Worcester Bosch R29 HE Boiler not working?

Hi all,

We have Worcester R29 HE condensing boiler - which until last night was working fine.

It seems to fire up (the red ign light comes on) - then there is a banging sound from within the boiler followed by hissing from the top accompanied with some steam or vapour. Then nothing? The boiler feels warm to touch though none of my radiators do? The display shows '99', but this counts down to around 70 when the boiler attempts to fire up again? The dials were set by the installer in 2004 and we have not touched them. The boiler was serviced late 2008 or early last year.

I have tried to re-set it but all I get is the code (FD) telling me I've hit the re-set button by mistake. I am sometimes getting a display that looks like two lower case letter 't's back to back? But I am not getting any other error codes?

I have checked and am satisfied that the condensate pipe is not frozen.

I've switched it all off now as I am concerned that the constant cycle of banging and hissing is not healthy?

Any advice would be greatly appreciated.

Many thanks

Derek