a friend of mine has a thirty year old central heating system which recently had a bad leak from the hot water tank.he called a heating company in and decided the tank would need replacing.they fitted a new tank and all was well for the first day but the next day no hot water.they came back and checked for blockages and flushed everything through and everything fine but still no hot water.in conclusion the engineer said the new tank must be incompatible with an old gravity system and to use the immersion heater and possibly in future fit a pump.my friend has no idea about these things and i'm not familiar with these old systems i said i would ask the experts for some advice.could their be an issue with fitting a modern tank onto and old system or something else.the system does not have a pump or motorised valve and prior to the tank leaking was working perfectly ok.cheers in advance.