Health and Safety: Combustion, Carbon Monoxide, and Indoor Air Quality
Health and Safety is the domain that can end a project, or a client's life. Items cover combustion safety and combustion appliance zone (CAZ) testing, carbon monoxide action levels, ventilation requirements, and recognition of asbestos, lead, mold, and radon hazards.
CAZ testing
Worst-case depressurization means closing doors and windows, then operating every exhaust fan (bath, kitchen, dryer) and the forced-air air handler before measuring pressure in the combustion appliance zone. Observe draft and spillage at each appliance. Programs commonly set a worst-case CAZ depressurization limit in the 3–5 Pa range; sustained spillage or a failed draft reading is a red tag regardless of the number. Fix the pressure problem - add makeup air or combustion air - rather than adjusting the appliance.
Carbon monoxide
- Ambient CO action levels commonly used: 35 ppm short-term, with 9 ppm as an 8-hour reference.
- Undiluted flue CO measured air-free above roughly 400 ppm is a common action level.
- Air-free CO = measured CO × 20.9 ÷ (20.9 − O2%).
Worked example. A natural-draft furnace reads 120 ppm CO with 4.5% O2 in the flue. Air-free CO = 120 × 20.9 ÷ (20.9 − 4.5) = 120 × 1.274 ≈ 153 ppm. That is elevated but below the 400 ppm air-free action level, so the next step is to find the cause - flame impingement, dirty burners, or spillage - and verify draft under worst case. If the reading had been 420 ppm air-free, the appliance would be shut down and tagged.
IAQ and ventilation
ASHRAE 62.2 sets the whole-house rate: 0.03 × conditioned floor area + 7.5 × (bedrooms + 1) CFM, delivered continuously or intermittently with the same average. Keep indoor relative humidity roughly in the 30–50% band. Radon action level is 4 pCi/L, and air sealing changes radon dynamics, so test after major tightening.
Hazards to recognize
Asbestos in duct wrap, vermiculite attic insulation, and older floor tile; lead paint and dust in pre-1978 housing; mold from uncontrolled moisture; and CO alarms as a backup, never a substitute for testing.
Traps that cost marks
- Testing the CAZ with interior doors open and exhaust fans off, missing the worst case.
- Zeroing the combustion analyzer in flue gas instead of outdoor air.
- Treating smell as a CO indicator, or dismissing a reading because the client feels fine.
- Disturbing asbestos-containing duct wrap or vermiculite without assessment.
- Installing exhaust-only ventilation without checking the CAZ first.