).
- What’s left is your filter-bank budget. Compare it to the table above.
If you come up short, you have four levers: enlarge the carbon face, reduce smoke-mode airflow if the resulting ventilation rate remains acceptable, choose a lower-resistance final stage, or choose a ventilation unit with more available static pressure. Use the published fan curve for the specific model rather than assuming a brand or product family has a particular static-pressure capability.
Duct layout tips
- Use long-radius tees or sweep elbows at the bypass junctions where practical to reduce fitting losses; the exact savings depend on the fitting geometry and airflow.
- Consider upsizing the bypass loop duct by one size for extra margin.
- Keep several duct diameters of straight run before each damper where practical, following the damper manufacturer’s installation guidance.