Siphonage

This problem is best understood by looking the diagram. It can occur when two flue outlets are close together, the wind blowing the smoke from one flue directly across the outlet of another. It can also occur when the mid feathers have collapsed inside the chimney or there are open joints linking two flues together. There are three possible solutions: 

i) Fit a taller pot to the working flue. 

ii) Line the chimney if the leakage is inside the stack. 

iii) Improve the ventilation in the room where the fire is. 
 
This problem can also occur when 2 rooms have been knocked into one, with, say, a cooker or gas fire in one chimney and an open fire in the other. The open fire will often pull fumes into the room from the other appliance. An open fire will always win a "tug of war" with a closed appliance.

Solution: increase room ventilation. We have also found in extreme circumstances, fitting new kitchens with high performance extractor hoods have created negative draught in flues and actually pulled smoke and fumes into  the home instead of them rising through the flue.

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