Do you have a twin channel programmer fitted?
If so check programmer is set to "off" as boiler should only fire for hot water in an "on" period.
If not, check programmer connections/wiring (this may not be something you can do yourself). Possible also a stuck relay on pcb.
If you have only a single channel programmer then the boiler can only work off the tank stat and will fire every time water is used.
Bunslinger
November 2008