STOVES, FLUES AND CHIMNEYS
Solid fuel stoves will be an integral part of the majority of huts which have heating, and they also represent one of the biggest dangers to inhabitants and environment.
Because of this they need to be designed and installed with the utmost care and you must conform to the Technical Guidance of the Domestic Scottish Building Standards (SBS) in its entirety to qualify for the Exemption from warrant (that is, Standards 3.17 to 3.22 inclusive).
This guide relates only to solid fuel stoves designed to burn wood, coal and peat in any combination, and their installation. It does not deal with any type of gas, oil, pellet or wood chip stoves as these are not generally compatible with low impact, low carbon hutting principles and registered installers should be used.
You are strongly advised to draw a section and plan of your solid fuel stove installation in order to design the whole installation correctly and identify any specific safety issues. This will also help you to quantify and specify the several parts you will require.
Always refer to stove and flue manufacturer’s own instructions. Stove manufacturers provide instructions on safe distances from side and back of appliance (including the flue) to combustible materials as well as for controlling hearth temperatures. This Guide does not cover stoves for whom manufacturers' data is not available, stoves that have been modified or stoves that are home-made which are strongly discouraged.
Note that the Building Standards (and guidance herein) may sometimes appear more onerous than manufacturer’s instructions and in these instances you should follow the former.
Many modern solid fuel stoves are raised off the floor by legs and guarantee a hearth temperature under the stove less than 1000C. This is the recommended type and the only type that this Guide covers. The stove manufacturer’s instructions should confirm this, amongst other important specifications, including minimum distances from the stove to walls, soft furnishings etc. Always consult these details very carefully and incorporate them early on into your design.
Whatever type of stove, a hearth must be provided made of completely non-combustible material, measuring at least 840 x 840 mm, extending at least 225 mm in front of the stove door (300 mm if can be used open) and 150 mm to either side.
Follow the diagram (figure 12B), which is the same as in the Domestic Building Standards (Section 3.19.5). If the floor is tiled with ceramic or stone with a thickness of at least 12 mm and fully grouted to the areas in figure 12B then it is not necessary to install an additional hearth.
Suitable materials for a thin hearth of 12mm minimum thickness are steel or toughened glass and the latter can be purchased already shaped and toughened for this specific purpose.. Alternatively a concrete hearth (inevitably > 12mm depth) could be cast in situ and tiled or not. This is often done within a plywood shutter leaving the ply base in the floor permanently. All these hearth types may only be used where the stove is guaranteed by manufacturer to give a hearth temperature of less than 1000C
PLAN OF APPLIANCE ON A HEARTH
SEPARATION FROM COMBUSTABLE & NON-COMBUSTABLE SURFACES
Neither the sides nor the back of a stove should ever be closer than 150mm to non-combustible surfaces. Separation to combustible surfaces and furnishings will be greater than 150mm and the manufacturer’s specification should be closely followed.
A combustible surface between 150 and 300mm from the side or back of a stove will require heat protection and this should be provided by 75 mm minimum of non-combustible material such as stone or concrete between stove and combustible wall. This protection should extend up the wall at least 300mm above the appliance or 1.2m above the hearth, whichever is the greater.
To provide heat protection from any un-insulated section of stove pipe (flue-pipe) - the maximum length of a vertical un-insulated flue-pipe is 1200 mm above the top of the stove - a non-combustible heat shield is required to extend above the height of the flue (refer figure 12C). This could be a continuation of the 75mm non-combustible material or a heat shield of non-combustible material such as steel or glass with an air gap of 12 mm between it and the wall. It may therefore be simplest and safest to install an insulated chimney all the way to the stove itself.
PROXIMITY OF FLUE TO COMBUSTABLE MATERIALS
FLUES & CHIMNEYS
The following does not refer to masonry chimneys of any type, whether hand built or other systems. It is traditional in many Scandinavian huts for instance to build such masonry chimneys founded on the ground itself within a timber structure. This is an excellent strategy to reduce fire risk but is beyond the scope of this Guide. If you choose this route you will need to follow the domestic Guidance for masonry chimneys (Sections 3.19 and 3.20 in the Scottish Building Standards). In Scottish huts, most chimney systems are expected to be metal and it is these therefore to which this Guide is addressed.
A twin-walled, insulated flue/chimney may be fitted from immediately above the top plate of the stove all the way to its termination above the roof. Alternatively a short (1200 mm max) uninsulated section may be installed immediately above the stove connected to the insulated flue above.
Flues are safest, most efficient and most economic when fitted into the top of the stove and travel vertically all the way up and out of the building. In this way the hut can benefit from the heat escaping from the stovepipe wall. Alternatively the flue may depart from the back of a stove for a horizontal distance of not more than 150 mm before bending 45 degrees or 90 degrees up an external wall. This does allow for a soot trap or inspection plate at the bend. (A vertical flue has to be swept down into the stove).
