As is customary for this time of year , i always write a wee reminder that spring is upon us once again and it won’t be long till Mr & Mrs Jack, D Aws will be scouting for a new home. Unfortunately that is most likely to be in a chimney and is one of the worst problem situations a solid fuel user can get.
Customers always seem to imagine a nest will be similar to that of a crows nest that you see in a tree. So they are almost always astonished when they realise it is more akin to a beavers dam and normally a minimum of 1.5 bin bags full of material. If they are interested i tell them my memories of the, “worst nest ever” back when i were just a young lad of about ten or so, let’s say early-mid 1980’s.
This nest was like several stories high and must have been generations returning and building on the old nests before as foundations. The run of the flu in this old building was quite funky too and presented my father and co , no insignificant problem. At some point they had to open up the bricks to make sure it was 100% cleared and in the end after 9 hours they had amassed 13 full bin bags of twigs, squashed coke cans, six inch nails, sheep’s wool, horsehair and other assorted nesting materials.
The moral of the story is , it is better to be safe than sorry…prevention is far better than cure in this case and so very much LESS expensive for you. The other thing is you should know that once you’ve got one , it cannot be removed till August unless you get a special permit. For more in depth information you can read our previous post on birds nests, filed under funnily enough ‘birds nests’
This article will serve as a general introduction to various pots, their insertions, cowls, and uses.
A standard terracotta electric cap aka elephants foot or a correctly sized roll top chimney pot is usually the best way to terminate a chimney. They are made of terracotta, have low wind resistance, are strong, last many years and most importantly do not impede the flow of Flue gases because there is no change of direction. Smoke is hot and it likes to travel vertically, not horizontally.
Cowls are typically fixed in one of three ways:
Strap fixing: this is by far the most secure and what sweeps typically recommend as it reinforces the pot and cannot easily be swept off.
Bolt fixing: These types of cowling are not favoured by many, the bolts pull the pot from the inside often opening cracks. They are also very easy to sweep off.
Gravity: many Terracotta inserts are held in place only by their weight. Care must be taken when cleaning them as they are easy to dislodge and Very heavy so can do a great deal of damage when landing from a fall.
It is worth noting some cowls (horizontal slats) can act as an additional 90 deg bend as the smoke is forced to change direction at a right angle before it can exit to the atmosphere. Smoke also quickly cools when it hits cold metal mesh and the top of the cowl causing creosote deposition within it. The cleanliness of cowlings should always be checked with binoculars after sweeping.
Chimney Terminals: The Rules
The internal diameter of a chimney terminal used for solid fuel open fires must be no less than 8” to comply for adequate gas flow and building regulations. Although Solid fuel terminals are typically suitable for gas the reverse is NOT true. Solid fuel terminals must be made from a material that does not easily or quickly corrode, especially where smokeless fuels are used.
ANY Mesh fitted to terminals servicing solid fuel appliances must be stainless steel and the hole size must be no smaller than 2.5cm x 2.5cm. Gas terminals often have smaller mesh and quickly block with soot when used incorrectly with solid fuel. This can cause smoking back and even carbon monoxide poisoning.
Any terracotta insert that sits inside a pot has a spigot which reduces the effective internal diameter. The spigot often makes them instantly unsuitable for open fires. There are indeed many inserts which are only to be used for disused chimneys.
One exception to the above is top hanging Cowls connected to a liner and attached to a stove. These can be as small as 5” 125mm internal diameter for some Defra exempted stoves however 6” 150mm is more typical.
Louvered pots are used to stop rain running down the flue and can be identified as the gap in the louver is quite large and they are also often formed as part of the pot. They must not be confused with GC gas cowlings that look similar but are absolutely not suitable for solid fuel.
GC terminals have a number for identifying them GC1, GC2 etc the number usually relates to the size of the cowling and its air flow rate. Some decorative fuel effect gas fires require chimneys that are as large as those required for solid fuel. It is important that sweeps always insist gas appliances are serviced immediately after sweeping by a gas safe engineer to ensure the relevant rules are met and the customer is safe.
H pots are one of the best anti downdraught designs, they come in two varieties; an inset which is not suitable for most solid fuel open fires and a Complete pot with the attached H that is. The inset version can be a great option for stoves that require a smaller diameter Flue. H pots have fallen out of favor because of the weight and price but really do work well for wind induced down draughts.
Rotary cowlings are designed to spin in the wind that may be causing a down draught and in doing so cause an up draught.
There are some downsides to these;
• They must be cleaned which means occasionally someone has the unenviable job of climbing on the roof to reach them.
