Hi All. For those who read my LLH thread, this question is because I'm weighing up whether my problems are just down to it being a Vaillant boiler. Quick background - I have 3 heating zones (u/s, d/s and hw). Any individual zone works fine. Multiple zones work fine when running together. The problems occur when the boiler is firing and individual zones open and close, because the return temp to the boiler suddenly cools, and it hits S53. If you imagine a typical heating system, zones open and close as the stat calls for heat, so depending on timings, sometimes the boiler can sit in S53 for hours, and the heating runs only luke warm. Other times the rads are at full temp. My question is this - is this a Vaillant problem? (specifically the 438). Or do all boilers have this issue? I just cannot believe in the modern day with people having multiple underfloor heating zones etc that this problem does not occur more often and for more people. Unless everyone fits a Low Loss Header when running multiple zones? Thanks for reading and appreciate any input.