Why Doesn’t My Stove Work? – Spring & Autumn Syndrome

There are a multitude of common misconceptions surrounding how a stove operates and in this series of Why Doesn’t My Stove Work? articles, we hope to dispel the myths and clearly underline the actual causes and simple solutions.

First and foremost, it is essential to recognise that the flue (chimney) is the engine that powers a stove. The basic fact is a closed heating appliance such as a wood burning stove without a flue will not work (and vice versa if you felt the need to try that).

There is a lot of ‘science stuff’ going on inside your flue system that powers your stove and below are three main driving factors:

  • It is not the heat of the stove or flue pipes themselves that draw air into your stove, it is the affect heat has on the density of the molecules within the gases. The warmer the gas, the lighter the molecules, the more encouraged they are to rise. Reversely, the colder the gases, the heavier the molecules and the more likely they are to slump or drop back down the system. Therefore, if the molecules within the gases inside the chimney system are warmer than the molecules within the air outside the chimney system, the gases inside the flue will rise as the air outside sinks and rushes in to fill the space left by the rising warm air (put simply), thus creating the magical draw!
  • Taking the above into consideration, the greater the temperature difference between the flue gases and the air outside, the greater the draw. Simple.
  • Thirdly (but crucially), the height of the flue system affects things as the taller the system, the greater the change in pressure which also encourages a stronger draw.

So what is Spring & Autumn syndrome? This is concerned with how quickly temperatures can change around these times of year. If the temperature outside should happen to rise quickly then this can easily surpass the temperature within the home. Warm outside vs cool inside equals a reversal of the draw as air flows down the system rather than drawing up.

Should this reversal of draw occur, you will not know until you attempt to light your stove. The lighting cycle will begin but the air coming down your system will force smoke out of the stove into your living space and prevent any hot air from rising into the flue to remedy things.

So how do you avoid this situation?

Firstly, test the direction of the draw before lighting it! Even without a fire burning, there is a draw of sorts happening in your flue and there are ways to find out which direction this is flowing; by hand or is a smell of soot being pushed out of the flue?

Secondly, should the draw be in reverse, you can encourage it back up the system by artificially warming the air within the flue. Some use a hair dryer, others a blow torch.

But, here at Glowing Embers we have a top tip! Stack your fuel in reverse. Logs on the base, kindling on top and firelighters on top of this. This is a tried and tested method in our showroom because the heat produced by initially the firelighters and then the kindling is directed up into the flue and not down into the fire bed, thus warming the flue above first. You will most likely experience a little smoke at first but if stacked and controlled well, the flue should warm reasonably quickly and the stove kick into life…

Do I Need To Insulate My Flexible Flue Liner?

There is NO Building Regulation that says you must insulate around a flexible flue liner, within a chimney stack. However building regulations do state that gases must be safely taken away from the building and by insulating your flexible flue liner, within the chimney cavity will ensure that the gases will travel directly up from the stove and out through the chimney top.

If the chimney is on an outside wall, or the stack is particularly tall, or the flue is of a large diameter then the answer to the question is probably YES ! By keeping the flue gases warm you will increase the speed of those gases escaping and help prevent condensation forming on the liner. Cooling gases can result in condensation flowing down the flexible liner and entering the stove itself. This may, over time cause damage to the flue and the stove.

A good quality Chimney Insulation is the ideal accompaniment to any flexible flue system, especially if your chimney is on an external wall. By keeping out the cold and maintaining the heat within the flue, Vermiculite Granules and FlexWrap or Rockwool Insulated Tubes will keep the hot gases rising for an efficient draw on your stove.

Vermiculite granules are supplied in a 100l bags and can be poured into the chimney cavity once the liner and register/closure plate are in place.

Please see our Vermiculite Calculator to estimate how many bags of Vermiculite you will need for your chimney. You will need to enter the dimensions of the chimney stack and the diameter of the liner you intend to use.

Vermiculite granules are only recommended if the chimney is watertight.

FlexWrap Blankets are supplied in 10m lengths for either 5-6” flexible liner or 7-8”. They are secured by Aluminium Securing Bands available in the two sizes as above.

FlexWrap is secured around the flexible liner before it is dropped down the chimney.

It is recommended that all joints are sealed using 50mm wide aluminium tape and then securing it with aluminium bands every 500mm.

The FlexWrap is 12mm thick and can withstand continuous high temperatures of up to 550°C as it is made from a non-combustible material.

This product can be cut with a Stanley knife.

Rockwool Tube insulation is available in 1000mm lengths to fit 5”, 6” and 8” flexible flue liner and is supplied as a tube. The tubes are manufactured from non-combustible rockwool which does not sustain vermin and will not encourage the growth of mould, bacteria or fungi.

They can also withstand continuous hot temperatures of up to 500°C.

It is recommended that all joints are sealed using 50mm wide aluminium tape. Each roll also comes with a self-adhesive tape which must be used for all joints.

To view Glowing Embers’ entire range of chimney insulating materials for all manner of installs, please click here…

The Purpose of a Roof Flashing

When a flue system passes through the roof of a property, the hole that must be cut in the surface will always be larger than the overall diameter of the flue pipe due to safe clearances to combustible materials. For example, a twin wall pipe with an overall diameter of 200mm will typically require a hole in the roof cut to at least a diameter of 300mm… This is where a Roof Flashing steps in.

A Roof Flashing is designed to provide a weather proof seal at this point by covering the hole with a base plate and providing a water tight seal around the flue with the silicone collar.

Generally, Roof Flashings can be separated into one of two types; suitable for tiled roofs or suitable for every other roof type such as profiled, corrugated, felt, wooden and so on.

