Do not worry they work perfectly fine, They work just the same as any other system or vented open flued boiler, they are just hidden out of the way and use the chinmey to safley evacuate the products of combustion. but as with all boilers it is very important they are serviced every year especially back boiler types as CO poisioning is the biggest problem of unserviced open flued boilers.The fire does not heat the water as previousley suggested it is the back boiler itself that heats the hot water and the radiators. The fire is just what it is a fire simple as that and cover's up the boiler.