The Draw Test for smoking fireplaces
This test is to establish if there is air movement up the flue and if not, why not?
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Tools required:
The test will take no more than 2 to 3 minutes. |
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Our process is to:
- Only start when the fire is not alight and hearth is cold
- Close all doors and windows to the room as you would in winter
- Use the lightweight cotton sheet to completely cover the aperture leaving no gaps down the sides or across the base (for large fireplaces such as Inglenooks we may use two sheets)
- Tape the sheet in position for a prolonged test but we make sure the tape does not damage any surfaces
- Wait for a minute giving the flue chance to act upon the sheet and observe any movement
- Sheet pulls in immediately and pulls tight against the fireplace and holds
- Sheet pulls in but returns a little and then pulls back in
- Sheet hangs loose with no movement
- Sheet pulls in but then blows back towards the room
- Sheet blows back towards the room and never pulls in
Diagnosis
Sheet being pulled into the opening (1 and 2 above)
This shows there is potentially a clear flue and favourable draw with the degree of strength being demonstrated by the pull on the sheet.
The wind will affect the pull as will the temperature difference between the room and the outside but as long as the draw on the whole is inward it's a good result.
We would then open and close the door to the room to see if this has an effect on the sheet. Sometimes closing the door will have an adverse effect for a few seconds if the air supply is not good.
All this diagnosis will enables us to establish whether there is a potential and adequate independent draw without influence from other parts of the building.
Sheet hangs loose (3)
This is always a worry unless the air temperature inside is the same as outside or if there is no wind to create a draw. Should this be the case we assume a chimney blockage or restriction.
In an attempt to get a response from the sheet, the window or a door on the windward side of the room should be opened.
If the the sheet continues to hang loose we would then consider examining for blockages in your chimney
If all looks clear we carefully light a small piece of news paper and hold it up the chimney (a hair dryer is a good alternative if the chimney is not too sooty). If the smoke/hot air rises we try the sheet again for signs of improvement.
To get better results it may be necessary to wait for a change in the weather conditions and try again.
If there are no improvements with the door shut, window open and a small amount of heat from a hair dryer or burnt paper further investigation using a camera may be necessary.
Examples
In the winter last year we had this negative sheet test response on a large fireplace with a tall brick 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 set. Unfortunately the slate was not removed afterward and more than 50% of the flue was blocked preventing any air movement.
A gentleman in Malvern, Worcestershire had failed to get his newly built fire and flue to work so in desperation and after completing 'Draw Tests' a camera survey was requested. To our amazement the survey revealed a MacDonalds bag containing Burger and Chips wedged on one of the bends blocking the flue. The brown bag was perfectly intact with no burn marks to be found, just a well dried out lunch!
In another case recently the flue had been altered at ceiling level and moved to the right horizontally for approximately 400 mm, soot falling on the flat area built up gradually reducing the capacity of the flue, the fireplace spilled smoke out after every six weeks of use .. almost to the day!
Sheet pulls in but then blows back towards the room (4)
Movement in the sheet usually indicates there is a clear flue but the degree of pull is being contradicted by something else and is often from an adjoining room, hallway or kitchen with an extraction systems fighting against the natural draw. The sway of the sheet can also be influenced by the wind blowing across the building, easily recognisable if in time with the sound.
An additional test placing the sheet over the doorway to the room would usually be necessary to try and establish whether the air is being removed in great quantities. With all other doors and windows in the room closed and using another sheet cover the door when open, you should be able to see easily which way the air is moving and to what degree.
The movement in the fireplace and the door may correspond showing where the problem lies. Try opening a windward window slowly and slightly, noting if the fireplace sheet pulls in and shows a positive draw, the area of the open window will also help to indicate the size of the air vent required to be fitted at a later date.
This may also be an intermittent issue and it is worth completing the test again on a still day but in situations like this it would be difficult to use anything else except a hair dryer to warm the air up the chimney to start the air moving inside the flue.
Sheet being pulled into the opening (1 and 2 above)
This shows there is potentially a clear flue and favourable draw with the degree of strength being demonstrated by the pull on the sheet.
The wind will affect the pull as will the temperature difference between the room and the outside but as long as the draw on the whole is inward it's a good result.
We would then open and close the door to the room to see if this has an effect on the sheet. Sometimes closing the door will have an adverse effect for a few seconds if the air supply is not good.
All this diagnosis will enables us to establish whether there is a potential and adequate independent draw without influence from other parts of the building.
Sheet hangs loose (3)
This is always a worry unless the air temperature inside is the same as outside or if there is no wind to create a draw. Should this be the case we assume a chimney blockage or restriction.
In an attempt to get a response from the sheet, the window or a door on the windward side of the room should be opened.
If the the sheet continues to hang loose we would then consider examining for blockages in your chimney
If all looks clear we carefully light a small piece of news paper and hold it up the chimney (a hair dryer is a good alternative if the chimney is not too sooty). If the smoke/hot air rises we try the sheet again for signs of improvement.
To get better results it may be necessary to wait for a change in the weather conditions and try again.
If there are no improvements with the door shut, window open and a small amount of heat from a hair dryer or burnt paper further investigation using a camera may be necessary.
Examples
In the winter last year we had this negative sheet test response on a large fireplace with a tall brick 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 set. Unfortunately the slate was not removed afterward and more than 50% of the flue was blocked preventing any air movement.
