Category Archives: General knowledge

Some Chimney Pots, Cowls and Caps

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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

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

Open fires versus Stoves ……some differences

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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 is an open fire ?

An open fire is just that, .. 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.

Efficiency

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 design but must have a minimum efficiency of 65%. Most modern stoves average around 80% efficiency with some exceeding 95%.

Safety

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 wood burning stoves are installed. Individual users often over-regulate the air supply causing the fire to slumber or they burn wet wood (>20% moisture).

Economy

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.

Aesthetics

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.

Environmental Impact

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 1950’s and 60’s. There is an argument today that wood burning 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.

Ventilation Requirements

An open fire must have an air vent somewhere 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 5 Kw do not require additional ventilation unless installed in a new build property post-2008.

Flue Requirements

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 200 mm 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.

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.

Using a stove thermometer to ensure optimum efficiency

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Your newly acquired wood/multifuel stove will give months of long, easy & warm cosy days where you can lounge around just enjoying yourself. However, as with all things that are subject to temperature & pH variations, maintenance checks are essential.
Using a thermometer gives you peace of mind over safety and the satisfaction that everything is running under optimum conditions.

Often times these, “checks” may seem as if they might drag you out of your comfort zone but the good news is, that once you’ve attached it to the stove top (or flue pipe arising from it) using a thermometer is as simple as making regular readings. They come in a few varieties and can be wire, hose clipped or magnetically attached.

 

stove thermometer

Figure:1 Typical stove thermometer with “optimum operation zone”


Used primarily to avoid over firing, (the resulting extreme temperatures create bad fuel efficiency) and are operated by having an eye gauge in both °Celsius and Fahrenheit. Keeping your stove burning in the Correct range: 115°C – 245°C or  (240°F – 475°F) will ensure the safest operation, whilst simultaneously giving maximum fuel economy.

Running Too Cool

Allowing your stove to reach burning temperatures below 115°C or (240°F) will lead to incomplete combustion creating carbon monoxide, tar, soot and creosote.

Creosote itself, is a condensation residue of coal and/or wood particles, hydrocarbons, gases and other airborne debris. It is formed as gases cool, for example when air in a chimney is not hot enough to push the particles out. Its appearance is of a thick hard black shiny goo which is flammable and reduces the bore of the flue by sticking to it.

Running Too Hot

When you allow your stove apparatus to run above burning temperatures of 245°C or (475°F) you risk damaging your flue/liner/cowl and possibly warping your baffle plate.  The risk of chimney fires is significantly increased now, especially if there has been creosote build up.

Further Info

Choosing to read our other articles under ‘stoves’ or burning ‘wood’ to gain the knowledge needed to use your appliances to maximum efficiency under safe conditions is advisable. Please always remember a significant part of this maintenance strategy is to get your solid fuel heating appliances in use, swept at least once a year by your chimney sweep.

You can buy thermometers and wood moisture meters Here
NB/ We are in no way affiliated with this company and the link is only just to show an example of what items you may want

Multi Fuel or Wood Burning stove.. What’s the difference?

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Traditionally, stoves were intended to burn wood and only wood, then multi-fuel stoves were created to give you the option of heating your home with either coal or wood.      So , ..    Multi Fuel or Wood Burning stove ?
First off, you should know that it is NOT advisable to burn both coal and wood in your stove at the same time as this can and will damage your flue lining and stove itself. 

Why is it not wise to mix fuels 

Why can’t you mix fuels ? because corrosive materials such as halogens and sulphur found in coal will combine with the moisture in wood creating nasty, Sulphurous acid & other solutions that stick to and then erodes your whole stove system.
Whilst multi-fuel stoves bring with them the versatility of being able to choose which fuel you’d rather use at a particular time , their one limitation is the use of  ONLY One fuel at a time! Mixing fuels is not cool ! 

Only coal as fuel? ..Simple use Smokeless

You should always check your owner’s manual for the final word from the manufacturer on what fuels are recommended for use. Although most multi-fuel stoves are equipped to burn normal house coal, (aka bituminous coal) often most stove manufacturers will advise against this because of the high amount of soot in house coal.
**High soot content results in your stove system becoming rapidly clogged up**.
Using smokeless coal avoids this problem by agglomerating particles and reducing the amount of smoke and soot going up your flue , is also better for the environment and thus suitable for use in smoke control areas.

