Low Water Content Boilers
This type of boiler only holds a only a small amount of water in it's copper heat exchanger, (sometimes made of a light cast iron) making it very economical to run and small in physical size. These must be fitted to a fully pumped system. Low water content boilers have been around for about 25 to 35years. They are tried and tested and work very well. They are economical to run and can be fitted to a sealed system, meaning these boilers will fit almost anywhere (even in a kitchen cupboard). Most modern boilers are fan flue with electronic ignition, so there is no pilot to re-light. but unfortunately these are no longer available in the UK
Advantages
Inexpensive to buy
Economical to run
Very few internal components
Very small so can fit inside a cupboard
Reliable
Disadvantages
Will not work on gravity Hot Water systems
do not give instant hot water except if used in conjunction with secondary circulation
no longer available