AGA Flues and Chimneys

Why do I need a flue?

Well, nowadays not all AGAs need a flue: the electric AGA just has a small 'oven vent' pipe that runs horizontally to outside, and the latest Total Control and Dual Control AGAs can be specified as 'room vented' not requiring any pipework from the cooker to outside.

However a flue is needed for any gas, oil or solid fuel AGA to exhaust the 'products of combustion'. AGAs with flues also vent their cast-iron ovens to outside through the flue too (which is why you don't get the cooking smells from the ovens in the room).

Which AGAs need Flues?

All oil AGAs need a 'Conventional Flue'.

All gas AGAs - this can be 'Conventional', 'Powered' or 'Balanced'. (Note: Balanced flue no longer available)

All solid fuel AGAs need a 'Conventional Flue'.

Electric AGAs DON'T need a flue but most models do have a 28mm copper 'Oven Ventilation Pipe' that runs from the back of the cooker, just below worktop level, horizontally (gently falling) to the fan box mounted outside.

Types of Flue

Conventional Flue (Oil, gas and solid fuel)

Probably what most people would consider as a flue: a brick or masonry built chimney, usually from a fireplace, that then gathers to run up through the house to a chimney stack on the roof.

You might think of a fireplace opened out to form a recess for the AGA, the cooker then having the black enamelled pipe (4" / 100mm diameter) rising up in to the chimney - this would be the 'Conventional Flue'.

A conventional masonry chimney is normally lined with a stainless steel liner to help keep the flue gases warm (reducing condensation and assisting updraught) and to ensure the integrity of the flue so no products of combustion leak in to the house.

Conventional flues can also be created using prefabricated 'twin-wall flue system'. Different grades are available for different fuels.
These are less popular now (most appliance manufacturers produce models that don't have to have a chimney) but are normally sections (lengths and bends, with various brackets and 'firestops' at ceiling/floor penetrations) installed up through the house to terminate above roof ridge level.
(The height of a flue generates updraught [bends reduce it] and there are various requirements that must be followed to ensure the flue works correctly and complies with what's needed.)

Powered Flue (Gas AGA only)

As the name suggests - the flue is powered (by an electric fan) which blows the flue gases to outside.

Since the fan is stronger than the natural uplift of a conventional chimney the flue pipe is smaller (54mm diameter) and can run horizontally to outside rather than having to be vertical like the conventional flue.

As the flue can run up to 6m in length (4m for the Dual Control Gas AGA) depending on the routing and number of bends. The AGA doesn't necessarily need to be on an outside wall; if there's a suitable route then the AGA can be on an internal wall with the flue running horizontally to and through an outside wall.

Balanced Flue (Two and four oven non-AIMS Gas AGA only, discontinued in 2015.)

This is a very delicate system which can only run up to 600mm directly from the back of the AGA through the wall to outside.

The AGA must be sited on an outside wall with suitable clearances around where the flue will terminate.

The balanced flue relies on the (very delicate) 'balance' of cold air being drawn in to the burner through one duct, the air being fed in to the burner, then the heat (less dense) of the products of combustion rising through a second duct to push them to outside.

This is easily upset by any small leaks in the system, and particularly by windy conditions outside working against the flue's small 'push'.

Balanced flue gas AGAs were first made in the late 1960's but they seem to be slowly being phased out: the powered flue AGA (launched in the early 1990's) is a much better solution - it can be sited anywhere a balanced flue one could be (and more besides) and isn't as prone to blowing out when it's windy.

AGA haven't launched any new models with balanced flues: they can't be fitted with AIMS and they're not available as a three oven version.

The balanced flue AGA was discontinued in 2015 and is no longer available.

Technical bits!

For a flue to work properly (and meet the required rules and regulations) it needs to be installed correctly.

The different configurations have different requirements so we always recommend having a home survey to determine the suitability of the proposed location for the AGA before committing to or deciding on a particular model.

Below is a brief summary and guide to the technical aspects of the different types of flue:

Conventional Flue Requirements

Ideally vertical, with as few bends as possible.

Bends, if they are needed, must not take the flue more than 45 degrees from vertical.

Must be a minimum effective vertical height of 3m (gas) and 5m (oil). (Effective vertical height is not the same as the actual height - more height is needed to generate more lift to overcome the resistance created by any bends fitted in the run.)

The flue must terminate high enough to clear any turbulence created by the roof, building (or any close-by buildings or obstacles such as trees) that could affect it. Normally this means being at least 600mm above the MAIN roof ridge. (If you're in an extension that's lower than the main part of the house the flue needs to clear the MAIN house roof. This can sometimes cause 'issues' securing it against strong winds and may look terrible too. If this is the case it's normally best to have a powered flue that doesn't need a vertical flue.

For any fuel of AGA a masonry chimney needs to be lined with the correct grade of flue liner with the correct cowl fitted.

If there's no masonry chimney it is possible to install a twin-wall metal flue system instead. This then works in the same way as the masonry one with a flue liner. Nowadays though it's usually cheaper (and aesthetically better) to install a powered flue AGA - the flue running to outside horizontally rather than a quite 'industrial-looking' metal pipe up the outside of (or through) the house to terminate above the roof ridge.

Combustible materials need to be kept away from the flue pipe and special requirements apply to chimney voids where no combustible materials are allowed.

It's quite common to fit a 'flue draught stabiliser' to an oil fired AGA; this will help reduce excessive updraught in strong winds and maintain a stable draught at the correct speed through the cooker.

Powered Flue Requirements

Can run up to 6m with up to 6 bends.
(4m and 4 bends for the Dual Control with gas power flue ovens).

Combustible materials should be kept 15mm away from the pipe.

The flue terminal outside needs to be away from openings to the building and above ground level - we can check the proposed position at a home survey visit.

An electric supply (just a 13amp socket) is needed for the cooker's power supply to plug in to. (If the power goes off the fan will stop and the flue gases can't be exhausted, in this case a safety device shuts the main burner down in the cooker until power is restored and the fan is working again. You don't need to relight the cooker - the pilot light stays on through out and will re-ignite the main burner when needed.)

Note:The power flue fan is just audible (like a fridge is audible) and runs '24/7' to clear both 'products of combustion' and the cooking smells / steam from the AGA's cast-iron ovens.

Balanced Flue Requirements

The AGA needs to be on an outside wall, no thicker than 600mm.

The position for the flue terminal outside must meet the manufacturer's requirements and be the correct distances away from openings to the building / ground level etc.

The flue must terminate in a zone with very stable wind conditions (no strong winds blowing at or across it).