The chimney can prevent the whole heating system from operating safely and efficiently if not sized properly. Chimney’s position and height should ensure removing dangerous gases and smoke from a fireplace or other appliance. So, how tall should a chimney be ?

As a general rule, chimney’s height depends on its distance from the roof ridge:

1. If the distance between the chimney and the roof ridge is less than 5 feet (1.5 meters), a chimney’s height above the roof ridge should be at least as much as the horizontal distance from the roof ridge is.
2. If the distance between the chimney and the roof ridge is 5 to 10 feet (1.5 – 3 meters), a chimney should be at least 3 feet tall (1 meter), but at the same time, the top of the chimney should be above the roof’s highest point.
3. If the distance between the chimney and the roof ridge is greater than 10 feet, the top of the chimney should reach an imaginary line that starts from the top of the roof and is tilted 10 degrees relative to the horizontal plane.

On top of this, you should take into account that the minimum overall chimney height (including connector pipe on the appliance) should surpass 15ft. Also, you should add 3% of the chimney height for every 1000ft elevation above MSL.

Also, ensure that the effective height of any chimney with bends (vertical distance between appliance and terminal) is at least twice the horizontal distance between the appliance and terminal.

Please note that for geolocations with strong winds, high surrounding buildings and/or trees, stacked houses, separate calculations should be done, based on all the factors on the location.

What if the chimney is lower than it should be?

If these recommendations are not met, counterflow air caused by the slope of the roof plane will prevent the proper flow of smoke, hence reduce the efficient operation of the entire heating system.

And, what if the chimney is too high?

If the chimney is too high, there is a danger of excessive cooling of the smoke at the exit of the chimney which reduces the flow, and also, the chimney is more difficult to access for inspections and cleaning. The higher chimney typically translating to more draft, which in some cases can push outside gases and smoke back into the house.

10ft – 2ft – 3ft chimney height rule

Widely known 10 foot, 2 foot, 3 foot rule can be used to determine chimney’s height, although it is not as precise as the recommendations we mentioned above. This rule basically says that a chimney should be at least 3 feet higher than the point it passes through a roof, but at the same time, it should exceed any object (a roof ridge, a part of the building, etc) within 10 ft radius by 2ft.

What is the correct chimney diameter?

Similar to the chimney’s height, the wrong chimney diameter can prevent the heating appliances from working properly. The appliance manufacturer’s chimney sizing recommendations should always be followed.

But, as a general rule, it is important to match the internal diameter of the flue with the outlet on the appliance. It should never be less than the outlet diameter of the appliance. 

For free-standing stoves, boilers, and cookers up to a maximum of 20kW, that are not in a fireplace recess, the minimum flue size is 5in (125mm) round or square, if the appliance has passed the requirements for smokeless zones, and 6in (150mm) if this is not the case.

For open fires with a standard fire opening up to 20in wide by 22in high (500mm x 550mm) the minimum required flue diameter is 8in (200mm) round or 7in (175mm) square.

For larger open fires, such as inglenooks, dog grate installations, or special appliances and stoves designed to operate with a fire opening greater than 20in x 22in (500mm x 550mm), the flue size should be at least 15% of the free unobstructed area of the fire opening (including sides if open).

Many Decorative Fuel Effect gas fires (DFE’s) that imitate coal or log burning open fire require the same chimney arrangement as for solid fuel open fires.

Avoid pipe bends

A straight vertical chimney performs better, so it is recommended that bends in the chimney should be avoided. If bends are required, there must be no more than four in the length of the chimney. The angle of the bend should not be greater than 45° from the vertical, with the exception that 90° factory-made bends or tees, which may be treated as being equal to two 45° bends . It is important that there is easy access for cleaning at any change of direction in the flue.

Also, ensure that the flue pipe connection from the appliance rises vertically for at least 600 mm before any change of direction is contemplated. The reason for this is that the flue draught is crucial nearer the appliance because of the higher flue gas temperature. Any horizontal or angled runs at the bottom of the flue will create severe restrictions to flue gas movement and affect appliance operation.

In order to maintain high heating system efficiency, and avoid excessive cooling and risk of condensation, you should try to position the chimney inside the building. It is proven that the chimney inside the building performs better than one outside the house.