Chimney Liners for Thatched Properties
Sarsen Energy recognises that installing a wood or multi-fuel appliance within a thatch property requires careful consideration. We endeavour to install appliances and chimney liners that will assure that the property is left safe from potential thatch fires.
Contrary to most believing that thatch fires are started from a spark emitting from the chimney and igniting the thatch, we believe that the most probable cause is the heat transfer through the chimney wall to the thatch that will be tinder dry. On older properties the dividing wall between the flue and the thatch can be as little as 100 mm / 4 inches, the brickwork and mortar can be soft and holes could have appeared. Unlike an open fire that is inefficient taking large amounts of air from the house cooling the flue gasses, a wood or multi-fuel stove is far more efficient but the flue gasses can be extremely hot in in excess of 300 degrees in normal operation and higher if a chimney fire occurs, therefore heat transfer can be greatly increased.
If the thatch is old and wet and has not been maintained well another possible reason of ignition could be “Pyrolysis” this is a thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen (or any halogen). It involves the simultaneous change of chemical composition and physical phase, and is irreversible.
We always follow document J and HETAS guide lines installing and appliance into a thatch property. If there is less than 200 mm clearance from the flue to the thatch we install a twin wall insulated flue system where the flue passes through the thatch sandwich. We also assure there is ventilation top and bottom of the chimney stack allowing for cooling air to assure that the area around the flue liner will not over heat. also suitable access for inspection.
We also, where applicable, raise the height of the chimney to a minimum of 1800 mm above the thatch. Although this is a requirement under building regulations document J, there is some controversy as to whether this is a requirement when undertaking repairs and modifications to the dwelling or whether this applies to new build properties only. You will receive contradictory information from many different sources.
Insurance companies will often have no requirement to raise the chimney height, although if the worst case happens they will soon use this regulation as a tool to offset responsibility.
Planning and conservation will also supply differing opinions (They are more interested in the aesthetics of the building and whether it impacts the local surroundings). Raising the chimney height to 1800 mm is often not allowed.
Below is a quote taken from a letter supplied to one of our customers from Wiltshire District Council a “Conservation Consultation Response” following a planing request to raise the height of the chimney.
“Chimney: There is no mention of this in the D&A. There would be significant visual impact of this proposal, and there is no argument made for it. The building regs height of 1.8 m relates to new-build properties and is accompanied by a range of other fire security measures, and does not apply retrospectively. This element should be omitted”.
This statement is contradictory to HETAS and other governing bodies.
We would advise that if installing into a thatch property then extensive investigation should be sought and written assurances are received prior to commencement of works.
Example installation undertaken by Sarsen Energy
Thatch has been striped back the existing chimney is in poor repair, structural timbers touching the brickwork, the brickwork is only 100 mm and therefore heat transference would be apparent.
The chimney is repaired and re-pointed above and below the thatch level.
A Twin wall insulated flue is installed with an air gap around, a vent is installed to allow for air cooling
Finally the chimney height is raised to the required height of 1800 mm