Is my Flue Size causing my fire to smoke?
Checking your flue is the correct size
This calculation provides additional information relating to the Draw and Tinfoil tests. We present it here in summary and just as an indication of the process we go through. Experience has shown that calculation is never enough, experience is invaluable to properly interpret the results.
If you have a flue system functioning successfully and wish to enlarge the fireplace dimensions or add a flexible flu liner to an old square flue then you can easily upset the fine balance between flue size, chimney height and fireplace opening causing the fire to smoke.
Part J of Building Regulations 2010 is not helpful when it comes to sizing flues for fireplaces larger than 500 mm x 500 mm and can be misleading because it will not take into account flues over 4.5 metres high.
There are various charts available on the Web which can assist with the ratios or you can contact us for assistance.
Is my new fireplace opening too small, can it go bigger?
If the Draw and Tinfoil tests have been completed, and the fire has been working for over half an hour without spillage, the new opening size of the fireplace has been successfully established. However, it is always best to monitor the situation in different weather conditions and with different configurations of doors/windows open.
The size of your flue is needed to estimate the extent of the problem you have if it could be related to the flue size.
If the size is not known it has to be measured and to do this we normally use a piece of card as a gauge taped on to the top of a garden cane or drain rod in a T shape.
We start by cutting the card to a width of 225mm and with it attached (taped) to the rod guide it into the flue where it becomes a uniform size usually 600 to 900 mm above the opening. We change or cut the card down until it fits because the smaller the flue the more important it is to get an exact size.
Most flues built before 1967 are square or rectangular and their size can be established by counting the bricks internally or by using a gauge. Large fireplaces or Inglenooks will usually have rectangular flues but if your fireplace is in excess of a metre square then expert advice is essential!
This is where we have to apply mathematics and the Laws of Physics that control air flow to see if the flue and fireplace opening are in the correct ratio and there is nothing else causing a problem.
The mathematics is simple with a flue to opening area ratio of 1-10 but we have given some examples below to compare against as a rough guide.
So for example:
Round flues
What if it's not quite right,
If your measurements show a large discrepancy where the new area is much less than expected for the flue size then the design of the flue could be an issue or there may be a restriction in the flue or at the pot and assistance will be needed and work will have to be done.
If however you are content with the new ‘functioning’ dimensions from the Tinfoil Test then the new opening will have to become permanent. This can be achieved by adding stone slips, a canopy, glass panel or even by building the hearth up by the same distance the foil came down.
Examples of a restriction:
While working on a fireplace in Cheltenham in the summer of 2016 we gained up to a 20% increase in performance by removing a pot that was too small for the flue. The pot was replaced with a larger version and this extra exhaust capacity was enough to allow the fireplace to draw and function correctly without any alteration.
This calculation provides additional information relating to the Draw and Tinfoil tests. We present it here in summary and just as an indication of the process we go through. Experience has shown that calculation is never enough, experience is invaluable to properly interpret the results.
If you have a flue system functioning successfully and wish to enlarge the fireplace dimensions or add a flexible flu liner to an old square flue then you can easily upset the fine balance between flue size, chimney height and fireplace opening causing the fire to smoke.
Part J of Building Regulations 2010 is not helpful when it comes to sizing flues for fireplaces larger than 500 mm x 500 mm and can be misleading because it will not take into account flues over 4.5 metres high.
There are various charts available on the Web which can assist with the ratios or you can contact us for assistance.
Is my new fireplace opening too small, can it go bigger?
If the Draw and Tinfoil tests have been completed, and the fire has been working for over half an hour without spillage, the new opening size of the fireplace has been successfully established. However, it is always best to monitor the situation in different weather conditions and with different configurations of doors/windows open.
The size of your flue is needed to estimate the extent of the problem you have if it could be related to the flue size.
If the size is not known it has to be measured and to do this we normally use a piece of card as a gauge taped on to the top of a garden cane or drain rod in a T shape.
