Smoke testing – DEFRA Exemption
Carrying on from the article on stove testing in the Aug ’25 edition of Sweep Stuff magazine, here we deal with Smoke Testing and Safety Testing. See Stove Testing Part 1 here.
There are a completely different suite of tests for an appliance to be recommended as DEFRA exempt for use in a smoke control area. It’s important to note that an appliance does not have to undergo the smoke testing procedure in order to achieve UKCA/CE certification. It is a UK specific test and is only carried out if a manufacturer wishes to have the appliance as recommended for use in a Smoke Control Area, i.e. DEFRA exempt. The test procedure is the same as outlined in the last article, but the smoke measurement apparatus is different. The same principle of a filter being used to capture the soot generated from an extracted sample is generally used, however the sample point is located not in the test flue, but in the extraction system after the extraction hood and the flue gas has been diluted. The reason for this dilution is to cool the flue gas. There are other measurement techniques which may be used such as electrostatic precipitator, but the test process is the same.
As mentioned in part 1, soot is a complicated thing to measure and understand. There is a difference between the PM measured close to the stove and PM measured after the flue gas has been diluted, with the latter being most like what you and I refer to as ‘Smoke’. The former is really only the precursor to smoke. As the flue gas cools, it agglomerates with some OGC (Organic Gaseous Compounds – hydrocarbons) and forms what we would consider to be visible smoke. The dilution ratio is monitored so that the results can be corrected back to the pre dilution numbers.
For DEFRA exemption testing, 5 tests are carried out at the nominal air control setting, and 5 are also carried out at a ‘Low’ heat output setting. It is this low setting that determines where a ‘DEFRA Stop’ must be positioned. The DEFRA smoke limit is calculated based on the appliance nominal heat output and the formula used is 5 + (Heat Output/3) g/hr, i.e. a 5kW appliance has a smoke limit of 6.6g/hr and an 8kW appliance has a limit of 7.6g/hr.
If the appliance is not capable of passing the low output tests then the DEFRA stop will be positioned at nominal air setting (assuming it has actually passed at nominal). This can cause some customer dissatisfaction when they realise they are unable to shut down the air control below nominal heat output. This becomes even worse when you consider that the DEFRA tests are carried out at 12Pa and if we assume a 9m tall flue in the real world, a draught of 30-40Pa can easily be achieved, i.e. there’s a lot more air for combustion available in the firebox in the real world than in the laboratory. This really is something that needs to change as manufacturers are forced to test at 12Pa and then get blamed for having a rubbish stove that can’t be shut down when it is installed in DEFRA mode on a 12m flue on a hillside coastal property.
Safety Testing – Distances to combustible materials
Safety testing is used to determine how close combustible materials may be placed to the appliance. The process quite long and typically takes an entire day. If I had free reign to change any aspect of the CE/UKCA marking process it would be this, as it is the most unrealistic aspect of the entire thing. The fuel load is not calculated by the nominal heat output, it is determined from the firebox floor area, i.e. the larger the firebox the larger the fuel load regardless of the nominal heat rating. If a stove has a nominal fuel load of 1kg Beech, it could be that over 2.2kg is used for safety testing, more than double the heat output. The test specifies that softwood ‘fir’ timber must be used and it must be 50mm x 50mm or 60mm x 40mm batons at least 2/3 the width or depth of the firebox, whichever is greater. In practice this means CLS construction timber (pine, fir, spruce) is used cut to the appropriate length. It must be stacked in a lattice formation in the firebox with at least 1cm gaps between the individual pieces. The air controls must be set to maximum, and the flue draught is increased to 15Pa.
The temperature limit for the trihedron walls is 65C above ambient, (90-100C depending on time of year).
The trihedron walls (sides back and floor of the chamber) are manufactured from 20mm thick plywood with 50mm PIR (e.g. Celotex) insulation board glued to the back. The whole lot is painted matt black for maximum radiative heat transfer. Thermocouples are stuck to the surface of the plywood facing the appliance and they must be located at the hottest point of the surface. The test engineer positions the walls as close as possible to the appliance and begin the test, typically it takes several hours and possibly all day for the wall temperatures to stabilise.
If the surface temperatures go above the 65C greater than ambient, the walls are moved back until the temperature decreases again. The test is only considered complete when the temperature stops increasing. This can mean 6-8 hours of overfiring the appliance with an increased fuel load that is positioned in the firebox to burn as quickly as possible. The appliance generally gets so hot that flue gas temperatures can be in the range of 400-600C and heat protective gear must be worn to refuel.
My personal feeling is that it is not a representative test for how the majority of users would actually use an appliance. How many consumers have fireplace chambers made from insulated plywood painted matt black and will over fuel their appliance in a very specific fashion to ensure maximum heat output for a period of 6-8hours? It is true that somebody ‘could’ operate their appliance in this way but it’s not clear why manufacturers need to state their distance to combustible materials based on an irresponsible user.
One of the changes from the 13240 to 16510 standard (basically the old to the new) is the addition of a ‘ceiling distance’, i.e. how close can a combustible material be to the top of an appliance. This did not exist in 13240, manufacturers could ask for a test to be carried out but it was up to the test houses how best to rig up the apparatus to get the numbers. 16510 now specifies how this testing must be done and if it is not done then the manufacturers must state a minimum ceiling distance of 750mm above the top of the appliance in the instructions. The wording of the legislation makes it clear that ‘any combustible fabric of the building’ cannot be within 750mm of the top of the appliance. This would include timber beams above a recess opening for example. However, looking at the way the test must be conducted, it looks as if the legislation was considering an actual ceiling, not a timber beam.
Figure 4 shows the schematic for how the trihedron walls set up. There is a caveat in the procedure that if the highest temperature detected on a surface is within 150mm of the edge, the trihedron wall must be extended. The test setup is very much that of a ceiling, not a beam, and all of the heat being trapped above the stove, and off the uninsulated part of the flue pipe is being trapped by the ceiling. If a point on the surface of the trihedron chamber is on the radiation sight line to the firebox, it is a certainty that it will be warmer than the points that are not. However, very few manufacturers are yet to state ceiling distances of the most recently introduced stoves which have been tested to 16510 and have managed to get a ceiling test done, the quoted safety distances are in excess of 750mm. This is not at all surprising given the requirements of the test procedure and apparatus.
The appliances I have seen have been aimed at the European/Scandi market, tall, cylindrical and freestanding. It’s quite likely that in the near future we will see stoves designed for the typical British installation in a chamber quoting ceiling distances below 750mm on the proviso that a beam is installed out of the sight line of the firebox (i.e. horizontally closer to the appliance rather than farther away).
There has been quite a bit of chatter online about the 750mm ceiling distance being a new regulation that is to be applied to all installations including current installations. It is not currently a building regulation, it is a requirement of the manufacturer if, and only if, their appliance has been tested to 16510 to state the ceiling distance in their installation instructions. Anything that was tested to 13240 does not need to meet this requirement so the rule does not apply in these cases. Most people used a distance of 450mm which in most cases was appropriate. In addition, if the manufacturer has tested for a ceiling distance and has determined a number to be greater or less than 750mm, that is what must go in the instructions. It is very much a grey area at the moment and the guidance that has been issued to now has not been at all clear.
The next article in this series will delve into some more of the practical implications of the change in test standard from 13240 to 16510 and what it means for placing appliances on the market.
