My Saunier Duval SD 30e is 10 year old it has started to fault when I run hot water such as a shower. On start of the shower water is nice and hot but two minutes or so later the water goes cold, waiting a couple of minutes without switching the shower off the water becomes nice and hot again, this may happen two or three times during the shower. likewise if I run hot water into the sink for washing up the same fault occurs.The central heating has never faulted, even in our recent very cold winter.

David R Hancock, March 2011

I suspect it is 1 of the flow switches which have rubber diaphragms inside, there are separate ones for hot water and central heating. That probably explains why heating is ok but hot water isn't. Either that or the hot water thermistor is faulty. This senses the temperature of the hot water the boiler produces and regulates the heat going to it. Either 1 of these need replacing by the sound of it. Hope this helps!

Taliskerman, March 2011

if other taps are ok suggest it's a flowrate problem try turning sink tap on more if that works clean shower head and always turn shower to max flow

brian, March 2011