i have a combie 82 boiler,no hot water running only when boiler is on?

i have a combie 82 boiler,there is no hot water coming from the system only when boiler is on or centeral heating is on,can any one help me to fix this problem,as i m having difficulties to have a bath or having to wash dishes in cold water,i think they maybe something wrong with the presure guage.oh ps. even when the central heating is on,the hot water is not continuos.