you can download the manual from the vokera website. If the problem is running hot-cold-hot (with the burners cutting out completely during the cold bit, but the fan staying on full) then suspect dirty flue venturi causing differential air pressure switch drop out (you can observe this with an appropriate meter across a pair of connections on the controller box). If the temperature is OK but the flow limited it could just be the time of year - the incoming water needs more warming than in the summer, or it could be that there is a blockage somewhere in the path of the water on its way to the tap. The blockage may be in the small white valve behind the hot water diaphragm, bits of rubber may have spalled off the degrading diaphragm and be blocking the valve behind.
If you only get hot water when the central heating is on you will get little hot water as the central heating burn rate is lower, suspect a holed DHW diaphragm or broken DHW microswitch if this is the case (hint, turn to summer only mode and see if hot water stops completely)
While it is ok to have a go at the plumbing bits, opening the sealed box is meant to be a job for a corgi man. When your corgi man does it he is meant to measure flue and burner pressures, as well as hoover the dead wasps from the bottom of the combustion chamber.
c
January 2010
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.