A conventional flue gas fire uses air from the room for combustion and sends the combustion gases up a suitable chimney or flue using natural draught. Conventional flue fires can be open-fronted or glass-fronted, depending on the appliance.

A balanced flue gas fire is room-sealed and normally glass-fronted. It takes combustion air directly from outside and sends combustion gases back outside through a concentric or twin-wall flue system.

Normally, no. A balanced flue and conventional flue appliance should be treated as different appliance specifications unless the manufacturer specifically states that the model can be converted using an approved kit or procedure.

This is where the terminology gets confusing. Many manufacturers offer the same fire range in separate balanced-flue and conventional-flue versions, but that does not mean you can simply change the flue connection on one appliance. For example, Onyx offers Avanti fires in both BF and CF configurations with different flue arrangements and performance figures.

Yes, but there are two very different types of “conversion”.
Some specific or older appliances have had manufacturer-approved kits that physically convert the appliance from one flue arrangement to another. You should never assume this is possible unless the appliance manufacturer explicitly approves it.

What is far more common with modern gas fires is a chimney renovation kit. This does not turn a conventional-flue fire into a balanced-flue fire. Instead, it allows a balanced-flue appliance to use an existing masonry chimney as part of its balanced-flue system. Gazco, for example, offers chimney renovation arrangements for compatible balanced-flue models, and the current Onyx Avanti range is also compatible with a chimney renovation kit.
I would make this distinction very clear on Bonfire’s existing flue guide, because customers frequently call both things a “conversion kit”. Your current article already discusses specialist chimney kits, so this would be a worthwhile clarification.

Balanced flue gas fires are generally more efficient, particularly when comparing equivalent modern glass-fronted appliances. Because the combustion system is sealed from the room, less heated room air is lost through the flue.

The difference can be significant. The current Onyx Avanti 85, for example, is quoted at up to 91.9% for balanced flue compared with up to 77.9% for conventional flue. However, efficiency varies considerably between individual appliances, so the actual manufacturer’s figure should always be compared rather than assuming every BF fire will outperform every CF fire.

There isn’t one answer for every property.
Balanced flue is usually the better option when efficiency, installation flexibility and a room-sealed appliance are priorities. It is particularly useful in new builds, renovations and properties without an existing chimney.
Conventional flue can make more sense where there is already a suitable chimney, particularly when the customer wants a traditional fireplace arrangement or an appliance specifically designed for that chimney. Conventional flue also gives access to certain open-fronted gas fire designs that cannot operate as balanced flue appliances.

No. This is one of the biggest advantages of balanced flue.
The flue can often run horizontally through an outside wall or vertically through the building and terminate through the roof, subject to the particular appliance’s permitted flue lengths, bends and terminal positions.
This means balanced flue fires can often be installed into false chimney breasts, purpose-built fireplace structures and properties that have never had a chimney.

Yes, with certain appliances. You do not necessarily need to abandon a good chimney simply because you have chosen a balanced-flue fire.

Approved chimney renovation kits can use a liner or liners within the existing chimney to create the required room-sealed combustion-air and exhaust arrangement. The exact system varies by manufacturer and appliance. Gazco specifically offers this approach on compatible balanced-flue fires.
This is actually a very good FAQ for Bonfire because people often wrongly assume:
Existing chimney = conventional flue only.
That isn’t necessarily the case.

A conventional-flue fire requires a suitable chimney or flue system, but that does not always mean an old-fashioned brick chimney.

Depending on the appliance, suitable options can include an existing masonry chimney, an appropriately sized flue liner, a prefabricated gas flue or certain precast flues. The appliance manufacturer determines which chimney and flue types are permitted. Gazco, for example, offers conventional-flue products for several different chimney/flue configurations.

The existing chimney should therefore be assessed rather than simply assuming that because there is a chimney, any conventional-flue fire will fit it.

A balanced-flue appliance normally does not rely on room air for combustion, because the combustion chamber is sealed and air is supplied from outdoors.

A conventional-flue appliance takes combustion air from within the property. Whether additional purpose-provided ventilation is required depends on the appliance, its input, the installation and the applicable regulations/manufacturer’s instructions.

I would avoid saying online that “all conventional-flue fires require an air vent”, because that is too broad. The correct ventilation requirements need to be established for the particular installation. HSE emphasises that gas appliances must have adequate combustion air and an effective flue and that ventilation must not be obstructed.

They are much more flexible than conventional-flue fires, but not literally anywhere.
The appliance still needs an approved route to outside. The flue may terminate through an external wall or roof depending on the appliance, but there are restrictions on flue length, the number of bends and where the terminal can sit in relation to windows, doors, boundaries and other parts of the building.

This is why a balanced-flue fire is particularly useful for new builds, extensions, rooms without chimneys, false chimney breasts and contemporary built-in fireplace designs, but the flue route should be considered at the beginning of the design rather than after the fireplace has been built.