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Volume 1 | Issue 1 | 11th June 18

A regular chimney pot is usually best

A standard terracotta cannon or roll top chimney pot that is correctly sized  is usually the very 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 as there is no change of direction. Smoke is hot it likes travel vertically and not horizontally.

Some cowlings in some ways 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 as it hits the cold metal mesh and the top of the cowl causing creosote deposition within. The cleanliness of cowlings must be checked with binoculars after sweeping.


Meshing must be 2.5cm x 2.5cm when used for solid fuel.

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 with building regs and for adequate gas flow. Solid fuel terminals are typically suitable for gas also however gas terminals are Not suitable for solid fuel. Solid fuel terminals must be made from a material that does not easily or quickly corrode .

Mesh Size

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, causing smoking back and even carbon monoxide poisoning.  This goes for any meshing  fitted to the terminal serving an appliance that burns sold fuels.

In  order to  guesstimate mesh size from the ground with binoculars a 25mm ball must be able to fit through the mesh gaps that’s roughly thumb width.

Pot inserts

Any terracotta insert that sits inside a pot has a spigot which reduces the effective internal diameter.   Often making them instantly unsuitable for open fires. There are 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.

A GC2 Gas Cowling insert. All GC type inserts are Only suitable for certain gas appliances

Right: a louvred pot suitable for a solid fuelled open fire or similar. Not to be confused with the GC gas cowling on the left.

Louvred Pots

Louvred pots are used to stop rain running done the flue, and can be identified as the gap in the louvre is quite  large and they are 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 Cowls have a number for identifying them GC3, GC4 etc the number usually relates to the size of the cowling and the air it can flow. Remember GC cowlings are NOT suitable for solid fuel, some  decorative fuel effect gas fires require chimneys that are as large as those require 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.

Left: H pots, you can clearly see the insert type that is not suitable for open fires here. It’s harder to notice when up on a roof


H pots are one of the best anti down draught 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 favour because of the weight and price but really do work well for wind induced down draughts.

Rotary cowlings

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 axis.
• They only work in the presence of the wind
• Eventually they will stop spinning and require replacing
• 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.

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. As we all know a traditional chimney relies on positive pressure inside the room where the appliance is to help push the fumes up and our and help create natural draught when combined with the heat of the fire. Occasionally, usually in public houses there is a minus ambient pressure situation cause by mechanical extract ventilation in the kitchens and this must then be offset by adequate intake air (vents) and a mechanical chimney fan.
There are some significant downsides to these too that you must be aware of:
• Only people competent to work on electrical appliance 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 smokey day indeed.

Disused Chimney Plugs

These types of terminal are not for use on live appliances. It is normally correct process 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 warning notice should be raised if any of these disused chimney terminals are fitted to a live chimney you are working on.

Above Disused chimney terminals not for use on live appliances



Cowlings are typically fixed one of three ways: 

  1.  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.
  2. 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.
  3. Gravity: many Teracota 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.


The main regulations for chimney cowlings are BSEN-1856-1:2003, and all gas chimney cowls to comply with BS5871. BSEN-1856-1:2003 is the main regulation required for chimney cowls.  temperature ratings T-450 for solid fuel and T-250 gas.