Technical Terms - a Glossary

It's easy for us to talk about all the little bits that go together to make up your AGA or Everhot and the technical stuff associated with them, and it's easy for us to forget that although it's our speciality, not everyone knows what we're talking about so here's a list of the technical terms and what we mean by them. (Don't forget to let us know if there's something we've mentioned but not explained so we can add to the list, thank you.)

Flues and Chimneys

Open Flue

Designates that the flue (and appliance's burner) is 'open' to the room where the appliance is situated.

The air for combustion is drawn from the room, as is the air that replaces that which is drawn through the flue to outside. (As opposed to a 'room sealed' flue that has a pipe to bring this 'combustion air' in directly from outside.

Open flues are normally not 'powered', although AGA's Power Flue models have a fan to push the flue gases horizontally to outside but also draw their combustion air from the room so are 'open flue' appliances.

Open flues normally need a 'combustion air inlet' (air vent) to provide a continuous supply of fresh air for the burner and flue to work properly and safely. They are adversely affected by the installation of extract fans in the same room. (The extractor draws on the room's air supply and sucks it to outside - if there's not enough ventilation then it will pull the air (and flue gases) down the flue with potentially very dangerous consequences.

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Balanced Flue

A balanced flue (normally horizontal but some vertical ones for other appliances are available) can also be referred to as 'room sealed'.

There are normally two pipes between the appliance and outside air - one for the flue gases to go out and the other to let fresh air in. Often these are combined in a concentric pipe (one inside the other). There's normally a 'terminal' and terminal guard on the wall outside.

The term 'balanced' referrs to the fact that the amount of air coming in is equal to (or 'balanced') by the products of combustion going out.

Some are 'powered' and use a fan to circulate the air/flue gases (most modern central heating boilers are like this) and others (like the AGA) aren't; they use the very limited natural push/pull effect and as a result can only have a very short flue (normally just straight through the wall behind and no more).

There are rules set by the manufacturers (and Gas Safety Regulations) regarding the positioning of terminals in relation to openings to buildings, other flue outlets and other items that may affect, or be affected by the flue; it's always important to refer to the manufacturer's installation instructions when positioning a balanced flue appliance.

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Conventional Flue

This is what we'd all recognise as a chimney. There is a vertical pipe off the top of the appliance that then rises up through the property to terminate above the highest point.

'Lift' (also known as 'draw' or 'pull') is created by two things - very slightly higher air pressure at ground level in comparison to the top of the chimney causes movement of air upwards, and the warmer (less dense) 'products of combustion' rise up through the cooler, more dense air.

A taller flue will always have more natural lift (without the hot air effect) as there is a greater difference in air pressure between the top and bottom.

Conventional flues work best when they're vertical. Bends and angled sections add resistance so slow the flow of the products of combustion to outside.

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Power Flue

A flue that could be 'open' or 'room sealed' but that has a fan to push (or sometimes pull) the flue gases to outside rather than relying on the natural lift of a chimney.

For a more detailed explanation of this and other flue configurations see Rayburn's Flue and Chimney Guide.

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More Flue-related Terms

Air Vent (Combustion Air Inlet)

A vent through the wall to allow fresh air in from outside. Appliances with 'open flues' need a supply of air to replace that used by the burner to burn the fuel and to replace the air that is taken up the flue.

These are needed not just by design but also under the gas/oil regulations too.

They are measured by their 'free area' and more is needed the higher powered the burner in the appliance is.

Note - an extractor fan IS NOT an air vent; it is infact the complete opposite! An extractor takes air out of the room, an air vent lets air in. If you have an extractor then you'll need a bigger air vent to compensate for it.

See also the air vent installation guide page.
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Effective Flue Height

The height of a chimney if it were to be vertical and straight with no bends or 'fittings'.

Any part of a chimney that isn't vertical (bends and sections running at an angle) adds resistance to the flow of the flue gases up it, as do any fittings, adaptors, connections and cowls.

A calculation is made which accounts for the resistance of these additional fittings / angled sections to give the 'effective flue height'.

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Products of Combustion

These are the flue gases produced by the burner.

They're different for oil, gas or solid fuel appliances but all need to be safely and completely exhausted to outside through the appliance's flue.

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Soffit Board

The board fitted to close the underside of the chimney in a recess/fireplace opening.

This MUST be a non-combustible material (we recommend 10mm thick Glasroc Multiboard as it's relatively easy to trim to shape and form a hole for the flue pipe to go through). Some prefer a metal plate, which whilst perfectly acceptable is difficult to cut and fix. (Normally a template would need to be made.)

