DIY HEPA H13 Air Purifier — Instructions and Build Options

A HEPA H13 air purifier filters 99.97% of particles 0.3 microns and larger — dust, pollen, mold spores, and some viruses. Commercial units cost $60–300. A DIY version with the same filtration specs costs $10–35 and takes 1–3 hours to build. This guide covers three build options: from the ultra-simple box-fan design to a two-stage carbon pre-filter system.

What is HEPA H13 and Do You Need It

HEPA (High Efficiency Particulate Air) is a filtration standard. Class H13 captures at least 99.95% of airborne particles in a single pass. H14 captures 99.995%. For home use, H13 is more than sufficient.

A DIY air purifier makes the most sense in these situations:

• Allergies to dust, pollen, or pet dander
• Home renovation — drywall dust, concrete, wood particles
• Wildfire smoke or heavy urban pollution
• Mold risk from humidity or water damage

The key metric is CADR (Clean Air Delivery Rate, m³/hr or CFM). For a 200 sq ft room with 8 ft ceilings, you need at least 100–130 m³/hr (60–80 CFM).

Option 1: The Simplest Build — Box Fan + HEPA Panel

Cost: $10–18 / Build time: 30 minutes / Skill level: beginner

This design was popularized by Dr. Richard Corsi — the "Corsi-Rosenthal Box." A box fan pulls air through a HEPA panel attached to its intake side.

Parts list

• Box fan or axial fan 8–10 inch — $8–15
• HEPA H13 filter panel sized to match the fan — $5–12
• Aluminum HVAC tape or duct tape — $3

Assembly

1. Hold the HEPA panel against the intake side of the fan. Trace and trim if needed.
2. Press the filter against the fan intake and seal all edges with tape — no gaps, or air will bypass the filter.
3. Run the fan on high. Air should pull through the filter, exit the back of the fan clean.

Expected CADR with an 8-inch fan: 80–120 m³/hr. Good for a room up to 160 sq ft.

Option 2: Plywood Box Purifier

Cost: $20–35 / Build time: 3–4 hours / Skill level: intermediate

A wooden enclosure solves the aesthetics and noise issues: the fan is recessed into a plywood box, walls dampen vibration, and the unit looks like a purchased appliance.

Materials and tools

• Plywood 10mm or MDF 8mm, approx 2×2 ft sheet
• Centrifugal or axial fan 120–150mm (computer case fan works great) — $5–15
• HEPA H13 panel 200×200 or 250×250mm — $6–12
• Corner brackets ×10, wood screws 3×16mm
• Jigsaw, drill, screwdriver, tape measure

Step-by-step assembly

1. Cut the box panels. For a 250×250mm filter: box dimensions 280×280×120mm. Cut 4 side panels 280×120, one top 280×280, one bottom 280×280.
2. Cut the fan hole in the top panel using a hole saw or jigsaw.
3. Drill intake holes in the bottom — 4×4 grid of 15mm holes for airflow.
4. Assemble the box with screws and brackets. Glue joints with PVA for an airtight seal.
5. Place the HEPA panel inside on the bottom. Secure with small wood strips.
6. Mount the fan in the top, screwed flush. Check airflow direction — fan should pull air up through the box.
7. Test for leaks — hold a lit stick of incense near the seams while the fan runs. Smoke should only enter through the bottom grid.

Expected CADR: 150–200 m³/hr. Sufficient for a room up to 300 sq ft.

Option 3: Two-Stage Carbon + HEPA System

Cost: $28–50 / Build time: 4–6 hours / Skill level: confident maker

Adding an activated carbon pre-filter absorbs odors, VOCs (volatile organic compounds), and formaldehyde. Essential if you have new laminate flooring, plywood furniture, or cigarette smoke.

Airflow sequence

Air → carbon pre-filter G4 (traps large dust and absorbs odors) → HEPA H13 (captures fine particles) → fan → clean air out.

Additional parts

• Carbon filter sheet G4 or carbon-impregnated foam 250×250mm — $4–8
• Same box from Option 2, plus an extra 35mm-tall section for the carbon layer

Installing the carbon layer

1. Cut the carbon material to box dimensions — 250×250mm.
2. Place it on top of the HEPA panel, carbon side toward HEPA, fabric face toward incoming air.
3. Secure with wood strips to prevent shifting.

Carbon layer: replace every 2–3 months. HEPA filter: every 6–12 months depending on air quality.

How to Choose the Right Fan

Centrifugal (blower-style) fans create higher static pressure — better suited for pushing air through the dense HEPA media. PC case fans in 200mm size are the most affordable entry point.

Common Mistakes to Avoid

• Gaps between filter and frame — air bypasses the filter entirely. Seal with aluminum tape, no exceptions.
• Wrong airflow direction — some fans blow the wrong way. Always check before installing.
• Underpowered fan — HEPA media has high resistance. A weak fan won't push enough air through.
• Unverified "HEPA" filter — cheap filters labeled "HEPA-style" may not meet H13 specs. Check the classification on the packaging.