A forge chimney draws only by buoyancy: with a 5 m chimney and flue gas averaging 100 C the pull is about 13 Pa, while a 5 m/s gust presses with about 15 Pa, so the chimney must clear the roof ridge and the smithy needs a permanent air inlet or the smoke and carbon monoxide come back into the room. Size the flue at one-tenth of the hood opening and place the forge on an exterior wall so the flue is short, straight and made of stone or brick.
Part of Workshop & Forge Design
A forge hood and chimney collect the smoke and gas from the firepot and carry them above the roof, driven by the lower density of hot gas against outside air.
The Draft Is A Few Pascals, So Height And Position Decide Whether It Works
The workshop guide puts the forge against an exterior wall or in a corner with a chimney directly above, of stone or brick, extending above the roof peak, with a hood that reaches at least 30 cm beyond the forge on all sides. The forge setup guide hangs the hood 60-90 cm above the firepot (Forge Setup). The chimney draft guide gives the flue as one-tenth of the opening area and the minimum height as 3 m (Chimney Draft). This article checks those numbers with the buoyancy formula the rocket-stove chapter uses (The Chimney Effect): Δp = (ρ_out − ρ_gas) × g × h, with ρ = 1.204 × 293 ÷ T for T in kelvin.
Worked Example: Draft, Flow And Wind
Inputs: outside air 20 C (293 K); firepot 0.8 m above the floor; chimney top 5.8 m above the floor, so h = 5 m; hood 1.5 m by 1.2 m, which is a 0.9 m by 0.6 m hearth plus 30 cm on every side, so hood area = 1.5 × 1.2 = 1.8 m²; flue area = 1.8 ÷ 10 = 0.18 m², about 42 cm by 42 cm.
The mean temperature of the gas in the chimney is not given by any source in this corpus; it is an assumed input, and room air drawn in under the hood dilutes the hot gas, so three values are tried.
| Mean flue gas | ρ_gas (kg/m³) | Δρ (kg/m³) | Δp = Δρ × 9.81 × 5 | Ideal speed √(2Δp ÷ ρ_gas) |
|---|---|---|---|---|
| 50 C (323 K) | 1.204 × 293 ÷ 323 = 1.092 | 0.112 | 5.5 Pa | 3.2 m/s |
| 100 C (373 K) | 1.204 × 293 ÷ 373 = 0.946 | 0.258 | 12.7 Pa | 5.2 m/s |
| 150 C (423 K) | 1.204 × 293 ÷ 423 = 0.834 | 0.370 | 18.2 Pa | 6.6 m/s |
Flow at 100 C, taking a discharge factor of 0.6 for friction and bends (the value the stack-effect chapter uses for openings, Stack Effect & Thermal Chimneys): Q = 0.6 × 0.18 m² × 5.2 m/s = 0.56 m³/s, or about 2,000 m³/h. Through the 1.8 m² hood face that is 0.56 ÷ 1.8 = 0.31 m/s.
Wind: dynamic pressure = ½ × 1.204 × v². At 2 m/s it is 2.4 Pa; at 5 m/s it is ½ × 1.204 × 25 = 15 Pa. A 5 m/s gust therefore matches or beats the draft at a mean gas temperature of 50 C or 100 C. A chimney whose top sits in the wind shadow of the roof can have the gust press down it instead of draw up it.
Method: Place The Forge And Build The Flue
Step 1: Choose the wall. Pick an exterior wall or corner so the flue is as short and straight as possible. Bends and long horizontal runs cost draft that the figures above show you cannot spare.
Step 2: Set the chimney top above the roof. Standard house-code practice, recalled from US residential code and not checked against a local one, is a top at least 0.9 m above the point where the chimney leaves the roof and at least 0.6 m higher than anything within 3 m. Either keeps the outlet out of the roof’s wind shadow.
Step 3: Build the flue at one-tenth of the hood area. A flue that is too large lets cold air fall back down it; too small and the fire is starved (the chimney draft guide lists both).
Step 4: Provide the air the chimney removes. The 0.56 m³/s must enter somewhere. Cut a low inlet, on the side away from the forge, large enough that the air is not felt as a gale; at an assumed face velocity of 0.5-1 m/s that is 0.56 ÷ 1 = 0.56 m² to 0.56 ÷ 0.5 = 1.1 m². This is a design choice, not a sourced figure.
Step 5: Keep the chimney away from wood. The fire guide asks for at least 5 cm of non-combustible material between a chimney and any wood framing (Fire Prevention in Settlements). Treat that as the least, and cover the roof around the chimney with metal, tile or sod, not thatch.
Step 6: Check the draft cold and hot. At this point a hand held under the hood should feel air moving up. If it does not, warm the flue with a small fire first; a cold chimney has no buoyancy.
Common Mistakes
- A short chimney that ends at the ridge line. It sits in the dead zone, and gusts reverse it.
- Closing the inlet in cold weather. With no air to replace what the chimney removes, the draft collapses and smoke enters the room. This is the winter carbon monoxide case.
- A spark-arrestor mesh with no allowance for soot. A mesh adds resistance to a draft of 5-13 Pa and clogs. Where thatch lies downwind it is still needed; make it larger than the flue and inspect it (Chimney Cleaning Schedule).
- Using the kiln chimney rules. A kiln and a forge differ in fuel rate and gas temperature; size the forge flue from its own hood.
Safety
A weak draft puts carbon monoxide in the smith's face
Carbon monoxide is odourless, and the forge makes it whenever the fire is starved or the draft fails. A draft of 5-13 Pa can be overcome by a gust, a closed door or a second fire competing for air. If smoke spills from under the hood, or the smith has a headache or dizziness, stop, let the fire die down and go into fresh air. A chimney that glows or throws sparks on to a roof is also a fire hazard for the neighbouring buildings (see the spacing in Fire Breaks & Building Spacing).
What’s Next
- Learn next: Ventilation Systems — why general ventilation cannot replace the hood
- Combine with: Chimney Cleaning Schedule — keeping the flue open and clean