Knowing which stove is which , just by what you can see 

Multi Fuel or Wood Burning stove , the most obvious examples you can see immediately by eye are the:

1. External primary PRC & SRC secondary air flow regulator controls and 
2. Internal raised grate of multi fuel burners or the flat bottom non grate of wood burners.

Coal burns best on a ‘raised grate’ since it needs an air supply from below. Therefore , the primary airflow intake is below and its manually adjustable Primary Regulator Control outside (usually, somewhere near the bottom of the stove door & is normally either a sliding slat or turning knob/disc) is used to burn it effectively.

multi fuel burner showing SRC & PRC annotated in green Multi Fuel or Wood Burning stove

1. The PRC [below discs]  &  SRC [above slats] Regulatory Controls of a multi-burner

Wood doesn’t need this additional air supply below therefore , wood burners come with only a ‘flat bottom‘ . This limits the air supply to the fuel, resulting in a slower burn and you will see they ONLY have a top secondary air flow intake with manually adjustable Secondary Regulator Control. 

****        The adjustable multifuel grate in the picture below shows it can be both raised (top) allowing upwards air flow or it can be flattened (bottom) stopping upwards air flow from below.    ****

multi fuel adjustable grate Multi Fuel or Wood Burning stove

2. Multifuel adjustable Stove Grate

You will find that wood burns faster on a multifuel stove than on a normal wood-burning stove because of the extra oxygen and air flow from below. 

side view of multifuel stove showing air intakes Multi Fuel or Wood Burning stove

3. Primary, Secondary & Tertiary air intakes .. stove door to Left of pic

So what do I want to buy then?

In short , what to buy depends on what you are going to burn? If you’re thinking of using wood as your main fuel then it’s best to buy a wood-burning stove. By buying a multi-fuel stove, you are already almost committed to coal being your main fuel (but keeping the option open to burn wood if need be, albeit at a lower efficiency) and wood as an option.

Therefore , for the reasons already mentioned above , smokeless coal is your prime candidate fuel for a multi fuel stove. So you will be considering Anthracite, as well as a host of brand name alternatives such as Taybrite and Phurnacite. 
However , as you will see in our article on stainless steel liners there are some drawbacks to using these smokeless fuels. 

 Things to keep in mind::

1. Normal house coal is cheaper but too dirty for stoves 
2. Smokeless is expensive and more corrosive but burns clean enough & is better to use in stoves than bituminous
3. Unless you have a free source of weathered hard woods then having a multifuel means using smokeless coal IS the option. 
4. Don’t ever use wood and coal together at same time, water in wood dissolves sulphur in coal creating sulphurous acid !! ALL coal has sulphur , some worse than others.
5. Best to have a china hat/cowl to deflect water ingress. All lum/flue dampness causes sulphurous acid corrosion, esp with smokeless fuel
6. Smaller (within stove capacity) hotter hardwood fires are cleaner and don’t ruin baffle-plates or warp stoves
7. Read about liners on this site, 316 SS for wood and 906 for Anthracites NB/ sulphur ~ halogen corrosives
8. In olden days damper plates were used to control burning[often slumbering Tutt Tutt!!] by choking the fumes exhaust rather than the air intake itself reducing combustion. This is dangerous due to the accumulation of toxins rather than reducing their creation in combustion control itself. 

Using a wood [you see SRC only ! Flat Grate] only stove:-
You would be controlling the flames ONLY by the Secondary air flow intake [see picture 1 above] by manually adjusting the Secondary Regulator Control. Correct settings would be seen as curling yellow flames emanating from the mixture of gases combusting above the fuel on the grate.

Using a multi [you see PRC & SRC Raised Grate] fuel stove:-
You would control the main flames by using the Primary Regulator Control [see picture 1 above] then adjust further also with more tighter control to satisfactory conditions by the SRC [see picture 1 above]. Visually blue/orange flames would be seen below and yellow ones above the fuel.