The Soffit (and anything above it in what's technically the flue void) must be non-combustible. This means using metal (NOT WOOD) battens to fix it (or screw to the masonry if you can) and you can't fix lights in the soffit or run cables above it in the flue void; if you want lights above the AGA then it will be necessary to fit the soffit board at a higher level to create space to hide the light fittings behind the lintol / beam. Alternatively fit the 'real' soffit at high level to close off the chimney, then a second one at a lower level (with the required clearance to the flue pipe) that can accept the light fittings.

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Consumer Unit

Also known as the 'Fuse box'.

Older systems did have fuses but most nowadays have either MCBs, RCDs, RCBOs or a combination of the aforementioned circuit protective or isolating devices.

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MCB (Miniature Circuit Breaker)

Essentially very similar to what most people would recognise as a fuse.

It will disconnect the power if it detects that the current flowing in the circuit is too great.

MCBs normally have a resettable 'switch' which makes them much easier to put back on when the fault has been rectified. They're also usually more reliable than a fuse.

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RCD (Residual Current Device)

Often known as a 'trip'.

It senses the amount of electricity flowing in and out of the circuit (this needs to be equal). If there's any 'leakage to earth' it means there is a fault and it will disconnect the power.

'Earth leakage' usually means there's a fault, which could be an exposed wire or connection that then allows electricty to find its way back to 'earth' via a route other than the return wire it should be going down (if someone touched the bare wire for example, the path could be through them). Since this is never a good thing the RCD is sensitive enough to cut the power before a dangerous level is reached.

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RCBO (Residual Current Breaker with Overcurrent)

Works as both an 'MCB' to detect and disconnect the electric supply in the event of an 'overcurrent' (or overload) fault, and as an 'RCD' in the event of an imbalance in the electric circuit where it will also cut off the power.

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Switched, Fused Spur Point

A switch to carry current generally on a par with the 'socket circuit' (more than lights, less than a cooker).

Used where you need to provide isolation for a single outlet / appliance but don't want to see a plug/socket or visible cable. There's usually a small holder for a fuse (the same size fuse as fits in a plug-top) built in to the outlet.

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Cooker Outlet / Connection Point

The cooker switch (or isolator) may be on the wall above the worktop however it would be unsightly to run a cable on the surface up to the switch.

A cooker outlet / connection point provides an intermediary point to connect the cooker. A cable is buried in the plaster from the switch to the connection point, the connection then gives proper insulated terminals to connect the cooker to the supply.

These are often fitted behind a standard electric cooker but this shouldn't be done for an AGA - the AGA can't be moved so the connection point should be to its side.

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Cooker Switch

A heavy-duty switch (normally rated at 45-50 amperes) that provides a point to isolate the electricty to the cooker.

Some are just a switch (usually red in colour) and some also incorporate a 13amp socket outlet as part of the switch/outlet.

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Gas and Oil Burners

Fire Valve

Usually fitted to oil supplies.

Older type ones had a wire strung around the (usually oil) boiler with a 'fusible link' above the most likely part to cause a problem (the burner) this had two parts stuck together with a glue that melted when the temperature started to get above safe levels. This, through a spring loaded pulley and wire system caused the fire valve to snap shut, cutting off the oil supply before a fire could take hold (think of a Wallace and Gromit type contraption and you're not far off!)

Some are simply a wheel-headed valve (looking like a tap) that has a spring loaded 'fusible' head which would snap shut when the 'glue' material melted. These are limited though by the fact that they only sense the temperature (and shut the oil supply off) where they are.

Modern equivalents do away with the wire / pulley / fusible link / fusible head and have a remote sensor (mounted over the burner or 'most likely' source of any problems) that connects with a capillary tube to the valve (which is usually sited outside the building) to shut the fuel supply off in the event of a problem.

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Thermocouple 1 - Gas

A thermocouple fitted to a gas burner has a tip that sits in the 'pilot' flame.

It's constructed of two dissimilar metals that when heated generate a tiny electric current.

The current is just strong enough though to hold open a spring loaded 'flame failure valve' which then allows the gas in to the main burner of the gas appliance.

The idea being that gas can only flow to the burner when the pilot flame is lit; if it goes out, the thermocouple cools, the tiny current stops and the flame failure valve snaps shut, stopping the gas from flowing and making sure you don't pass neat, unburnt gas in to the appliance or room.

These thermocouples often need to be replaced - the tip sits in the lit pilot flame all the time which burns it away.

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Thermocouple 2 - Electronic

Working in a similar way to the 'gas' burner thermocouple however this is one that connects to an electronic device which reads the voltage and interprets it to let the electronics know the temperature at the tip of the sensor.

