I have a 2.5 yr old vaillant 37kw system boiler which has been great. Now there's a prob on the heating side. When the burner kicks in it heats up to about 50C then shuts off again with the pumps spinning. After about 1 hr of doing this on and off, it eventually heats up to the preset temp, currently 70C and all seems well. It passes through a drayton zone valve which appears to open/close and is not noisy. There are NO probs on the HWT tank side, boiler heating up to the preset temp straight away and continuing. Vaillant came and said boiler fine and was most likely the zone valve restrictin return to boiler so there is too much difference in heat between flow and return.. help... why does the flow return to normal after an hour or so?