We start by cutting the card to a width of 225mm and with it attached (taped) to the rod guide it into the flue where it becomes a uniform size usually 600 to 900 mm above the opening. We change or cut the card down until it fits because the smaller the flue the more important it is to get an exact size.
Most flues built before 1967 are square or rectangular and their size can be established by counting the bricks internally or by using a gauge. Large fireplaces or Inglenooks will usually have rectangular flues but if your fireplace is in excess of a metre square then expert advice is essential!
This is where we have to apply mathematics and the Laws of Physics that control air flow to see if the flue and fireplace opening are in the correct ratio and there is nothing else causing a problem.
The mathematics is simple with a flue to opening area ratio of 1-10 but we have given some examples below to compare against as a rough guide.
So for example:
- If the fireplace opening left by the tin foil measured 600mm x600mm its area would be 3600cm sq and we would expect the flue to be either 200mm or 225mm diameter or a 175mm to 200mm square if built before 1967. We have allowed for a flue height of 6 metres but if your house is taller the flue size could be smaller or the opening bigger.
Round flues
- 2000 cm sq expect to have a 175 mm diameter flue,
- 3000 cm sq expect to have a 200 mm diameter flue,
- 4000 cm sq expect to have a 225 mm diameter flue,
- 4900 cm sq expect to have a 250 mm diameter flue,
- 3000 cm sq expect to have a 175 mm square flue,
- 4000 cm sq expect to have a 200 mm square flue,
- 4900 cm sq expect to have a 225 mm square flue,
- 8000 cm sq expect to have a 250 x 325 rectangular flue,
What if it's not quite right,
If your measurements show a large discrepancy where the new area is much less than expected for the flue size then the design of the flue could be an issue or there may be a restriction in the flue or at the pot and assistance will be needed and work will have to be done.
If however you are content with the new ‘functioning’ dimensions from the Tinfoil Test then the new opening will have to become permanent. This can be achieved by adding stone slips, a canopy, glass panel or even by building the hearth up by the same distance the foil came down.
Examples of a restriction:
While working on a fireplace in Cheltenham in the summer of 2016 we gained up to a 20% increase in performance by removing a pot that was too small for the flue. The pot was replaced with a larger version and this extra exhaust capacity was enough to allow the fireplace to draw and function correctly without any alteration.
Above: the camera is inside the flue looking up at the base of the pot. The disc is the camera guide but note the large horizontal shadow above the guide. This was an obstruction that could only be investigated from above.
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The pot had to be removed to gain access to the obstruction which, surprisingly, was the wall of the adjoining flue! The pot had been centralised for looks ignoring the reduction in exhaust capacity and as a result the fire stopped working.
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Most Victorian houses and properties built before 1967 have brick built flues measuring 225mm x 225mm but if lined with a modern flexible liner the flue will most likely be a 175mm diameter, the smallest used for solid fuel fires but much too small for the original Victorian fireplace to function correctly.
Houses built after this date will have clay liners with a minimum diameter of 175mm ranging up to 360mm depending upon the fire opening size and flue height but these were designed to be safer and not to deteriorate as easily as the brick flue. As a result they are not often lined and if found to be too small it is more likely that the fireplace has been adjusted and built too big for the flue size to function correctly.
Help?
If there are still anomalies which you cannot understand it is worth contacting us with the results of your tests for further assistance.
Sending photographs and results by email will be helpful to refer to in conversation. See Contact in the main menu
Houses built after this date will have clay liners with a minimum diameter of 175mm ranging up to 360mm depending upon the fire opening size and flue height but these were designed to be safer and not to deteriorate as easily as the brick flue. As a result they are not often lined and if found to be too small it is more likely that the fireplace has been adjusted and built too big for the flue size to function correctly.
Help?
If there are still anomalies which you cannot understand it is worth contacting us with the results of your tests for further assistance.
Sending photographs and results by email will be helpful to refer to in conversation. See Contact in the main menu
