Understanding Chimney Draft
Chimney draft is the driving force that pulls air into a combustion appliance and evacuates smoke and combustion gases safely out of a building. This natural phenomenon occurs due to the density difference between hot gases inside the chimney and cooler outside air, combined with the vertical height of the chimney stack.
The draft pressure created in a chimney is crucial for safe and efficient operation of fireplaces, wood stoves, boilers, and other heating appliances. Too little draft results in poor combustion and smoke spillage, while excessive draft can lead to rapid fuel consumption and heat loss.
Formula
The chimney draft pressure is calculated using the following formula:
[\text{Draft} = h \times \rho \times \left(1 - \frac{T_{o}}{T_{i}}\right) \times g]
Where:
- h = chimney height (m)
- ฯ = air density (kg/mยณ)
- Tโ = outside temperature (ยฐC in absolute terms)
- Tแตข = inside temperature (ยฐC in absolute terms)
- g = acceleration due to gravity (9.81 m/sยฒ)
The result is expressed in pascals (Pa).
Example Calculation
Let's calculate the draft for a residential chimney:
Given:
- Chimney height: 20 m
- Air density: 1.292 kg/mยณ (standard at sea level)
- Outside temperature: 25ยฐC
- Inside temperature: 50ยฐC
Calculation:
$$\text{Draft} = 20 \times 1.292 \times \left(1 - \frac{25}{50}\right) \times 9.81$$
$$\text{Draft} = 20 \times 1.292 \times 0.5 \times 9.81$$
$$\text{Draft} = 126.69 \text{ Pa}$$
This 126.69 pascal draft pressure is strong and would provide excellent draw for most residential heating appliances.
Key Factors Affecting Chimney Draft
Height: Taller chimneys create more draft. Doubling the height roughly doubles the draft pressure, all else being equal.
Temperature Difference: The greater the temperature difference between inside and outside, the stronger the draft. This is why chimneys work better in winter than summer.
Air Density: Air density decreases with altitude, so chimneys at high elevations produce less draft than identical chimneys at sea level. Air density also varies with barometric pressure and humidity.
Flue Diameter: While not directly in the formula, the diameter affects flow resistance. A larger diameter reduces friction losses but may cool gases more quickly.
Practical Applications
Residential Heating: Ensuring adequate draft for fireplaces, wood stoves, and furnaces to prevent smoke from entering living spaces.
Industrial Boilers: Calculating required stack heights for power plants and industrial boilers to achieve proper combustion air flow.
HVAC Design: Sizing chimneys and vent systems for gas appliances, ensuring code compliance and safe operation.
Troubleshooting: Diagnosing draft problems in existing installations and determining if mechanical draft assistance is needed.
Understanding chimney draft calculations helps homeowners, HVAC professionals, and building designers ensure safe, efficient operation of combustion appliances and proper ventilation of combustion products.