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If you have bought a house and have no experience as to the performance of the chimney be very
careful, ask the previous owners and try to gather as much information as possible before you commit
yourself.
As a rough rule of thumb consider that if you have a 90 bend in the flue you will need an additional 1m
of height and for every metre of horizontal flue you will need an additional 2m of height.

LINING OF EXISTING CHIMNEY’S

Most old, leaking chimneys need lining and insulating with special materials designed to:-
1. Cure Leaks.
2. Reduce the build up of tar and soot on the walls of the flue.
3. Withstand the tremendous heat generated when tar and soot catch fire.
4. Generate a steady and controlled flue vacuum.
Old unlined chimneys are not suitable, if wood burning appliances are used on these chimneys the
following may happen:
Tar builds up on the brick faces of the internal chimney walls.
This build up can take from 12 months to 5 years, and as time passes the tar gets thicker and thicker,
in really bad cases the tar can work its way through the chimney walls into the plasterwork of
adjoining rooms staining the wallpaper or plaster and causing a pungent wood smoke smell which can
contaminate the affected rooms.
If a chimney in this state catches fire the results are severe causing bricks and mortar to crumble and
drop down the chimney, and tar to ouze through the affected walls.
Often, with a chimney fire of this nature it could well be necessary to remove the complete stack
taking out and replacing all the walls affected by the tar impingement.
Severe damage can also occur on some types of twin wall stainless steel chimneys, causing
distortion and leaks of flue gases.

A chimney cap is a device that is normally placed on top of a flue outlet for the purpose of facilitating
dispersion of the combustion products; it must satisfy the following requirements;
-have a useful exhaust section that is at least double the section of the flue outlet on which it is
inserted;
-have a shape that prevents the entry of snow or rain into the flue outlet;
-be built in such a way that venting of the combustion products is guaranteed regardless of wind
direction.

CHIMNEY HEIGHTS AND TERMINAL POSITIONS

Fig 1 is a rough guide about chimney heights and terminal positions.
Many manufacturers will call for a minimum flue height of 4.5 meters.
Document J of the building regulations provides details of the required statutory flue heights and
terminal positions, the British Standard for chimney height calculations is BS5854:1980. (1996)

It would be illegal to install any appliance in a residential dwelling without complying with
Document J or the manufacturers instructions.

In the schematic shown in fig1 chimney locations are graded on a scale of 1 to 4
4. being the optimum position.
3. being the next
2. being not very good and likely to cause problems
1. Being not capable of complying with current legislation.
Note that the fig 1 illustration is not accurate and should only be used as a general guide.
Before any instalation work is carried out, accurate compliance with Doc J of the Building Regs should
be ensured.