Enter the height of the chimney, the density of the air, the temperature inside the chimney, and the temperature outside the chimney into the calculator to determine the chimney draft. (Temperatures can be entered in °C or °F; the calculation uses absolute temperature in Kelvin.)
Chimney Draft Formula
The following equation is used to calculate the Chimney Draft.
D = h \rho g \left(1-\frac{T_o}{T_i}\right)- Where D is the chimney draft (Pa)
- h is the height of the chimney (m)
- ρ is the air density (kg/m³) (varies with conditions; for reference, dry air is about 1.225 kg/m³ at 15°C and 1 atm, and about 1.292 kg/m³ at 0°C and 1 atm)
- To is the outside absolute temperature (K)
- Ti is the inside (flue gas) absolute temperature (K)
- g is the acceleration due to gravity (≈9.81 m/s²)
What is a Chimney Draft?
Definition:
A chimney draft is a measure of the pressure differential between the inside and outside of a chimney shaft.
How to Calculate Chimney Draft?
Example Problem:
The following example outlines the steps and information needed to calculate Chimney Draft.
First, determine the height of the chimney. For this example, the height is measured to be 15m.
Next, determine the density of the air. For this example, assume approximately 1.292 kg/m³ (about the density of dry air at 0°C and 1 atm).
Next, determine the temperature inside the chimney. This is measured to be 40°C (which is 313.15 K).
Next, determine the temperature outside the chimney. In this case, this is measured to be 30°C (which is 303.15 K).
Finally, calculate the chimney draft using the formula above (with temperatures in Kelvin):
D = h*ρ*g*(1-To/Ti)
D = 15*1.292*9.81*(1-(303.15/313.15))
D = 6.07 pascals
