I have had the same problem several times, You say you have replaced the microswitch, have you looked to check if the small white plastic lever that comes out of your diverter valve is moving up and down to operate the switch if so then make sure its set in the correct position to switch the switch.
If it is have you tried replacing the dhw diaphragm? this is done by switching the boiler off, draining it and then you remove the white cover on the very bottom of the boiler and look up. You will see 6 brass screws, on the bottom of the brass diverter valve along with a nut and a screwed thread. Using a good philips screw driver undo the 6 brass screws. You will then see the diaphragm. Remove the diaphragm and check to see if its holed if so then replace it. If its ok then it sounds like its the diverter valve stuck.
Mine has stuck twice and the diaphragm has been ok each time. My experience why this sticks is because the diverter valve is like a small needle valve that is chromed and over the summer whilst the heating is not used the valve remains in one position and the chrome is damaged in a minute spot causing it to jam when the heating is switched on.
You have two options, 1 - replaced the diverter valve but this is expensive and usually not required. The other is buy a kit from Wolseley Parts Center (Parts center No. 475669, Vaillant number 14 0352) this kit contains everything you need to overhaul your diverter valve and the job is "doable" if you have any mechanical confidence. You will need to strip the valve down and replace all the component parts and grease the o-rings with the silicone grease provided and rebuild and replace it. To remove the diverter valve though you will first have to remove the pump.
One thing to remember though depressuirze, drain and isolate your system electrically first as this will prevent any unwanted water damaging any sensitive electrical components.
Hope this helps Steve
Steve
October 2007
howtomendit.com for free repair help, information and advice.
