Miljøprojekt Nr. 1192 2007 – Vurdering af omfanget af dårlige skorstene til private brændeovne og brændekedler, regelgrundlag og løsningsmuligheder

Vurdering af omfanget af dårlige skorstene til private brændeovne og brændekedler, regelgrundlag og løsningsmuligheder

Summary and conclusions

The pollution from burning wood in stoves and boilers in private homes and the environmental and health related consequences, are determined by a complicated interaction between a number of factors, among these, habits when using the stove, applied fuel, type of stove, chimney and chimney positions in regard to the surroundings.

According to a questionnaire submitted by the Danish Environmental Protection Agency in 2006 to 79 chimney sweeps, 19 % of all wood burning stove owners have poor habits, 15 % have a poor quality stove, 9% use unsuitable fuel and 6 % have too low chimneys.

The purpose of this project has been to discover as to which extent poor chimneys contribute to local problems with smoke nuisances and pollution, and assess and recommend possibilities for remedial treatment and prevention both by actual improvements and by rendering proposals for further adjustments, e.g. by giving inputs for a new Danish Building Research Institute (SBI) recommendation.

Poor chimneys – contribution to local problems

The chimney is the engine of the wood burning stove, operating the combustion. Without a well functioning chimney with an efficient chimney draught, the wood burning stove will not function at its optimum, which may cause light-up problems, smoke in the living room, poor combustion and potential problems for neighbours. It is the heat in the smoke that creates the chimney draught and the hotter the smoke the better the chimney function, and the better the chimney is insulated the better it will hold on to the heat in the smoke. Thus it is important that light-up takes place during heavy combustion and at high temperatures, so that the chimney is quickly heated to high operating temperatures. In such cases, steel chimneys are usually better than brick-built chimneys as they are easier and more quickly heated, and they retain the heat better, due to better insulation.

The reason for poor chimneys is to a great extent that they are not high enough and in rare cases leaks in the chimney or problems with the air-intake to the stove, e.g. if the house is too airtight or leak-proof, or a ventilation system or exhaust hood cause under pressure, so that not enough air is led into the combustion.

Smoke nuisances for neighbours are either caused by poor habits with too little combustion air, too low chimneys or an inappropriate position on the house, short distance to the neighbours or a combination of these factors.

To the extent that the poor chimneys result in poor combustion, this will lead to a significantly higher emission of particles, soot CO, PAH, unburned gases etc.

Exact measurement of emissions from wood burning stoves is very difficult and expensive and it is impossible to perform measurements without great preparation and planning the operating time of the stove. The smoke’s colour and smell may be observed downwind, which may give good indication of potential pollution, but it cannot be directly connected to the size or amount of the ongoing pollution. It is considered reasonable to assume that both the smoke’s colour and smell to a certain extent are proportional with the concentration of particles, soot, CO, PAH, unburned gases etc.

Odour measurements on a two-year-old wood burning stove and an even older fuel boiler show that the odour emissions from ordinary combustion is three-times the size in an old wood burning stove than in that of a new one. This was not a case of poor chimneys, but ordinary average installations and heights. Range calculations on these odour measurements/air emissions show that offensive odours may occur at ordinary burning, especially during light up and burning. The chimney’s location in the house is very important to the spreading of smoke, just as the neighbours’ location in relation to the chimney and in relation to the prevailing wind direction are significant factors for the extent and degree of nuisance.

Rules applicable for chimneys

Building regulations include performance requirements for chimneys and they must be high enough to ensure sufficient chimney draught and spreading of the smoke, so that it does not cause problems or nuisance. Specific directions for chimneys attached to solid fuel boilers and wood burning stoves may be found in the SBI direction.

