Definitions & Explanations

Fluing

Modern fluing generally involves concentric flues these days where the inner 'tube' carries the exhaust while the outer 'tube' provides the air for combustion and cooling. The addition of a fan, which again most modern domestic boilers have, can mean that this flue can be in excess of 15m long in many cases and can  incorporate a number of bends. In addition they can be vertical or horizontal, so siting boilers is not as restrictive as it used to be.

As boilers are now, almost without exception, 'condensing' due to legislation governing their efficiency, there is ann added complexity to the flue system in that it has to allow for the condensed fluid (or condensate) to be taken off to a drain.

Having said all this, the easiest, and therefore least expensive, place to put a boiler is on an outside wall where the flue can go straight through the wall.

Open Vented Heating Systems

Vented systems employ a small water tank (a Feed & Expansion or F & E tank) in the loft which does what it says on the tin - feeds water into the system and takes up the expansion of the water in the system as heats up. Generally speaking the tank will have a feed pipe coming out of the bottom of the tank (and into the heating system), and a pipe that loops above the tank and points back into it - an open vent. The tank will have a ballcock to ensure that it doesn't run dry.

Unvented Heating Systems

Unvented systems operate in much the same way as modern car cooling systems do - ie they're sealed. In theory, then, no water gets in or out of the system once it's set up. Initial filling is achieved via a 'filling loop' which allows mains water to flow into the system until a certain pressure (normally 1 to 1½ bar) is reached after which the filling loop is closed and disconnected.

An expansion vessel (a metal cylinder containing an air pocket) is provided to allow for the expansion of the water in the system as it heats up.