DIY Plan: Turning an FFU into a Horizontal Flow Hood

2025-09-16

1. Pick the Right FFU

Size: A 2x4 ft FFU (600x1200 mm) is popular—big enough for jars, plates, and spawn bags.
Filter rating: Certified HEPA 99.97%+ at 0.3 microns minimum. ULPA (99.999%) is overkill and pricier.
Airflow: Around 0.3–0.5 m/s (60–100 ft/min) output.
Noise rating: Some FFUs can roar like a jet engine—check dB levels if you value your sanity.

2. Decide the Orientation

FFUs are designed to blow air downward (vertical). For mushroom growing, you’ll want it horizontal (side‑blowing).
The fix: Mount the FFU on its back so it blows across your workspace.
Make or buy a stable stand/frame so the unit doesn’t topple (remember, they’re heavy!).

3. Build the Housing

Think of the housing as the “bench body” that the FFU plugs into. This channels airflow smoothly toward your work area.

Frame: Plywood box, steel frame, or heavy‑duty plastic. Dimensions just large enough to house the FFU at the rear.
Work surface: Smooth countertop (stainless steel, melamine‑coated wood, or sealed laminate). Needs to be flat, stable, and washable.
Sides/top: Add side panels and a “roof” to enclose the working area partly. This prevents room drafts and keeps the clean zone stable. Acrylic or polycarbonate are nice if you want see‑through panels.

(Imagine a desk with three walls on the sides and back, then the FFU forms the “back wall” blowing clean air forward.)

4. Seal & Smooth

Use weather stripping, silicone, or gaskets around the FFU’s frame so all airflow passes through the HEPA and not around it.
Smooth out inner surfaces—fewer ledges = fewer dust/spore traps.
Paint raw wood with sealant or epoxy for easier wipe‑downs.

5. Add Pre‑filtration

Attach a pre‑filter (G4 or similar) on the intake side of the FFU.
This captures coarse dust and extends HEPA lifespan

6. Test Airflow

Use an incense stick or smoke pen: hold it in the work area. The smoke should travel smoothly in straight lines across the bench, not swirl or bounce back.
No drafts sneaking in from gaps. If you see turbulence, extend the side panels or adjust positioning.

7. Ergonomics & Comfort

Height: Build so the work surface is at your elbow level when seated. You’ll be doing fine sterile work; comfort matters!
Lighting: Overhead LED strip or mounted lamp outside the airflow zone (never inside, or it becomes a germ hotel).
Space: Leave ~12 inches of clear sterile airflow between the HEPA outlet and your work (e.g., agar plate, jar mouth). Don’t work right up against the filter face.

8. Maintenance

Replace/clean pre‑filter regularly (monthly or as it clogs).
Replace HEPA according to hours of use or pressure drop readings (often 3–5 years).
Before every session, wipe interior surfaces with 70% isopropyl alcohol.

You’ll have a DIY horizontal flow hood powered by a professional‑grade FFU. It won’t look as sleek as a factory bench, but in practical mushroom terms it does the same job: a reliable, sterile air shield protecting your cultures.

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