Rules and directions in the building regulations and the SBI direction are basically sensible and reasonable, but it looks as though the performance requirements are not often met, since the directions in the SBI direction are not being enforced by the authorities. Many chimneys have been approved even though they do not meet the requirements. The municipal authorities and chimney sweepers need to take a more firm approach so that the requirements in the direction are observed to a much greater degree. Furthermore, general knowledge is needed as to the smoke spreading around low buildings and as to chimney height directions and location so as to avoid inconveniencing neighbours

Spreading of smoke from low buildings

When the wind blows past a building over-pressure will occur in front of the house, and under-pressure will occur to the rear of the house. Furthermore, whirlwinds will occur mostly at the back of the house. If the chimney opens into the whirlwind zone, the smoke could easily be dragged down to the ground behind the house and spread out along the ground.

If the chimney opens into the over-pressure zone, in general this would reduce the chimney draught. If it opens into the under-pressure zone it would result in a better chimney take-up in principle, but downdraughts might occur, i.e. whirlwinds blowing down the chimney and spoiling the chimney draught.

If the building downdraught is to be avoided the chimney must open above the whirlwind zone, as stated in the SBI direction. The most appropriate chimney location is on the ridge, as this gives the best smoke dispersion. The further away from the roof the chimney is located, the higher it must be to achieve the same dispersion of the smoke, as with a location on the ridge. On houses with flat roofs the most appropriate location is in the middle of the roof. Flat roofs may be problematic and often require higher chimneys than houses with high pitch if the same spreading conditions are to be achieved.

Apart from the wind velocity, the wind direction is also significant for the spreading conditions. The largest building downdraught on the building’s lee side is seen when the wind hits the house at an angle between 45 and 75°. If the wind blows along a building with pitch, the conditions will be like those of a flat roof. If this is the prevailing wind direction, and neighbours are located in that exact direction, there is an increased risk of smoke nuisance.

Adjustment of emissions from wood stoves and boilers

There are three important pieces of information on wood burning stoves in the Danish Guidelines for Air Emission Regulation /54/:

According to section 42 of the Environmental Protection Act, the municipal council can order a wood stove owner to reduce pollution.
Orders as a consequence of the Environmental Protection Act cannot be appealed against another administrative authority
Only clean wood may be used in wood burning stoves. However, newspaper may be used for light-up.

In connection with complaints over smoke nuisance, the supervising authority also has the possibility of inspecting the wood burning stove, and the owner must give all relevant information on the system. If the person is not willing to cooperate, and will not allow the inspection, section 87 of the Environmental Protection Act permits the supervising authority to seek help from the police, whilst observing the Due Process of Law Act .

In the US there is a general requirement concerning air pollution from wood stoves and it is not allowed to expose neighbours to smoke from wood stoves and emissions of very visible smoke from wood stoves are prohibited. According to state regulations in the state of Washington, the smoke from wood stoves may not exceed 20% opacity for more than 6 consecutive minutes.

A similar set of rules might be considered in Denmark, where joint guidelines and assessment methods for handling complaints could contribute to faster and better solutions. Such rules could be applied for nuisance caused by poor chimneys, bad placing and incorrect use.

Chimney-oriented solution possibilities

Chimney sweeps are often involved when problems occur with the wood stove, both as regards the actual installation and when the problem is the neighbours complaining about the smoke. Many problems could be solved with instructions for better burning techniques, which may enhance the chimney performance, as better light-up and burning give higher temperatures in the chimney, and enhance combustion which will involve less emissions and nuisance.

However, some chimneys, both new and old do not work properly, even if the fuel is burned correctly, as they are too low. In such cases physical changes must be applied to improve the chimney draught. The problem is present both with new but too short chimneys and with recycled brick-built chimneys that are connected to a new wood burning stove. The latter is often caused by the more efficient new stoves that require larger chimney draughts to suck up the combustion air through the stove, and also the fact that the smoke temperature is lower, as it is cooled down much more efficiently than in the old stoves. There might also be problems with the installation of an insertable unit in an open fireplace, where the chimney is often not particularly high.

Basically, possible solutions would be to extend the chimney or to install an electrically driven exhauster, which would always ensure the necessary air up-take, provided of course that it is switched on. The disadvantage of an exhauster is the power consumption and the fact that the stove cannot be used in a power failure.

