Vaillant 828 Eco Max. Boiler. When running hot water for a bath or to fill the sink the water goes from hot to cool and eventually back to hot. Why ?

Keith, May 2015

Primary heat exchanger need acid cleaning ,drain system and close main water in let ,take out heat exchanger when you open cover it's right in front undo two screws to take out ,make sure four O ring in place .....change NTC hot water if after cleaning still problem

ltd, May 2015