Workshop ventilation works in three tiers: capture the gas at the hood and chimney, supply a permanent low inlet to replace the air the chimney removes, and add high openings for general air. General ventilation alone cannot protect a smith, because if even 0.1% to 1% of the carbon burned becomes carbon monoxide, diluting it to the 10 mg/m³ 8-hour guideline in a 120 m³ smithy needs about 1,170 to 11,700 m³ of fresh air an hour, which is 9.7 to 97 air changes.
Part of Workshop & Forge Design
Workshop ventilation is the planned movement of air through a building: the hood and chimney that carry fire gases out, the inlets that let air in and the high openings that clear heat and smoke.
Capture At The Source, Because Dilution Cannot Keep Up
The workshop guide names the forge’s carbon monoxide, sulfur dioxide from coal and metal fumes as a life-safety issue, and asks for a chimney with enough draft, a hood extending 30 cm beyond the forge on all sides, open walls or large doors on the sides away from the chimney for cross-ventilation, and shutters in cold weather to balance ventilation against heat. Its only detection rule is a symptom: headache, dizziness or nausea while working means carbon monoxide, so stop and ventilate. Symptoms are a late warning; carbon monoxide also dulls judgement, and the carbon monoxide guide notes that drowsiness is exactly what a cold, tired person expects (Carbon Monoxide Risks). The design has to prevent the exposure, and arithmetic shows how.
Worked Example: Carbon Monoxide Against Room Air
Inputs. Fuel burned: 5 kg of charcoal an hour, the middle of the 4-6 kg of general forging (Forge Fuel), counted as 5 kg of carbon (an overstatement, since charcoal is not pure carbon). Carbon’s molar mass 12.0 g/mol; CO 28.0 g/mol. Room 6 m × 5 m × 4 m = 120 m³. Exposure guidelines from the CO guide: 10 mg/m³ over 8 h, 35 mg/m³ over 1 h, 100 mg/m³ over 15 minutes, and 1,200 ppm (about 1,375 mg/m³) immediately dangerous to life.
The fraction of carbon that leaves as CO instead of CO₂ is not given by any source in this corpus; the CO guide says so itself. It is therefore a parameter, tried at 0.1% and 1%.
- Carbon burned: 5,000 g ÷ 12.0 = 417 mol an hour.
- CO released at 1%: 4.17 mol × 28.0 = 117 g = 117,000 mg an hour. At 0.1%: 11,700 mg an hour.
- Air to hold the room at the 8-hour guideline: 117,000 ÷ 10 = 11,700 m³ an hour at 1%, and 1,170 m³ an hour at 0.1%. In a 120 m³ room that is 97 and 9.7 air changes an hour.
- Closed room, no exchange, at 1%: concentration rises by 117,000 ÷ 120 = 975 mg/m³ an hour. It reaches 100 mg/m³ in 100 ÷ 975 = 0.10 h, about 6 minutes, and 1,375 mg/m³ in 1,375 ÷ 975 = 1.4 h, about 85 minutes. At 0.1% these times are ten times longer.
- Natural ventilation for comparison, with the stack formula of the stack-effect guide (Stack Effect & Thermal Chimneys): v = √(2 × 9.81 × H × ΔT ÷ T). Take a 3 m rise between a low inlet and a ridge opening, 10 K warmer inside, T = 293 K: v = √(2 × 9.81 × 3 × 10 ÷ 293) = 1.4 m/s. Two openings of 0.5 m² in series act as 0.5 ÷ √2 = 0.35 m². Flow = 0.6 × 0.35 × 1.4 = 0.30 m³/s = 1,070 m³ an hour, about 9 air changes. That barely meets the 0.1% case and falls ten times short at 1%.
The conclusion does not depend on the true fraction: any CO that escapes into the room is far more than the openings can dilute, so the gas must go up the hood and chimney.
Method: Three Tiers
Step 1: Capture. Hood and flue as in Forge Placement & Chimney. This tier removes most of the gas and is the only one that scales with the fire.
Step 2: Make up the air. The chimney draws air out, so the same volume must come in. A low, permanent inlet on the side away from the forge, at an assumed 0.5-1 m/s face velocity, is 0.56-1.1 m² for the 0.56 m³/s of that article’s example. It has no shutter that can be closed in winter; if the draft is too strong, narrow the inlet with a fixed grille, never close it.
Step 3: Add high openings for heat and smoke, a ridge gap or clerestory, as the workshop guide asks.
Step 4: Test the draft with a hand or a thread held at the hood face before every long session.
Step 5: Work in pairs in winter. A second person outside the smithy checks the smith at intervals, because a smith poisoned by CO may not notice.
Common Mistakes
- Closing doors and shutters in the cold. This removes the makeup air and collapses the draft. It is the usual way a winter smith is poisoned.
- Counting cross-ventilation as the safeguard. Wind can reverse it.
- Relying on symptoms. A headache is a late warning, and carbon monoxide dulls the judgement needed to act on it.
- Ignoring coal. The workshop guide notes sulfur dioxide from coal; use charcoal where you can.
Safety
Carbon monoxide has no smell
At the assumed 1% yield, a closed 120 m³ smithy passes the 15-minute guideline in about 6 minutes and the immediately dangerous level in about 85 minutes. If anyone has a headache, dizziness or drowsiness, put out the fire, open an inlet and an outlet and get everyone into fresh air; rescuers ventilate first and do not enter a closed smithy to carry someone out without airing it. A person who has been poisoned can relapse and needs watching afterwards.
What’s Next
- Learn next: Fire Safety & Suppression — the other life-safety system in every workshop
- Combine with: Forge Placement & Chimney — sizing the capture tier