The need for further regulations

Experience from several parties, chimney sweeps and municipal authorities shows that the chimney height is often too low to avoid smoke downwash and neighbour nuisance, especially in residential areas with densely built, low buildings, if the rules of the building regulation are kept, such as they are presented in the SBI direction.

It would be fully in accordance with the building regulations and the SBI direction to refuse approval of a chimney placed by the eaves, if this is close to neighbours in the prevailing wind direction, and thus there is a great risk of problems, unless the chimney is so high that nuisance is avoided (perhaps 4-5 meters above the roof).

Enforcement of the rules can and should be tightened, in order to avoid the poorest chimneys, which would be beneficial to neighbours, sellers and buyers of wood stoves and chimneys. Even if the buyer in some case will have larger expenses for the wood stove installation, neighbour disputes will be avoided as well as tiresome extra expenses for subsequent improvements that are inevitably more expensive than if they had been installed initially.

Existing chimneys may very well be improved, since according to the present rules the municipal authorities may order a house owner to extend his chimney, if it causes problems.

Basically the present rules should be enforced. One of the means to do this could be to add a section to the building regulations with the wording that the instructions in the SIB direction must be observed, unless it is documented that the function requirements are being observed in another way. In order to carry out such a rule in practice it would be appropriate to have a clear division of responsibilities the municipality and the chimney sweeper, so that he who inspects and approves or rejects a chimney also holds the authority to do so.

This could then be followed up by more specific directions to supplement those that are already in the SBI direction. There are many different possibilities, which with the present level of knowledge may be difficult to prioritize.

Determining an absolute minimum height for chimneys, e.g. in meters of steel chimney, calculated from the top of the stove and additional lengths for other chimney types, bends etc.
It would be beneficial for the chimney draught to have a maximum length for the first piece of non-insulated duct.
The wood burning stove manufacturer should state a minimum chimney height for each burner type.
A construction permit for establishing chimneys could help ensure good chimneys and less nuisance for neighbours, as the chimney’s height and location on the house and proximity to the neighbouring houses is assessed and approved (by the municipal environmental authority/chimney sweep) before establishment.
Calculating the chimney height with a German PC program according to EN 13384 should be considered, however this requires closer investigation of the effect of the chimney height, also in relation to location on the house, and in relation to neighbours.

Conclusion

Many nuisance wood stoves have improved their performance either by increasing the chimney height or by mounting an exhauster, improved burning techniques, prohibition against use in certain wind directions etc. Many have improved with the help of the chimney sweep, whereas in other cases notices or municipal orders have been given, and a few cases have ended in the courts.

There are still many chimneys that do not comply with the function requirements of the building regulations, as the regulations not interpreted as strictly as in the wording of the SBI direction. It may also be difficult to assess whether the function requirement has been or can be met, since basically it is an assessment not measurement of a parameter that has to be higher than a given value.

Basically lacking in the present rules is a specification of the fact that the SBI directions must be followed, and if not, it must be documented that the function requirement has still been met.

It should also always be taken into consideration that in many cases a higher chimney than what is stated in the SBI directions must be established in order to meet the requirement that the smoke discharge must not give any smell or health deteriorating effects.

The verall conclusion is that if the present rules and regulations were followed correctly, most chimneys would be acceptable and they would have a height and location so that neighbour problems can be avoided to a large extent.

However, this will not remedy all the problems that poor burning habits cause between neighbours. A chimney and the spreading of smoke from it will never be better than the oven or boiler that is used and the applied burning techniques facilitate. Poor combustion results in strong foul smelling smoke, which will cause problems, no matter how good or well placed the chimney is. Information may contribute to improving the conditions, especially if supplemented by clearly defined rules and requirements to the wood stove owners that they are obliged to burn as cleanly as possible and that it is their responsibility that neighbours are not exposed to smell and health deteriorating elements from their stoves. This could be accompanied by a requirement for the smoke’s appearance/colour (opacity), as practised in the US, which would make it easier for both neighbours and authorities to determine, whether or not combustion is satisfactory.