Flame Failure Valve (FFV)

The valve that the thermocouple connects to.

It has a spring loaded gas valve that's held open by the tiny current generated by the thermocouple and will snap shut and cut the gas off if the thermocouple is disconnected or cools down to the point where the current is too low.

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Mostly fitted to gas appliances to regulate the gas pressure down to a set level.

Used if the incoming pressure is higher than needed for the appliance's burner.

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Building Terms


A popular modern method of plastering internal walls.

Instead of 'wet plastering' (first applying a thick 'scratch coat', allowing it to dry, then a thin finish 'skim' coat and waiting for that to dry) plasterboards are 'dabbed' with special cement-like adhesive to the walls (known as dot and dab giving a smooth flat and dry base for the skim coat to be applied to.

It's much quicker to do, quicker to dry and adds a little bit more insulation to the walls, in fact some plasterboard is supplied with an insulation backing to further increase the insulation in the wall.

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Dot and Dab

See also 'Dry-Lining'.

A method of fastening plasterboards or Glasroc Multiboard to a masonry wall using blobs of special cement-like adhesive to stick the boards.

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Glasroc Multiboard

A sheet material that's similar to plasterboard but that has no paper backing and is completely non-combustible. It can be tiled on to or plastered with a 'skim' coat, or left plain white.

The trade name is 'Glasroc' however it is 'Glass Reinforced Gypsum' (board). Available in 6mm, 10mm, 16mm and 20mm thicknesses, usually a 2400mm x 1200mm sheet. Most builders' merchants keep it.

It's safe to use for high-temperature locations such as lining recesses where a solid fuel / woodburning stove will be fitted, as a board behind an AGA where the manufacturers ask for a 'non-combustible' wall (it can be plastered with 'skim' and be dot and dabbed to the wall).

It can also used as a 'soffit board' to close the bottom of a chimney / recess/ fireplace where an AGA will be installed (plasterboard is not suitable due to its (combustible) paper backing and wood can't be used either as it is also cobmustible.

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Sleeper Wall

A supporting wall in the underfloor void of a 'suspended floor'. Installed to add strength and ensure the floor joists don't have to span too great a distance.

An extra sleeper wall built under where an AGA or Everhot will be installed is often used as a floor support to ensure it can carry the weight of the cooker.

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Suspended Floor

A 'traditional' floor construction comprising of floor joists set in to the wall (and sometimes supported part-way across the span with an additional 'sleeper wall') with floorboards nailed or screwed to them.

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Floating Floor

A floor consisting of a solid concrete 'sub-floor' (usually cast at the early stage of building the house), to which insulation of 50-100mm thickess is laid on, then interlocking sheets (normally 8' x 2') are laid 'floating' on the insulation with the floor finish (laminate, carpet) on top of the sheets.

Floating floors are very insulating but because they can allow some movement they're not usually tiled and have a low load-bearing capacity. It's not normally possible to install a heavy cooker like an AGA on a floating floor; the best advice is to cut out the area under the cooker and make it solid, or use our specially adapted metal base (for AGAs only, not Everhots).

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Worktop Height (From Finished Floor)

The height of the worktop measured from the top of the finished floor. (This is the top of the tiles / laminate flooring or whatever floor finish you have, not the concrete sub-floor or floating floor's boards!)

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Finished Floor Level

The top of the finished surface of the floor. This is the top of the tiles / flags / wood boards / laminate flooring that you'll walk round on when the project is complete.

If you still have the concrete floor with no tiles (or 'finish') when the cooker is installed then you'll need to ADD THE THICKNESS OF THE FINISH to this to get your finished floor height.

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Worktop Upstand

An upstand at the back of the worktop set vertically. Normally about 100mm-150mm (4"-6") high and usually about 20mm (3/4") thick. Most commonly fitted in conjuction with granite / quartz / slate worktops - the upstand is often the same material but not quite as thick as the worktop and replaces the first couple of row or two of wall tiles.

If the upstand is to run behind the cooker then it will 'push' it forward (the upstand MUST NOT rest on top of the cooker) - we'll need to know its thickness so we can set the cooker in the correct position when it's installed.

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Similar to the Upstand but usually the 'backsplash' refers to the section behind the cooker. It's often higher too (to catch more splashes).

Backsplashes could be a tiled area or made from granite (or similar worktop type material), or be glass or stainless steel.

As with the worktop upstand we need to know its thickness so we can allow for it when installing the cooker.

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More Installation Questions Answered - see the links below