Understanding Chimney Downdraught & Summer Chimney Syndrome – Hodgsons Chimney Sweeps

Downdraught refers to the reverse flow of air down a chimney or flue system, contrary to the intended upward draft that carries combustion by-products out of a building. It occurs when external or internal pressure and temperature conditions disrupt the buoyant flow of warm gases, causing atmospheric air or smoke to be forced back into the appliance or living space.

Mechanisms & Contributing Factors:

1. Pressure Differential Reversal:
   Under normal operation, the hot gases generated by a fire create a positive pressure differential between the inside of the flue and the cooler external atmosphere, enabling upward movement (stack effect). A downdraught occurs when this pressure gradient reverses due to higher external pressure at the chimney terminal, overpowering the buoyant force of the warm flue gases.
2. Thermal Inversion:
   In warmer ambient conditions—such as during summer—external air temperature may exceed the internal temperature of an unused flue. This eliminates or reverses the thermal draft, causing denser outside air to sink into the flue system.
3. Wind-Induced Pressure Zones:
   Wind striking a building can create positive pressure zones on windward walls and negative pressure zones (suction) on leeward sides. If the chimney is located within a positive pressure region or if wind eddies cause turbulence around the chimney pot, this can force air back down the flue.
4. Flue Blockage or Design Faults:
   Short chimneys, cold flue walls, oversized terminals, or obstructions (e.g., nests, creosote buildup) can restrict the exit of gases and make the flue more susceptible to reversal. Poor insulation can also contribute to rapid internal heat loss, weakening the stack effect.
5. Mechanical Interference (Negative Indoor Pressure):
   Extractor fans, tumble dryers, or combustion appliances can create negative pressure within the building envelope, drawing external air down through the flue to compensate—especially if adequate make-up air is not provided.

Key Formula Reference:
The draft pressure (∆P) generated by a chimney under natural conditions is typically estimated by:

∆P = C × h × (Ti – Te) / Te
Where:

• ∆P = pressure difference (Pa)
• C = constant based on gravity and air density
• h = height of the chimney (m)
• Ti = temperature of flue gases (K)
• Te = external air temperature (K)

A downdraught is indicated when ∆P becomes negative (Ti ≤ Te or when wind pressure dominates the terminal zone).