A composting toilet chamber needs a vent pipe of 10 cm diameter rising above the roof with an insect screen. At 3 m tall, with the chamber 10 K warmer than the outside air, it draws only about 6.6 L/s by stack effect, a weak flow that a 2 m/s wind can overrule. Ventilation carries smell and moisture out and keeps the chamber aerobic, but the cure for odour is dry carbon cover over every deposit, not stronger suction.
Part of Composting Toilet Systems
Chamber ventilation is a passive vent pipe that lets air pass through a composting toilet vault, removing gases and moisture and keeping the waste aerobic. It runs on the warm-air-rises effect, so it is gentle and depends on the weather.
The Vent Pipe: What The Parent Page Specifies
A 10 cm diameter pipe from each chamber, extending above roof height, with a fly screen on top. The first-month latrine page puts a vent pipe at least 50 cm above the roofline. The pipe does three jobs: it carries smell above the roof instead of into the privy, it removes moisture so the cover stays dry, and it draws flies up toward the light at the top where the screen traps them (the parent page’s claim that a well-managed system has essentially zero fly problems).
How Much Air Does It Move? Worked Example
Use the stack-effect relation from Stack Effect & Thermal Chimneys: draught speed v = sqrt(2 × g × H × ΔT ÷ T), pressure Δp = ρ × g × H × ΔT ÷ T, and flow about 0.6 × area × v.
Inputs (the 10 K and 3 m are assumptions for illustration, not measurements): H = 3 m, ΔT = 10 K, T = 298 K, ρ = 1.2 kg/m³, pipe diameter 0.10 m.
- Δp = 1.2 × 9.81 × 3 × 10 ÷ 298 = 1.19 Pa.
- v = sqrt(2 × 9.81 × 3 × 10 ÷ 298) = sqrt(1.975) = 1.41 m/s.
- Area = π × 0.05² = 0.00785 m².
- Q = 0.6 × 0.00785 × 1.41 = 0.00664 m³/s = 6.6 L/s = 24 m³/h.
Compare with wind: the same page gives wind pressure as 0.5 × ρ × V², which is 0.6 Pa at 1 m/s, about half this vent’s 1.19 Pa, and 2.4 Pa at 2 m/s, twice it. So a cap that turns wind into suction matters, and so does siting the pipe clear of roof eddies, above the ridge.
When The Draught Reverses
The flow depends on the chamber being warmer than the outdoor air. At night in summer, or whenever the outside is warmer than the chamber, ΔT is zero or negative and air can flow down the pipe and push chamber air out through the seat. Painting the pipe black and placing it on the sun side raises ΔT; a chamber in cool shade does the opposite.
Air Needs A Way In
A pipe only moves air if replacement air can enter. Leave a screened inlet low in the chamber, such as a screened vent in the access door, or draw air in through the seat hole when its lid is open. Without an inlet the pipe simply chokes and the chamber breathes through leaks.
Reading The Smell
| What you smell | Likely cause | Fix |
|---|---|---|
| Rotten eggs | Wet, airless waste: hydrogen sulphide | More dry cover; check for pooling liquid; clear the vent |
| Ammonia (sharp) | Too much nitrogen, often urine | More carbon cover; divert urine |
| Sour or musty | Too wet | Dry carbon; check drain and roof leaks |
| Smell only in the room, not at the vent | Lid or seat leaks, or blocked or reversed vent | Seal the lid; unblock the screen |
The parent page’s rule holds: if it smells, there is not enough cover.
What You Need
- A vent pipe — 10 cm diameter, bamboo, plastic or metal, long enough to clear the roof
- A fly screen with openings small enough to stop insects, plus a rain cap
- A screened air inlet low in the chamber
- A sealed seat lid that closes tightly
Common Mistakes
- Relying on the nose as an alarm. Hydrogen sulphide deadens the sense of smell at dangerous concentrations.
- A blocked fly screen. A clogged screen stops the draught; inspect it.
- Bends and long horizontal runs. Each reduces flow; keep the pipe straight and vertical.
- Venting at the eaves, where wind eddies push smell back into the privy.
Safety
The air in a vault can be toxic, oxygen-poor and flammable
Hydrogen sulphide deadens the sense of smell at dangerous concentrations, so a chamber that “no longer smells” may be more dangerous, not safer. Methane is flammable. Never put your head into a chamber or enter one. Open the access door and let it ventilate before emptying, with a second person outside. Never test the air with a flame, and keep fires and sparks away from the vent outlet.
What’s Next
- Learn next: Cover Materials & Management — what actually controls odour
- Combine with: Dual-Chamber (Batch) Design — the vault the vent serves