“Fresh-Air” Intake vs. Real Filtration – What We’ve Learned
Note: I’m a homeowner piecing this together while a CBH warranty ticket is still pending (no repair date yet).
What We Found in the Factory Setup
| Item | Observation |
|---|---|
| Fresh-air flex duct | Runs from an exterior hood into the main return plenum. In theory this should supply outdoor make-up air. |
| Small blue filter screen | Was missing during the home inspection. Our independent home inspector flagged the absence, so CBH added a light‑blue screen afterward. It arrived with no label, no MERV rating, and no guidance on when or how to replace it. The screen also had holes and gaps around the edges, letting air bypass it. |
Because CBH never provided a rating—or even basic maintenance instructions for the screen—it was impossible to know whether the material could handle smoke, pollen, or fine dust.
My Interim Upgrade
I replaced that unknown screen with a darker-blue MERV-1 pad (photo below).
All incoming outdoor air now passes through this pad before it mixes with return air from the house, but MERV-1 only stops coarse debris. Fine smoke and pollen still get through.
Original Unrated Filter
Updated MERV-1 Filter after only 2 months during fire season (Summer 2024)
Air-Quality Monitor Results
| Scenario | Before any pad | With MERV-1 pad |
|---|---|---|
| Boise wildfire smoke events | Indoor readings jumped to “Unhealthy” within an hour. | Peaks now settle near “Moderate.” An improvement, but still not ideal. |
| High-pollen days | Noticeable spikes on the monitor. | Spikes are lower, yet still present. |
Extra Twist: Detached Duct in the Attic
During an attic check we discovered the flex duct has partially pulled away from the exterior wall jack, so the system is grabbing a mix of attic air and outdoor air. We’ve opened a warranty request; CBH hasn’t scheduled the fix yet.
Detached duct in the attic
Planned Fix After the Duct Is Re-Attached
- Install a prefab filter plenum directly after the exterior intake, using a cartridge in the MERV 8–13 range to tackle smoke and fine dust.
- Keep the standard whole-house filter at the furnace as a second layer.
- Re-check air-quality readings during the next smoke event to confirm the improvement.
Take-Aways for Fellow Homeowners
- A fresh-air duct only helps if it’s sealed and uses a properly rated filter.
- If the builder supplies an unlabeled screen with no maintenance guidelines, assume you’ll need to upgrade.
- In wildfire or high-pollen regions, filtering the intake air to MERV 8–13 before it hits the return plenum can make a big difference.
- Periodically verify that the duct is still clamped to the exterior hood; vibration and settling can loosen fittings.
Bottom Line (Homeowner View)
Until the duct is re-secured and a real cartridge filter is in place, outdoor smoke and pollen will keep sneaking indoors—and our HVAC equipment will keep ingesting debris it was never meant to handle.