Gas Central Heating

Gas central heating boiler, also known as Heat Generator can be compared to an engine of a car as both provide heat that the object requires to warm itself.
The size of the boiler must be in proportions with the size of the property. If oversized, it will lead to excessive fuel bills. On the contrary if undersized, it will fail to generate enough heat. Boiler must be able to generate enough heat on coldest day of the year; one can have a rough estimate about the ideal size of a boiler from this parameter. Most of the boilers are oversized to 30%, the reason being the card calculator used to have a judgment of size. Emphasis being on energy conversation now days, there is no need of oversize as calculations can be performed on computer. It leads to smaller radiators and thus, less water in the system can be filled which reduces the size of boiler and finally cuts the installation and fuel bills.

Boiler is not only the determinable factor governing the amount of radiators which can be fitted in the system. Power of the pump being used along with adequately sized pipes through which circulation can take place also have their say in calculating the amount of radiators in boilers. Thus, total output of all radiators, systems and pipes determines the final size for the boiler.

Flueing & Ventilation Sufficient amount of air is a requisite for gas-burning appliances and to get rid of gas which gets burnt via a flue. Modern gas central heating are known as room sealed (Balance Flue) as air required for combustion of gas is taken from surroundings through a duct and second duct flushes out burnt gas present at outside back (generally air duct holds the flue duct). Open Flue boilers are also available as alternatives. Open flue boilers are generally not recommended as they take air required for combustion form an air grills which in fitted in room where boiler is located. Fumes of burnt gas then go upwards through flue which ends over roof. If due to any reasons either flue gets damaged or the ventilation grill chokes, or any fault gets developed in boiler, poisonous fumes can enter the house. Thus the use of open flue gas central heating is generally avoided while fitting or replacing central heating boiler, though new open flue boilers carry extra safety devices but being on a safer side is always better. 
These days, all boilers are either room sealed or fanned flue, which are the safest and best option. Some boilers employ fan along with open flue and as from 1st April 2005, all new boilers which get installed should be efficient over 90% unless exceptional circumstances  are present.
Extra air is required by most of gas central heating boilers to keep them cool; especially if they get installed in a small size compartment though this is not a requisite in modern boilers. Heat input of boiler, source of air and type of flue determines the position and size of ventilation.
You must consult a Corgi registered installer if you have any doubt regarding the flue and ventilation as lot of regulation and safety measures are employed for the flue and ventilation. Also, you must check your appliances for safety once a year and must get the faults get repaired, if found.