A gentleman in Malvern, Worcestershire had failed to get his newly built fire and flue to work so in desperation and after completing 'Draw Tests' a camera survey was requested. To our amazement the survey revealed a MacDonalds bag containing Burger and Chips wedged on one of the bends blocking the flue. The brown bag was perfectly intact with no burn marks to be found, just a well dried out lunch!
In another case recently the flue had been altered at ceiling level and moved to the right horizontally for approximately 400 mm, soot falling on the flat area built up gradually reducing the capacity of the flue, the fireplace spilled smoke out after every six weeks of use .. almost to the day!
Sheet pulls in but then blows back towards the room (4)
Movement in the sheet usually indicates there is a clear flue but the degree of pull is being contradicted by something else and is often from an adjoining room, hallway or kitchen with an extraction systems fighting against the natural draw. The sway of the sheet can also be influenced by the wind blowing across the building, easily recognisable if in time with the sound.
An additional test placing the sheet over the doorway to the room would usually be necessary to try and establish whether the air is being removed in great quantities. With all other doors and windows in the room closed and using another sheet cover the door when open, you should be able to see easily which way the air is moving and to what degree.
The movement in the fireplace and the door may correspond showing where the problem lies. Try opening a windward window slowly and slightly, noting if the fireplace sheet pulls in and shows a positive draw, the area of the open window will also help to indicate the size of the air vent required to be fitted at a later date.
This may also be an intermittent issue and it is worth completing the test again on a still day but in situations like this it would be difficult to use anything else except a hair dryer to warm the air up the chimney to start the air moving inside the flue.
Example:
We had a case like this in Sussex where the fire did not respond that well even when the window was open. The air was been drawn out of the room and up into the stairwell toward a Velux roof light which tended to be left open. The stairs and open window acted like a chimney and fought against the lounge fireplace but once the window vent was shut the fire worked well.
We had a case like this in Sussex where the fire did not respond that well even when the window was open. The air was been drawn out of the room and up into the stairwell toward a Velux roof light which tended to be left open. The stairs and open window acted like a chimney and fought against the lounge fireplace but once the window vent was shut the fire worked well.
Sheet blows back and never pulls in (5)
This could be downdraft but it is very rare to have a proper case of this and it is usually noticeable by the overwhelming smell of acrid soot as opposed to smoke as soon as you walk into the room.
Finding the reason why air is being pulled down the chimney is now a priority and usually it is caused by mechanical extraction or low pressures around the property. A natural draw without any heat from a fire is dependent upon air movement across the chimney top or a thermal rise from a warm room and as a result is no match for strong low pressures.
We normally start by isolating the fireplace, closing and covering all the doorways with a cotton sheet preventing the influence of other parts of the building. Any vents are checked with newspaper or a smaller sheet for inward air movement.
If there is more than one fireplace in the room then this should be sheeted too to establish whether it's the culprit, robbing all of the available air or if it is also sacrificial to a stronger vacuum.
If we can easily find a reason for the constant movement of air down the chimney then a cure can be established but if ideas have run out there are other things to consider such as:
All of these issues can be tested without resorting to guesswork but it is always worth remembering that there may be more than one factor causing the problem.
This could be downdraft but it is very rare to have a proper case of this and it is usually noticeable by the overwhelming smell of acrid soot as opposed to smoke as soon as you walk into the room.
Finding the reason why air is being pulled down the chimney is now a priority and usually it is caused by mechanical extraction or low pressures around the property. A natural draw without any heat from a fire is dependent upon air movement across the chimney top or a thermal rise from a warm room and as a result is no match for strong low pressures.
We normally start by isolating the fireplace, closing and covering all the doorways with a cotton sheet preventing the influence of other parts of the building. Any vents are checked with newspaper or a smaller sheet for inward air movement.
If there is more than one fireplace in the room then this should be sheeted too to establish whether it's the culprit, robbing all of the available air or if it is also sacrificial to a stronger vacuum.
If we can easily find a reason for the constant movement of air down the chimney then a cure can be established but if ideas have run out there are other things to consider such as:
- poor flue position in relation to the adjoining roof or neighbouring building
- height of the flue overall
- a courtyard or garden area acting as a low pressure zone pulling air out of the house.
All of these issues can be tested without resorting to guesswork but it is always worth remembering that there may be more than one factor causing the problem.
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Example:
A customer in the Cotswolds had a metre square fireplace opening served by a flue that was too small. We completed a tinfoil test for smoke spillage but even with the smallest opening smoke spillage kept occurring. After resorting to a sheet test on the lounge door we quickly found that air was being drawn out of the room and hallway by a low pressure area outside the front door. This could not be stopped, so more vents fitted on the windward side of the house brought air in to compensate. This together with the fitting of a draft excluder to the front door solved the issue. The fire is now working correctly. |
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What next?
We should by now have a good idea of what is happening and why and be thinking of ways to resolve any of the the problems we have found. The 'Tinfoil Test' is next and is just as important to establish how well the flue system will perform as an exhaust system.
Help and Advice
If you need help and advice please get in touch. Sending photographs and results will be helpful to refer to in conversation but please note you will not receive an emailed reply.
We should by now have a good idea of what is happening and why and be thinking of ways to resolve any of the the problems we have found. The 'Tinfoil Test' is next and is just as important to establish how well the flue system will perform as an exhaust system.
Help and Advice
If you need help and advice please get in touch. Sending photographs and results will be helpful to refer to in conversation but please note you will not receive an emailed reply.