The Tinfoil Test

The Tinfoil Test for smoking fireplaces

The Tinfoil Test is the next stage after establishing that there is a steady upward draw to the flue system but it can be used as a stand alone test if we know the flue draws well.

With the Tinfoil Test you are simply going to reduce the fireplace opening to see how small it has to go before it works and if the end result is too small an opening then there may also be some restrictions or design faults to address.

Tools required:

Ordinary household cooking tinfoil and tape. We normally use Duct tape but use tape that will not damage any surfaces when removed
A torch and incense sticks are also useful if you have some
Dry kindling to make a small fire for up to 30 minutes

Warning! - Lighting a fire.
In this stage of testing we need to light a small fire so have already established that the ‘Draw testing’ is positive and a camera survey, sealed smoke test or previous use should have established that the flue is fit for purpose.
(N.B. To simply sweep the chimney DOES NOT indicate that it is safe to light a fire )

If you have lived with the fireplace working and smoke spillage is relatively new, perhaps after recent renovations, then we continue only as long as we are confident the system is safe.

The Tinfoil Test is only a temporary measure for testing and should not be left unattended while the fire is alight. Smoke and Carbon Monoxide alarms should be installed in the room prior to the testing process.

Example; On two occasions in 2015 we surveyed chimneys being brought back into service, both had large holes in the chimney, one exposing the attic timber work and the other the rear of timber frame work and plaster board of a child's bedroom wall! Both had just been swept !

The Tinfoil Test

It's easy, effective, costs very little to complete and can be left in place over a period of days and through varying weather conditions to give proper results.

By using ordinary household cooking tinfoil and tape suitable for your fire surround and the fireplace opening, we add full width strips of foil horizontally to the top section of the opening, taping up all of the gaps and holes where possible.

On larger fireplaces with timber beams or openings with wooden surrounds we try to fix the foil to the rear of or behind the timber to prevent over heating or smoke staining especially if you are going to light several fires.

We usually blank off at least half of the opening height before lighting a small fire with kindling only but keeping the foil handy ready to add more strips until the smoke stops escaping and the fire is working well for at least an hour. On the other hand if the fire works immediately then the foil can be rolled upwards in increments making the opening larger until we find the point at which it will start to seep smoke - then we will know we've gone too far!
There are photographs below showing a variety of ways we complete a foil test

The finished opening size with the foil should now have balanced the ratio between flue height, size and fire opening but not necessarily with air starvation. This still may become apparent after 10 to 20 minutes, depending upon the size of the room, and cause the fire to smoke out unexpectedly. By opening a window slightly the effect is usually instant and the smoke pulls up the chimney again, we use our torch shining it from left to right under the edge of the foil to see which way the smoke is moving. Some times we will hold an incense stick or smouldering kindling under the edge of the foil to test for spillage.

If all is working well and safely, it is time to increase the fire size and place more strain on the flue capacity and air supply. We keep the foil handy to add more if necessary and use the torch to detect smoke if the foil is reduced. The air supply can be adjusted to the minimum and measured as an area (eg. the distance the window is open from its closed position multiplied by its height, the area of the open gap in mm.sq) for future reference when fitting a permanent vent.
We do not leave the fire unattended until it has been extinguished and we are satisfied it is safe to do so.

With the foil in place the fire can be tested over a period of several days to allow for different weather conditions and wind directions and if the foil position remains effective we record the dimensions of the new fireplace aperture.

What next?

You now have a temporary functioning fireplace and the decision now is whether to replace the foil with something permanent or to try and improve the draw even more and reduce the foil height slightly or completely:

To check the new opening size against what could be expected of your flue dimensions, this can be done in our Flue Sizing section
If the foil has only reduced the fire opening by an acceptable degree then the addition of a canopy, special glass panel or brick work on the whole hearth may be aesthetically possible otherwise the fireplace will have to be adjusted
On the other hand you still may have air starvation or restrictions within the flue which could be removed to increase the exhaust capacity allowing the fire to work without adjusting the opening. If the Tinfoil test was minimal in height , say approximately 100 mm, then this may be the best approach and by using a camera survey to identify potential blockage
See Restrictions

HELP?

We can offer advice about your results if you send detail and photographs of the foil testing process.

In the left hand photograph below the foil has been adjusted several times to establish the correct opening dimensions for a good draw. The right hand photograph shows the fire surround after remedial stone works to replace the foil had been completed and again it worked perfectly.

Various ways to complete the Tinfoil Test

Sides foil was fixed to the opening to maintain proportions

The base of the foil on the wide opening above was stiffened with a garden cane to stop it flapping and splitting.

Checking with a torch for smoking as the fire dies down