Chimney Height

Is chimney height important?

At Billing Chimneys we’re are constantly reiterating the importance of chimney safety. A key aspect of this is that a chimney or flue should create sufficient draft to take away the harmful gases created by the fuel burned by open fires and appliances. The efficiency of this draft is affected by the chimney height and because this is a safety issue, the recommended chimney height is set out in UK Building Regulations 2010 and found in Approved Document J. In addition, British Standards, BSEN 15287* and BSEN 8303** offer further guidance on best practice.

The Building Regulations class chimney and flue systems as ‘Controlled Services’ and combustion appliances, such as wood burning stoves and AGAs, are ‘Controlled Fittings’. Enforcement of the regulations are the responsibility of your local authority who must be notified of any work associated with such controlled services and controlled appliances.

It is important that anyone carrying out building work where there’s a requirement under the regulations must ensure that the work complies with those requirements. Work not carried out to the correct standard could end up costing more as the local authority can order further work, or even demolition and a rebuild. Failure to comply can result in fines too. A lot of chimney work does not need planning permission, but it does need to conform with the building regulations, therefore a building notice is often required, so, how can you ensure your chimney complies?

Exact height requirements and where to take measurements from are given in paragraphs 2.8 – 2.9 of Approved Document J (ADJ). Also vital in any design are flue termination heights and locations which are also set out in the document.

In addition to the regulations, all appliance installation instructions have a minimum flue height given. This needs to be considered, especially as fuel type also effects termination locations. Taking into account the correct flue termination heights and location is also vital in any design.

Chimney height is important in all instances. Firstly, chimney height will have been designed into the property when it was built. Instances of where you will need to consider chimney height include new buildings, conversions, rebuild of chimneys, in fact any situation where you are fitting, altering, or replacing an external flue or chimney should be checked. The position of the chimney is also important, it needs to be away from windows and adjacent properties and there are areas of a roof where a chimney should not be placed. As with chimney height, details of chimney and flue location requirements are laid out in the regulations.

How does chimney height affect draft?

A chimney works by draft, where the gases produced by burning are drawn upwards and out of the chimney or flue. It does this because the fire or appliance heats the air which rises to meet colder air above creating pressure. The higher the chimney, the better the draft. There does come a point however where a chimney might be too tall as there will not be enough pressure and the gases will revert down the chimney. Similarly, a system that’s too short will not produce enough draft. If insufficient draft is an issue a chimney height extension is something to consider.

Explaining this can become very technical, we’re talking about pressure differential in the flue or chimney, this is the difference in pressure between two points. Chimneys work on negative pressure; all flue systems have a designation, either N or P. These points in a chimney will be where the hot and the cold air meet. Chimney height will affect where these points will be and how effective the draft will be.

How does chimney height affect safety?

Chimney height dictates the safety of a chimney in different ways. As we’ve explained a chimney needs height to produce a good enough draft to draw out and expel the harmful gases created by combustion – burning of fuels. Failure to expel the gases can result in Carbon monoxide poisoning. The location of the outlet, the proximity of other buildings, topography and geography along with localised atmospheric conditions all need to be considered.

There are numerous other reasons for a chimney to be of the height stipulated in the building regulations. These include:

  • Preventing smoke re-entering a property, being drawn back down a redundant flue shaft or another live flue etc.
  • Prevention of smoke nuisance.
  • To avoid potential fires being caused by hot embers rising in the chimney and sending out sparks, so for example chimneys must be situated away from window frames.
  • Chimney height is also particularly important with thatched properties where chimneys often require to be heightened to meet current regulations.

How do I find out if my chimney is the correct height?

Details of the current building regulations for chimneys and flues are readily available, but the jargon can be confusing and the calculation procedure difficult. Chimney or flue height will depend upon the appliance installed and the manufacturer’s guidance should be followed. You may be surprised to know that local wind conditions are also factored into calculating a chimney height. Additionally, the area surrounding the building with the chimney might include structures such as tall buildings and even trees, and these can affect draft.

The chimney height above the roof is measured from what is termed the weather surface to the flue outlet. The weather surface means for example the roof tiles or felt, and the outlet measurement does not include any caps or cowls. The location of the chimney or flue will depend upon the design of the roof itself and it’s not always possible to simply extend a flue height. Also, a flue system must be continuous.

What about my wood burner?

These regulations apply for flues to wood burners too. The building regulations state that if all guidance is followed a minimum flue height of 4.5m, from appliance to top of the flue, could be acceptable, but it is best that a full assessment and calculation is undertaken. The outlet from the flue should be above the roof as the gases being emitted need to discharge safely and not present a potential fire hazard.

Helping the professionals

Billing Chimneys has many years and are experience and have been supporting professionals in ensuring chimneys are compliant with the building regulations and suggesting changes to chimneys and chimney design where necessary. The building regulations stipulate rules for the location of a chimney or flue. This is for safety reasons and includes distances from ridges and any roof windows.

We work with architects, designers and building contractors, so anyone wanting to include a chimney or flue into a design or want to make alterations that involve a chimney and want to ensure the local building control will be happy with proposal, we can help here. If in doubt, call Billing Chimneys!

Billing’s experts are the people to call

The height of your chimney is maybe something you have never given much thought to, but it is part and parcel of your heating system and needs to work safely, legally and at its optimum capacity. The physics behind this may appear daunting and too complicated to think about, which is why it’s best to engage professionals.

References

*BSEN 15287 – Refers to Chimneys. Design, installation and commissioning of chimneys. Chimneys for non-roomsealed

**BSEN 8303 – This British Standard specifies requirements for the design and installation of the following types of domestic heating and cooking appliances, rated up to 50 kW, burning wood, woody biomass and solid fuels in both new and existing buildings. a) open fires; b) room heaters; c) free-standing cookers; and d) independent boilers. The standard also specifies the procedures to be followed for on-site testing of the installations.

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