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In order to function correctly (and conform to safety regulations) fires and wall heaters need to be fitted to an appropriate flue. There are also extensive requirements for the installation and ventilation of fires and wall heaters.

Types of flue

Burning gas in air creates a number of by-products - the products of combustion. Where gas is burnt in any quantity (gas fires, water heaters and so on - but not cookers) these by-products will accumulate quite quickly - which may be harmful. They must, therefore, be removed. A flue takes the products of combustion out of the room and dissipates them in the outside air.

You must be able to distinguish between the following types of flue:

Open flue
Fan-assisted flue
Balanced flue

Open flue

An open flue is simply a flue which is open to the room it serves. The products of combustion rise up it by convection and by natural draughts drawing them up a chimney to the outside air.

Customers may already have or require one of the following types of open flue:

Brick Chimney Conventional brick-built chimney - combustion products rise through convection and are also drawn up by wind passing the top of the chimney, creating an updraught. These are built in to the original design of the house. - Class 1 Flue

Pre-Cast Flue Pre-cast concrete flue - common in newer homes, these are built in to the original walls - without increasing their thickness. - Class 2 Flue

Prefabricated flue - usually a later addition, this is a twin-skinned flue (made from stainless steel) which is attached to the inside of a wall and led away either through the roof or through an outside wall. These are sometimes boarded over to imitate a chimney breast.- Class 2 Flue if less than 152 mm

Open flues are always found on an outside wall - giving the shortest and safest route to disperse the products of combustion.

Fan-assisted flues

These allow fires or heaters to be sited on interior walls - although the flue terminal itself must be on an outside wall. Essentially, the flue is led from the product to a flue box which is open to the outside air.

The products of combustion are dispersed in one of two ways:

a fan in the flue box draws them along the flue from the fire or heater
a fan in the fire or heater pushes them along the flue until they emerge into the outside air.

Balanced flues

If no chimney or other open flue is available, fires or heaters can still be mounted, on an outside wall (only), using a balanced flue. This very effective flue actually seals the product off from the room it is being used in - the air for ventilation comes from outside and the products of combustion cannot enter the room. For this reason, they are called room-sealed products.

A balanced flue has two channels - one to draw in air to burn with the gas, the other to get rid of the products of combustion.

Ventilation

As you can see, a room-sealed product needs no other form of ventilation. The other flues need a supply of air within the room to burn the gas in - a ventilating air grill. If the appliance input is greater than 7kW check with manufacturer's instructions.

This is by no means a disadvantage. The action of convecting products of combustion up an open flue 'pulls' a current of cooler air into the fire. This provides a more natural and comfortable warmth - not stuffy or stifling. It also helps reduce condensation.

Installation requirements

It is vitally important that customers are aware of what they need in order to install particular products:

they must know what particular products demand by way of space and flues
they must be able to describe what they already have and decide whether it is suitable for the product they are looking at.

Flues

All gas fires require a suitable flue.

Brick or masonry chimney flues:

must be suitably constructed and in good repair
must be swept clean if previously used for solid fuels
must be inspected prior to installation
must have a cross-section (A) sufficient to contain a circle of not less than 175mm - usually 225mm x 225mm (or 9" x 9")
must have an adequate space (B) behind the fire - known as the catchment area.

Flues

These figures are for general guidance - always check the manufacturer's instructions for actual requirements.

If a fireplace has previously had a coal fire in it, it is almost certain that it will be suitable for a gas fire. The chimney will require to be swept.

Fanned and balanced flues are not subject to these constraints as neither allows the products of combustion to enter a room. They are, however subject to rules on where the flue end (the terminal) can be sited.

Note: - read manufacturer's instructions for requirements.

Terminal guards

Where balanced or fan-assisted flues come out of the wall less than two metres above ground level (or balcony level in flats) you must fit a wire terminal guard to prevent people or animals coming into contact with hot products of combustion.

Fireplace openings

The different types of fire require different sized openings to the flue:

Note: all dimensions are for guidance only - for any product check in the manufacturer's trade literature for actual figures.

Wall-mounted fires

Wall Mounted Fires

Hearth-mounted fires

Hearth Mounted Fires

Inset fires

Inset Fires

Hearth-mounted decorative fires

Hearth Mounted Decorative
Fires

Hole-in-the-wall mounted fires

Hole in the wall mounted fires

The chairbrick

The Chairbrick This is the chair-shaped brick arrangement at the back of the fireplace (the catchment area) which sticks into the fireplace and helps create an updraught in a coal fire. This may have to be removed if there is not a large enough gap to the back of the fireplace with the gas fire in place.

Hearths

Hearths Floor-standing gas fires cannot go onto carpet or floorboards, they must have a hearth. The hearth must:

be made of solid, non-combustible material - at least 12.5mm thick and 50mm above floor level
have a surface that must be relatively smooth and level
extend at least 150 mm beyond each edge of the naked flame or radiant
extend at least 300 mm from the nearest burner - effectively the front of the fire.

Ventilation requirements

Fires or heaters which take their air for burning gas directly from the room (with open or fan-assisted flues - not balanced flues) must have adequate ventilation.

For most fires, normal room ventilation is adequate. Fires with high input/low heat output (normally decorative gas fires), however, do require a permanent ventilation opening of 100cm2. Check with manufacturer's instructions.

Electricity

Electricity Some products require an electric point near to the chosen fixing point (for various effects or for ignition purposes).

Concealing pipework

The gas pipework to the fire does not have to run along a skirting or a wall - it can be concealed in a number of ways:

it can feed through the side of a chimney breast
t can be chased into the brickwork leading to the fire and then covered
it can be installed behind a surround or plinth.

Efficiency and running costs

Gas fires and (in particular) wall heaters are very cheap ways to provide heat in the home. If you compare the cost per kWh between gas and electricity and solid fuel you will see that gas is significantly cheaper.

Product efficiency

Efficiency ratings are a selling feature in that fires and wall heaters should have a Energy Efficiency Information label attached to them to give the customer an idea of likely costs.

More efficient products make better use of the energy in the gas and are, therefore, more economical and more environmentally friendly than other products.

These figures are based on the efficiency rating of the fire or heater. The most important information for the customer is the heat output - the amount of heat actually being pushed out into the room. The amount of energy going into the fire (the heat input) is the amount of energy in the actual gas.

% efficiency = Heat output (in kW)
Heat input (in kW) x 100

A fire, say, with a heat input of 5 kW and an output of 3 kW would therefore have an efficiency of 3/5 x 100 = 60%.

Fires and heaters are described by their heat output rating - a 4kW Fire, for example, has a maximum heat output of 4 kW (making it a fairly typical fire). The actual heat felt in the room (the comfort level) will vary, however, depending on:

the size of the room
levels of insulation
amount of ventilation
other heat sources in the room

Running costs

You can calculate running costs by multiplying the heat input rating by the cost per kWh.