Air starvation and smoking fireplaces
What is Air Starvation?
It's an insufficient volume of air coming into the room to replace that which is being drawn up the chimney by the fire.
The air supply has to be sufficient to keep the smoke moving up the chimney and if it isn't then the draw weakens and the fire smokes out into the room. Building Regulations recommend that the air supply to a room is based upon 50% of the surface area of the flue but this can vary depending upon how much residual air is available in the building.
Air starvation is responsible for over 70% of the problems we have to deal with but the cure is the most unpopular!
What causes air starvation?
To list just a few:
Symptoms
If you have insufficient air entering the room then smoke spillage occurs usually 15 to 30 minutes after the fire is first lit, unlike a restriction or a partial blockage where the fire smokes out immediately. Opening a window or door may instantly stop it but not it will not easily vacate the smoke now in the room.
The Cure
Old houses often have enough air entering to satisfy the fireplaces but newer or renovated houses which are well sealed will require a dedicated air supply in the room which is normally equal to 50% of the area of the flue.
Sometimes it is not just the fire that makes demands upon the air supply coming into the house and equally it can be the fire which causes air starvation to other appliances such as central heating boilers.
Test which way the air moves
We use a lightweight bed sheet and some tape to fix it to the door frame to the room.
Then we hang the sheet from the top of the door frame with the door open and make sure it covers the side frames and drags on the floor.
The degree the sheet bellows out like a sail will indicate the volume of air in motion between the two rooms.
Our simple process:
Example:
A house in the Cotswolds with a new large open fire tended to smoke after half an hour of being alight. There was an air vent in the room but it was small and the under floor supply pipe to it was long and contained several bends . If the door to the room was left open then the fire seemed to improve but the owners were suffering from headaches and wanted it cured.
With the door just off the catch the draw of the fire would pull it ajar so a window was opened and adjusted until the door didn't move thus giving the exact surface area of air required as a vent into the room.
As a matter of interest the air source to the room door was traced back to the kitchen and a vent supplying air to the Aga oil fired cooker. The draw on the open fire proved to be strong enough to overcome the supply from the vent and draw fumes down the Aga flue. Hence the headaches after a period of time!
At a hotel in Prestbury, Gloucestershire the Great Hall fire was smoking and we were asked to solve the issue.
There are two doors to the hall, one to a corridor and the other leading to the base of the main stairwell where the restaurant, back door to the garden and another door to the corridor all converge. We sheeted all four doors and they all pulled in toward the stairs except the door to the hall which moved both in and out. With the fire going well both doors to the hall behaved the same way when sheeted.
The fire was still receiving air and was not smoking out so we finished the sheet tests and decided that the staff would have to call us back if and when it started to smoke again. As the glass guard, normally in front of the basket, was placed back infront of the fire smoke immediately poured out, remove the guard and it behaved perfectly!
The guard was causing an eddy which pulled the smoke out of the fire but if we reduced the fire opening with tin foil then the smoke, with the guard in place, went up the chimney.
The air supply was in such a fine balance through out the building that as a result of standing a small glass guard close to the fire the smoke had been pouring out for months.
It's an insufficient volume of air coming into the room to replace that which is being drawn up the chimney by the fire.
The air supply has to be sufficient to keep the smoke moving up the chimney and if it isn't then the draw weakens and the fire smokes out into the room. Building Regulations recommend that the air supply to a room is based upon 50% of the surface area of the flue but this can vary depending upon how much residual air is available in the building.
Air starvation is responsible for over 70% of the problems we have to deal with but the cure is the most unpopular!
What causes air starvation?
To list just a few:
- adding new double glazing
- draft exclusion
- opening up another taller larger flue in close proximity
- kitchen extraction especially in public places
- new buildings or trees changing the air dynamics around the house
- vents being blocked or closed
- stair wells acting like chimneys drawing air upward and away from the fire
Symptoms
If you have insufficient air entering the room then smoke spillage occurs usually 15 to 30 minutes after the fire is first lit, unlike a restriction or a partial blockage where the fire smokes out immediately. Opening a window or door may instantly stop it but not it will not easily vacate the smoke now in the room.
The Cure
Old houses often have enough air entering to satisfy the fireplaces but newer or renovated houses which are well sealed will require a dedicated air supply in the room which is normally equal to 50% of the area of the flue.
Sometimes it is not just the fire that makes demands upon the air supply coming into the house and equally it can be the fire which causes air starvation to other appliances such as central heating boilers.
Test which way the air moves
We use a lightweight bed sheet and some tape to fix it to the door frame to the room.
Then we hang the sheet from the top of the door frame with the door open and make sure it covers the side frames and drags on the floor.
The degree the sheet bellows out like a sail will indicate the volume of air in motion between the two rooms.
Our simple process:
- We try this without lighting the fire first and see which way the sheet pulls..... into the room or into the hallway and to what degree
- If it pulls into the hall way, using the same method we try other doors and see where the air is being drawn to. Vents may be needed there too
- If it pulls into the room we open a window slowly and the sheet should eventually hang loose
- Now with the fire working we try the test again and see the difference. It is usually quite revealing
- If it still pulls into the hall then this can cause smoking quite quickly and further consideration for this air movement is required
- If it pulls into the room then we remove the sheet and close the door but not on the catch. We allow the door to be pulled open and then measure the area of the air gap around the door when it stops opening. We take the average of several tests and this indicates the area of free air ventilation required for the fire to be created in the room
Example:
A house in the Cotswolds with a new large open fire tended to smoke after half an hour of being alight. There was an air vent in the room but it was small and the under floor supply pipe to it was long and contained several bends . If the door to the room was left open then the fire seemed to improve but the owners were suffering from headaches and wanted it cured.
With the door just off the catch the draw of the fire would pull it ajar so a window was opened and adjusted until the door didn't move thus giving the exact surface area of air required as a vent into the room.
As a matter of interest the air source to the room door was traced back to the kitchen and a vent supplying air to the Aga oil fired cooker. The draw on the open fire proved to be strong enough to overcome the supply from the vent and draw fumes down the Aga flue. Hence the headaches after a period of time!
At a hotel in Prestbury, Gloucestershire the Great Hall fire was smoking and we were asked to solve the issue.
There are two doors to the hall, one to a corridor and the other leading to the base of the main stairwell where the restaurant, back door to the garden and another door to the corridor all converge. We sheeted all four doors and they all pulled in toward the stairs except the door to the hall which moved both in and out. With the fire going well both doors to the hall behaved the same way when sheeted.
The fire was still receiving air and was not smoking out so we finished the sheet tests and decided that the staff would have to call us back if and when it started to smoke again. As the glass guard, normally in front of the basket, was placed back infront of the fire smoke immediately poured out, remove the guard and it behaved perfectly!
The guard was causing an eddy which pulled the smoke out of the fire but if we reduced the fire opening with tin foil then the smoke, with the guard in place, went up the chimney.
The air supply was in such a fine balance through out the building that as a result of standing a small glass guard close to the fire the smoke had been pouring out for months.