Building It: Parts, Wiring and Automation
The parts list, the wiring that makes the system fail toward protection, and the automation that means you never have to think about it.
Materials List
Core system (all tiers)
| Item | Purpose | Approx. cost |
|---|---|---|
| Suncourt ZC damper (normally closed) — sized to your duct (links in the sizing table) | Straight run; closes during smoke mode / power loss | $78–95 |
| Suncourt ZO damper (normally open) — sized to your duct (links in the sizing table) | Filter loop; opens during smoke mode / power loss | $78–95 |
| Galvanized tees ×2, duct, fittings | Bypass loop | $40–80 |
| Custom or off-the-shelf filter housing | Holds the 3-stage bank | $50–150 DIY; see HVACQuick CFB inline filter boxes for pre-built options |
| MERV-8 pre-filter (16×16 commodity size, shares the carbon face) | Stage 1 — protects carbon & final filter | $10–15 |
| Carbon panel or DIY carbon tray (16×16 face, 2” bed) + bulk coconut shell carbon, 4×8 mesh | Stage 2 — VOCs and gas-phase pollutants | $50–150 |
| Final particle filter (per your tier) | Stage 3 | $15 (MERV-13) to $300 (HEPA) |
Fail-Toward-Filtration Wiring
The control concept below uses one relay to switch the damper transformers so that loss of control power returns the dampers to the filtered path. Exact 120 V and 24 V wiring must follow the actuator/transformer instructions, component ratings, and applicable electrical code; use a qualified electrician where required.
- De-energized (power loss, or smoke mode triggered): ZC closes the straight run, ZO opens the filter loop → air is filtered
- Energized (normal mode): ZC opens the straight run, ZO closes the filter loop → air takes the free path
Losing control power defaults the outdoor-air path toward filtration; it does not guarantee that the ventilation system itself continues operating, nor does it make every other failure mode safe.
Automation (Optional but Worth It)
| Item | Purpose | Approx. cost |
|---|---|---|
| Shelly 1 Gen4 WiFi relay | Home Assistant–controlled switch for both dampers | ~$25 |
| PurpleAir outdoor sensor | Real-time local PM2.5 — the automation trigger | ~$240–300 |
| PurpleAir indoor sensor (Touch) | Verifies your filtration is actually working | ~$210 |
| Aranet4 (or similar NDIR CO2 monitor) | Tells you when the house needs fresh air | ~$190 |
| AirGradient ONE | VOC trend monitoring — a rough indicator of changing gas-phase conditions | $138 kit / $230 assembled |
The automation logic is simple: when your outdoor PM2.5 sensor crosses a chosen threshold, Home Assistant de-energizes the relay and the system switches to filtered mode. Add hysteresis (for example, different trigger and release thresholds) so the dampers do not repeatedly switch when readings hover near the line. The threshold is an operational choice, not a universal health boundary. Even without automation, a manual switch can provide smoke mode.
Why the AirGradient? Activated carbon does not provide a simple universal end-of-life signal. An indoor VOC sensor can help identify changes in gas-phase conditions, but it cannot reliably diagnose carbon saturation: readings can change because of cooking, cleaning products, occupants, humidity and other sources. Use it as a trend indicator, not as a breakthrough alarm.
Total Cost
Pick your configuration and the estimator adds up the line items from the tables above:
The point of this approach is targeted retrofit rather than replacement of the entire ventilation system. Actual costs vary widely with equipment, duct layout, filter availability and labor.