Worcester 24i junior boiler about 5 years old making banging noise and losing pressure thro overflow can you help.

I'm assuming your talking about the water pressure in the system?

when i open the tap for hotwater i can see only the flashing light on and not hotwater at all the guy changed the fan but after changing its no use waste of money i still have this problem i dont want to spend more on it can some one let me knolw the problem so that i can fix ir my self .

When water is heated it expands. To deal with this all boilers are fitted with a small expansion vessel which is like a reservoir which holds the excess water.

This expansion vessel is like a balloon in a metal pot. When the water goes in the metal pot the baloon squashes a bit to make room for the extra volume of water. When it cools the balloon pushes the water back into the heating system.

What has happened is your expansion vessel no longer works. When the water heats up there is nowhere for it to go so the pressure jumps up to 4 bar. The Worcester boiler lets high pressure water escape (most boiler allow this to go through the safety valve at say 3.5 bar).

When the water cools the pressure drops very low as the volume of water contracts.

The problem is your expansion vessel - normally a little red painted metal cylinder inside the boiler.

http://www.plumbnation.co.uk/site/worcester-greenstar-junior-24i-combi-boiler-natural-gas/

Annie, I don't have experience in this boiler , but I found a link that has some information and numbers to get help. Thought I would pass it on and hope it will be helpful.

look at the water pressure gauge on the boiler it might need topping up ?

Pressure is ok? at 1 bar or above
I think on this one there should be a reset button [red] on the left hand top of the casing - or lower the control panel [ phillips screw on each side] and the button is on the left of the base plate. Press this and try the heating.or hot tap
P.S. if you're on a prepayment meter check you have gas.

Manual here
http://www.worcester-bosch.co.uk/homeowner/literature/discontinued-boiler-literature/discontinued-gas-boiler-literature/24i-junior-rsf-discontinued-aug-2007-literature

All these machines need 0,8 bar minimum pressure for one.
When significant drop of pressure call qualified repair man.

If this is not your regular work you are at mortal risk.

http://www.worcesterbosch.com/Go there see if you can get some help.

replace pressure release valve