• If the axel is even the tiniest bit bent then they will be very noisy and spin of the axis.
• They only work in the presence of the wind.
• Eventually, they will stop spinning and need to be replaced.
• They only work on wind induced down draughts and serve no benefit when the down draught is caused by positive pressure down draughts from being in a high-pressure zone. Extending the termination beyond the high-pressure zone is the recommended action for these.
Disused Chimney Plugs
These types of terminals are not for use on live appliances. It is normally the correct procedure to issue a Do Not Use warning notice when these are being used on live appliances. It is impossible to perform a satisfactory or accurate type 2 smoke test when these are installed. A standard terracotta electric cap aka elephants foot or a correctly sized roll top chimney pot is usually the best way to terminate a chimney.
Mechanical extract fans
Electrical extract fans can be used to offset positive pressure down draughts and can even make chimneys work when the pressure inside a property is a minus. Traditional chimneys rely on positive pressure inside the room where the appliance is helped push the fumes up by natural draught when combined with the heat of the fire. Occasionally, usually in public houses, there can be a minus ambient pressure situation caused by mechanical extract ventilation in the kitchens and this must then be offset by adequate intake air (vents) and/or a mechanical chimney fan.
There are some significant downsides to these that you must be aware of:
• Only people competent to work on electrical appliances can install them legally.
• They must be manually cleaned.
• Alone they do not make the installation compliant under J regs, merely make it work.
• They require servicing.
• If they fail or the electric goes out it’s a very smoky day indeed.
So i hope you have enjoyed this wee introductory article, please feel free to share it , steal it… modify it as folks do on the interwebs. Tom
An open fire is just that, it is a fire sat within an opening on a grate/ insert into an open flue/ chimney. The only control you have (other than a damper plate in some kinds) over an open fire is the amount of fuel you use in it.
What is a stove?
A stove is a closed appliance. Therefore, its air intake is controlled by the user by the means of primary and secondary air flow controls which control how quickly the fuel burns.
What are the differences in efficiency? A typical open fire will have an efficiency of approximately 5-35% on a 24-hour cycle; with a maximum of around 60% efficiency. The efficiency of a stove varies with each stove design but must have a minimum efficiency of 65%. Most modern stoves average around 80% efficiency with some exceeding 95%.
The stove is undoubtedly safer when in use, as the flame is contained within a cast iron or steel box and behind a piece of toughened glass. An open fire has no safety features except for maybe an aftermarket mesh spark guard. Perhaps surprisingly, statistically, chimney fires are much more likely when woodburning stoves are installed. Individual users often over-regulate the air supply causing the fire to slumber or they burn wet wood (>20% moisture).
The initial outlay is more expensive when installing a stove mainly due to the costs of a suitably sized liner but its’ running costs are very much lower due to its’ higher efficiency. This is achieved by being able to regulate the air intake on the appliance and having a physically smaller burning area than most open fires. Also, less fuel is required to get the same heat output from a stove.
The open fire is a clear winner here. An open fire is seen as a primal thing… you really feel like the fire is in the room with you and it certainly is the focal point of any room. The smell, crackle, and roar of a true open fire simply cannot be beaten. Whilst the stove has a controlled burn for efficiency, toast and melted marshmallows taste better on the open fire.
To put this in perspective open fires burning anything but smokeless coal have been banned from virtually every major city in the UK due to the smog they produced in the 1950s and 60s. There is an argument today that woodburning stoves produce particulate emissions similar to that of diesel cars but this is also true of open fires though and in much higher concentrations than in a quality closed appliance.
An open fire must have an air vent in the room of the appliance and this must have a minimum free air space of 50% of the cross-sectional area of the throated part of the flue. This is indeed a large vent and in most cases brings in lots of cold air. Whereas most stoves under 5kw do not require additional ventilation unless installed in a new build property post-2008.
Open fires and stoves work in different ways. Due to the uncontrolled nature of an open fire and the weak draught caused by diluted flue gases from the room. It must have a large air flow in order to evacuate the exhaust fumes from the room.
This is accommodated by having a minimum 200mm diameter sized flue. A stove, on the other hand, requires a powerful draught to draw air forcefully into the firebox through the small air intakes. This is done by having a much smaller diameter flue, often with a smooth internal surface that often gets hot and is helped by additional insulation around the liner. Both require N2 class flues rated T450 as a minimum.
The stats clearly show that the stove takes the prize for efficiency and economics but you really can’t beat a natural flame fire for the feeling of home comfort.