The correct separation between proprietary twin-walled insulated chimneys and any combustible material is given by the manufacturer and is often 50 mm but this should be carefully checked. That separation is maintained by proprietary support bands that are fixed to walls either internally or externally. Where a flue passes through a wall, platform floor, roof or hidden space such as a cupboard there is significant danger of ignition of adjacent combustible materials and it is essential to follow the flue manufacturer’s advice. In hidden spaces you should provide a guard around the flue to ensure that no material can come within 50 mm of the flue by accident.
Where the flue passes through any structure or cupboard, hidden attic etc, ensure that it is held securely in place by proprietary metal rafter support brackets or similar giving the required minimum separation and insulation from any combustible material.
Ideally the flue will be left as open and visible as possible. Consider this at an early stage of hut design. Do not take it through a platform floor or any other hidden space (such as a cupboard) unless it is really necessary. Ensure that there are no joints in the flue where an insulated flue has to pass through any structure. You will find it helpful to draw a section and plan the lengths of flue to be installed.
It is essential to ensure that the structural hole (in a roof or floor platform) for a flue is slightly over-sized so that it can accommodate a fireproof lining of a proprietary non-combustible material that meets the Class 0 standard in the Building Regulations.
The flue/chimney will be sealed to the roof covering with a non-combustible flashing either of the fireproof rubber type or a malleable metal type. In an insulated structure it will also be necessary to dress the waterproof roof membrane onto a metal separating plate fixed to the upper surface of the insulated roof structure. This is assuming the roof has been built correctly with a ventilated cavity between the insulated roof and the roof covering.
The height of the flue termination above the roof and ridge is shown in figure 12D. In many cases the flue will need to terminate a minimum of 600 mm above the hut ridge. But note that many manufacturers suggest a minimum overall flue and chimney height of 4.0 metres to achieve a satisfactory draught. In low, single storey huts this may mean extending the flue well above 600 mm of the ridge and, in these cases, you will need to stay the chimney well. There are proprietary systems for doing this or you may design metal structures or wires to achieve the stability required. Any movement, including long term vibration in a metal chimney will cause seals to break and allow precipitation and /or combustion gases to leak into the building.
All commentary in Flues and Chimneys is based on non-combustible roof covering. For combustible roof covering a flue outlet should be 1.8 metres above roof finish.
POSITION FOR FLUE OUTLETS
Combustion requires an adequate air supply and the stove will not operate efficiently without one. Furthermore it can produce toxic gases if inadequately supplied creating a severe danger to inhabitants (even a danger of death) and damage to the environment. A small, insulated hut space requires careful consideration as it may be sealed sufficiently well to restrict the combustion air available.
If the stove output is rated at 5kW or less it will be necessary to add up the fixed ventilation area (that is, permanent, unclosable ventilation openings) in the space occupied by the stove and ensure that it totals at least 12000 sq mm. This can be provided by slotted ventilators of a permanently open type. However, good practice is to provide a direct air supply from the exterior environment into the stove. This is generally achieved by use of a proprietary air supply kit supplied by the stove manufacturer.
The pipe for this kit can be installed either through the wall or the floor of the cabin if the space under the building is well ventilated and open to the external environment. Stoves with output of over 5kW are not recommended in huts (unless heating water in a back boiler - stoves with boilers) but where they are installed good practice will again require provision of a direct air supply into the appliance.
In such a small space as an insulated, reasonably well-sealed hut, an extractor fan may cause a problem to the proper functioning of an open-flued solid fuel stove by reducing air pressure. This Guide therefore strongly recommends that extractor fans are never installed in huts with stoves even in separate bathrooms and toilets. Rather ensure that there is an openable window or fixed ventilator to clear unwanted odours and any moisture generated Good practice is to carry out a spillage test on completion of installation.
STOVES AND BOILERS
Stoves with internal boilers present particular hazards and these are detailed in Electrics and Photovoltaics on washing and showering. The main concern with boilers is their ability to boil water when the stove is run too hard or the water supply is inadvertently interrupted as can occur where a private supply is not entirely reliable.
This can pose a significant danger to inhabitants. There must be provision to let pressure and steam escape safely from an overheating system (ie they should be fully vented to the outside of the hut). Systems should be designed so as not to overheat and may include ‘heat dumps’ to achieve this. Take professional advice if you are not entirely confident in your installation.
Section Fire details good practice in fire safety precautions which should be followed in all huts but with particular care where a stove is installed. Make sure that as a minimum you have fire extinguishers both inside and outside the hut, a smoke and a heat alarm within the hut as well as a carbon monoxide detector in every room containing a combustion appliance and any that could be used for sleeping. These must be sited and maintained according to the manufacturers’ recommendations.
If you are off mains then you must have spare batteries for your alarms and replace them as soon as indicated by the low power indicator on the alarm. Assuming the hut will be used by different parties, some of whom may have little or no experience of operating stoves, it will be essential to provide well displayed safe operating instructions along with the fire safety drill and emergency procedures.