Restrictions that cause a fireplace to smoke
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What do we mean by a Restriction ?
Anything that reduces the capacity of the flue causing a poor draw or flow rate causing the gasses to fill the chimney and spill out into the room. There are many causes and most are very obvious;
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The flat surfaces around this new flue prevented it functioning correctly.
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Restrictions.
A simple task of looking up the chimney with a torch can reveal obvious visible blockages like dampers, chimney valves, balloons or bags of straw but we also check the pot or terminal serving the fireplace and make sure there are no close fitting vents which will allow a draw but prevent normal function. The terminal below for example will not allow proper function but acts as a vent.
Other cowls may have been fitted in an attempt to make the flue system work such as Anti-downdraft, or Spinning Cowls can cause a restriction on large flues but I would not normally recommend their removal for testing unless it is easy to do so.
If the flue has been lined and there is a flat concrete pad supporting the flue invariably this will be the cause of smoke spillage but in most cases the Tin Foil test will overcome the restriction .
Pots may have been added that are too small or when fitted steel bars or slates placed across the flue to support the pot, these could be partially blocking the flue causing smoke spillage into the room .
A Camera Survey will help you to pick up all of these restrictions and equate them into your findings.
Examples
A simple task of looking up the chimney with a torch can reveal obvious visible blockages like dampers, chimney valves, balloons or bags of straw but we also check the pot or terminal serving the fireplace and make sure there are no close fitting vents which will allow a draw but prevent normal function. The terminal below for example will not allow proper function but acts as a vent.
Other cowls may have been fitted in an attempt to make the flue system work such as Anti-downdraft, or Spinning Cowls can cause a restriction on large flues but I would not normally recommend their removal for testing unless it is easy to do so.
If the flue has been lined and there is a flat concrete pad supporting the flue invariably this will be the cause of smoke spillage but in most cases the Tin Foil test will overcome the restriction .
Pots may have been added that are too small or when fitted steel bars or slates placed across the flue to support the pot, these could be partially blocking the flue causing smoke spillage into the room .
A Camera Survey will help you to pick up all of these restrictions and equate them into your findings.
Examples
- In the winter last year we had this negative sheet test response on a large fireplace with a tall chimney of original construction, the draw did not exist but the camera inspection revealed that the builder had used a piece of slate to support the chimney pot whilst the cement holding it set. Unfortunately the slate was not removed afterward and more than 50% of the flue was totally blocked.
- On one occasion after completing the initial tests in a relatively new house a camera survey revealed a bag containing a MacDonalds Burger and Chips wedged on one of the bends blocking the flue. Perfectly intact, no burn marks just well dried out!
- In another recently the flue had been altered at ceiling level and moved to the right horizontally, soot falling on the flat area built up reducing the capacity of the flue and it smoked out after every six weeks of use .. almost to the day!
Some examples of restrictions to a flue
This is the correct gap between the top of the 300mm flue and the Dovecote capping
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The 100mm gap above the flue prevented the system functioning .
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1. Looking down a blocked chimney pot
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2. The central pot had to be re-positioned
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3. The internal edges reducing the flue size on the other pots are being removed
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This is a venting cap or insert. It is only placed on top of the pot when the flue and fire are not in use.
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This view looking up the chimney from the fireplace shows bricks that have fallen from above and blocked the flue
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