domestic hot water usually comes from one of two sources :- instantaneous such as combi boilers and multipoints OR stored hot water from an immersion cylinder.
combi's and multipoints work at mains pressure.
CONVENTIONAL stored water cylinders work from a storage tank situated at a higher position and are therefore LOW pressure because they are working only under gravity of a few metres difference - the exception are the newer, mains pressure store hot water cylinders and there is no way you should work on these unless you really know what you're doing.
i can only assume that you've got a cylinder and tank arrangement, which means you could be air locked - has the water been drained down recently?
what i would try is to pull out your washing machine and use one hose to connect the cold to the hot - on the pipe work not the machine - turn on the bath tap and then blast cold water through the hose for no more than 3 seconds. hopefully the bath tap will continue to run.
andy the plumber, June 2008