Tiled Roof Flashings

These Flashings sit on a malleable, aluminium base, designed to be secured beneath the roof tiles for ultimate weather proofing. The silicone cone upper portion will be pre-set to a 20° angle to accommodate the standard pitch of a residential roof. As all flashings are designed to accommodate a range of pipe diameters, the silicone cone portion is simply then cut away to the desired width for your flue pipe.

Non-Tiled Roof Flashings

This form of Flashing features the same silicone upper cone but set at 90° for flat roofs. The base features an aluminium strip around the edge rather than a base plate to allow the product to contour to any roof type, especially handy when it comes to corrugated roofs. It is through this strip that you can secure the Flashing on top of the roof surface and again, as all flashings are designed to accommodate a range of pipe diameters, the silicone cone portion is simply cut away to the desired width for your flue pipe.

Low or High Temperature Flashing?

Depending on the type of pipe being used and distance from the stove, you may select either low or high temperature resistant Flashing. As the hottest point of any system is the connection to the stove itself, it is the distance from this that we judge which tHigh Temp. Flashingype of Flashing you will need:

  • Is your Flashing going to be within 5m of the stove itself? Then you require a high temperature Flashing

  • If your Flashing going to be over 5m of the stove itself? Then a low temperature Flashing will do.

It is a common misconception that using insulated Twin Wall flue means you only require a low temperature Flashing. This is incorrect and the above should be observed as a rule of thumb.

Tip – If you have a particularly steep pitch to your roof, it is advisable to purchase the next size up Flashing to accommodate this.

Glowing Embers supplies leading brands of Flashings and fixing kits to meet the needs of every install and roof type and a range of sealants such as Envirograf Silicone, perfect for sealing external applications.

Chimneys in Thatched Properties

The installation of a log burner and flue system into a property with a thatched roof can be a cause for concern for many consumers due to the highly combustible nature of the thatch and sadly, in recent years, the number of chimney related, thatched roof fires have slowly but steadily increased.

The two leading factors in thatched roof fires are:

  1. The transfer of hot flue gases through a brick stack with open or porous joints
  2. The presence of thick beds of thatch against a chimney, together with hot flue gas temperatures, which over time will result in the thatch being heated to the point where it will char or burn. This thatch then insulates the brick chimney stack, resulting in the heat being unable to escape and creating a localised hotspot in the centre of the hatch. However, more regulations have been introduced to help reduce (and hopefully stop) these potential dangers from happening.

Here is some simple advice to follow when it comes to installing a flue system in a property with a thatched roof, to help keep you and your property safe and compliant with Building Regulations and HETAS guidance.

First and foremost:

  1. Any work carried out on an appliance or a chimney must be reported to your local authority (this is commonly via the HETAS Compliance Certificate).
  2. Before any lining work is carried out, the existing chimney must be swept thoroughly, checked and any corrective repairs must be undertaken to ensure it is in suitable condition.
  3. Consult Approved Document J for Building Regulations, ensuring you have reviewed the distance to combustible material to the chimney.

What Type of Flue Should I Use?

  • Single brick stack chimneys with thatch tight to the brickwork must have a Twin Wall insulated flue system with a known distance to combustibles (typically 50/60mm) to comply with Regulations. We advise SFL SFlue or Shieldmaster Twin Wall for these applications.
  • Twin Wall systems can simply be assembled and fed carefully into the stack, ensuring it is locked at every joint and secured in the stack, following the manufacturer’s guidelines. You may need to break into the stack to attach the relevant support brackets and ensure the system is central.
  • For chimney stacks with bends and/or offsets, from the point the chimney starts within a living space up to the bedroom ceiling only, this can be relined with a Flexible Flue Liner, cast in-situ concrete lining or breaking into the stack and adding a clay refractory concrete or pumice lining ensuring it is the same diameter as the Twin Wall. These two flues must then be sealed securely.

How Large Must the Clearance Be Between my Flue and the Thatch?

  • For small 440mm square two brick stack chimneys, there must be at least a 40mm gap from the outer face of the brickwork to the thatch material, this can be done by constructing a non-combustible shield away from the face of the chimney before re-thatching takes place. They also are unlikely to fit a traditional liner and maintain the required distance to combustibles without adding this 40mm air gap.

  • For larger chimneys, where the distance from the inside of the flue liner to the outside of the brickwork is greater than 200mm, the thatch can be tight to the chimney, not requiring the 40mm gap. However, it is vital that the flue is fully centred all the way down the chimney.

  • To obtain the required 200mm distance to combustible material on a 6-inch (150mm) diameter lining, the external brickwork of the stack would need to be at least 550mm square and the liner would need to be fully centred.
  • Some local authorities restrict the removal or alteration of the thatch due to conservation requirements, it does make this situation more difficult but is not always impossible to overcome.

Is Extra Ventilation Required?

  • Back Ventilation can be used to cool the void between the Twin Wall flue and inner brickwork, this preferably should be ducted in from the outside of the building.
  • Depending on the type of systems used, a closure plate and/or support brackets may be required.
  • An air vent between a loft space and the void between the Twin Wall and brickwork should be used, with a high-profile vent in the chimney stack above any thatch to allow back ventilation.
  • It is recommended to install an inspection hatch in the stack, within the loft space so the externals of the System Chimney can be checked during any maintenance inspections on the combustion system.

Follow the guide above and it is perfectly safe to install a wood burning stove and flue into your thatched property but please do ensure your home Insurance company is made aware of any plans or installations, failure to do so may result in the home insurance being invalidated.