Clean Air Room Planner

Overview

Every wildfire season, three mistakes repeat at scale: people buy purifiers too small for their rooms (marketing "large room" claims assume only 2 air changes per hour), they run them on quiet mode (CADR drops 3–5×), and they put them in the wrong place. The engineering is simple — clean air delivery rate (CADR) must match room volume × desired air changes per hour (ACH) — but the industry makes it hard to do the math.

This skill is that math, plus the operational kit around it:

1. Purifier sizing — CFM/CADR needed from room dimensions and target ACH (5 for smoke events, 4–5 allergy season, 2–3 general freshness), with noise and CADR-margin guidance.
2. Adequacy check — is your existing purifier enough for THIS room? Verdict + how long a full air change takes + whether running it on low makes it useless (it usually does).
3. DIY box-fan filter sizing — the CR Box (corsi-rose boxfan + MERV-13) engineering: fan free-flow CFM, filter pressure drop, effective clean airflow estimate, and filter-area rules.
4. Pollution diary — log indoor/outdoor PM2.5 readings; the analyzer computes indoor/outdoor ratio, infiltration factor, decay constants, and event summaries (how bad, how long, what helped).
5. Filter-life tracker — real usage hours vs rated life with change-due dates instead of "check the color of the light".

When to Use

• Buying a purifier: "what CADR do I need for a 20 m² bedroom / 250 sq ft living room?"
• Smoke/pollen event: "is my purifier adequate, and where do I put it?"
• Building a DIY box-fan filter (Corsi-Rosenthal box)
• Deciding whether to run HVAC fan / recirculate during a smoke event
• "When should I actually change this filter?"
• Reviewing a PM2.5 log: how much smoke got in, did the purifier help?

Don't use for: CO / radon / VOC removal (HEPA does nothing for gases — activated carbon is a different mechanism; the tool says so explicitly), industrial cleanrooms, or medical-grade isolation.

Quick Start

# Size a purifier for a room (metric)
python3 scripts/clean_air.py size --area 20 --ceiling 2.7 --use smoke

# Same in imperial
python3 scripts/clean_air.py size --area 220 --height 9 --unit ft --use smoke

# Is my purifier enough? (rated 300 m³/h CADR)
python3 scripts/clean_air.py check --area 20 --ceiling 2.7 --use smoke --cadr 300

# DIY Corsi-Rosenthal box from a 20" box fan + 4 MERV-13 filters
python3 scripts/clean_air.py crbox --fan-cfm 1800 --filters 4

# Pollution diary
python3 scripts/clean_air.py log add --indoor 45 --outdoor 210 --note "smoke peak"
python3 scripts/clean_air.py log add --indoor 18 --outdoor 190 --purifier on
python3 scripts/clean_air.py log analyze

# Filter life
python3 scripts/clean_air.py filter new --name "Living HEPA" --rated-hours 2000
python3 scripts/clean_air.py filter tick --name "Living HEPA" --hours 8
python3 scripts/clean_air.py filter status

The Physics in Brief

CADR sizing

required CADR (m³/h) = room volume (m³) × target ACH

(ACH conventions from smoke-readiness guidance: 5 ACH keeps PM2.5 at a fraction of outdoor levels with windows closed; higher is better during peaks.)

Buy 1.5–2× the computed CADR if you want to run at low speed (quiet, overnight) — CADR on low is typically 20–35 % of the high-speed rating.

Placement

• Central location beats a corner; the far corner of a large room can be under-served.
• Do NOT blow cleaned air out an open doorway; point the outlet into the room's main volume.
• During smoke events: close windows, run purifier continuously, set HVAC to recirculate (no outdoor air intake) if it has a MERV-11+ filter.

DIY Corsi-Rosenthal box

A box fan (1500–2200 CFM free-flow) + 4–5 MERV-13 furnace filters as a cube on the intake side. Measured by academic teams (Corsi, Rosenthal et al., 2020–2022) at 600–900 CFM (1000–1500 m³/h) clean air delivery — beating most $400 commercial units for ~$60 of parts, at higher noise.

Pressure-drop rule of thumb: each MERV-13 filter adds resistance; effective airflow ≈ free-flow CFM × 0.35–0.5 depending on filter count and fan curve. More filter surface area (filters in parallel) = less resistance = more flow.

PM2.5 numbers that matter

Indoor/outdoor ratio with windows closed, no filtration: typically 0.5–0.8 of outdoor during heavy smoke. With adequate HEPA at 5 ACH in one sealed room: 0.1–0.2.

Filter life

HEPA media doesn't "clog" on schedule — it loads with dust. Rated hours (500–3000 h) assume average dust. The tracker uses real run-hours and a smoke-loading multiplier (each heavy smoke week counts ~3×) and schedules replacement before pressure drop kills CADR.

Common Pitfalls

1. Trusting "for rooms up to X m²" on the box. Those claims assume 2 ACH or less — adequate for dust, NOT for smoke. Always recompute from CADR.
2. Running on night mode during an event. CADR on low can be 4× lower; the room never reaches steady state.
3. One purifier for the whole apartment. Clean the bedroom (smallest room, longest occupancy); keep the door closed; that's your clean-air room.
4. Opening windows to "air out" during smoke. This resets the room to outdoor levels; only ventilate when outdoor PM2.5 drops below indoor.
5. Expecting HEPA to remove gases. Smoke smell/VOCs need activated carbon; HEPA handles particles only.

Verification Checklist

• size output CADR = volume × ACH exactly
• check output includes minutes-per-air-change and a clear verdict
• crbox effective CFM is 35–50 % of fan free-flow (sanity band)
• log analyze prints I/O ratio trend and event summary
• filter status shows % life used and change-due date

Examples

$ python3 scripts/clean_air.py size --area 20 --ceiling 2.7 --use smoke
Room: 54 m³ (20 m² x 2.7 m)
Target: 5 ACH (wildfire smoke)
Required CADR: 270 m³/h  (~158 CFM)
  buy margin: 400 m³/h to allow quiet-speed operation

$ python3 scripts/clean_air.py check --area 20 --ceiling 2.7 --use smoke --cadr 300
Your purifier: 300 m³/h in 54 m³ room → 5.6 ACH → ADEQUATE for smoke
  full air change every ~11 minutes; run on HIGH during peaks.

Conventions from public smoke-readiness guidance (EPA Wildfire Smoke Guide, AHAM CADR definitions) and peer-reviewed Corsi-Rosenthal box measurements. Local conditions vary; use a PM2.5 monitor to verify.