Hi, I just had the same problem with the lower model Baxi 80E. Changed diverter valve and then DHW would fluctuate in a cycle of hot then cold then hot again- NOT pleasant in shower! Anyway it turned out to be the DHW thermistor, It was not totally broken but not reading correctly as someone would be calling for hot water, So boiler would be heating up water then the thermister would think the water was too hot and tell the boiler to cut off, the flame goes out for a few seconds but the water was still being called for (running) the thermister then reads the waters lowering temp and tells boiler to fire up again. So cycling hot then cold then back to hot and back to cold etc. The person at the sink or shower begins to receive the effects of the few seconds loss of flame. IE Cold water.
It was a 20 minute job and £15. Job sorted with steady hot water. Hope it is as simple for you. It is worth a try for £15 pound.
Jo, February 2012
