Some Chimney Pots, Cowls and Caps

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 (block off a lum) 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 / Fixings

Cowls are typically fixed in 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 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 (with horizontal slats) can act as an additional 90 degree 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.

Mesh Size

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.

Pot Inserts

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 (e.g. bonnet cowl) 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

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 gas cowls are absolutely not suitable for solid fuel.

Figure 1: Louvered pot

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 chimney 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.

Figure 2: Gas Terminal/Cap GC2


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. 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 their weight and price but really do work well for wind induced down draughts.

Figure 3: H Pots

Rotary Cowlings

Rotary cowlings are designed to spin in the wind that might be causing a down draught and in doing so cause an up draught.

There are some downsides to these:
• They must be cleaned , so occasionally someone has to climb 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.

Figure 4: Disused chimney plugs & typical e-cap bottom r.h.s

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 should 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.

I hope you have enjoyed this wee general knowledge article , please feel free to do with it what you will ..because I know you will anyway. 😉  Tom