Chimney Pots making a smoking fireplace worse

Chimney Pots
Pots are the cause of many smoke related problems especially if they are fitted and sized incorrectly. A pot is not always necessary and on many occasions their removal has been an instant cure to smoke spillage.

Sizing Pots
If a pot is to be fitted then the smallest internal dimensions at the top or base should be a large as the flue so not offering any restriction.  Many think that the smaller the pot the faster the draw because of the increased speed of the gasses.  Yes, the air speed through the pot will increase but the overall flow or exhaust rate will decrease and it is the unrestricted speed of this column of air that provides the draw across the fireplace opening preventing smoke spillage.

The common internal size of a chimney pot is 200mm which is suitable for stove and small fireplaces.  These are placed upon circular flues or flue liners but in older properties.  With brick built flues they are often too small and 250mm and 300mm pots have to be used.

For example a common 225mm square flue found in houses before 1964 fitted with a 200mm internal diameter pot can reduce the exhaust capacity of the flue by up to 38%. Add an offset or supporting slates left as a result of careless fitting  and this can easily increase. ​

​The problem caused by the introduction of a pot is widespread;  one customer was advised by a Conservation Officer to fit two pots to a functioning large inglenook fireplace in a listed building. The pots were fitted and the inglenook then didn’t work, so the opening had to be reduced to suit the reduced air flow!

Is my Chimney Pot too small?
If we are testing to see if a chimney pot is too small without actually measuring it we light a small fire making sure there is no spillage across the opening.  If the smoke is shooting out of the top of the pot in a straight column like a steam train for at least a metre in height that is normally indicative of there being a restriction.  Increasing the fire will also usually result in smoke spillage after a few minutes as the capacity to remove the volume of gases has been reduced.

Our remedy is to change the pot or remove it and fit a Dovecote to prevent rain ingress but allow full exhaust capacity.  
The  gap between the slab and flue should be equal to the flue diameter for a functioning fireplace.  See photograph here. 
Note: the Dovecotes in the  background have little clearance above the flue because the fires are not being used. 

Fitting Pots
Builders, please do not restrict the flue by using slates or bars across the base of the pot to support it while cement is setting! The restriction shown in the photograph here are  caused by slate and a bar completely prevented any flue function but even a small area of blockage can make a big difference to performance.
​The remedy  for careless work like this is to knock it out which of course is more expense to the customer.