Remember, though, that in a zoned system, the boiler will always be oversized. My heat loss upstairs may only be 18KW, but I still run my boiler at 34KW and it still maintains a delta T of 20 (ok, 21 to be exact). The problem is the 400 series Vaillant seems very sensitive to flow-return temps, especially on start up, because it doesn't modulate down quick enough. The new PCB may have helped with this, but what I've learned from my struggles is that minimum flow rate is key - and the only way to achieve this is to have an adequately sized pump to deliver this flow rate at the hex resistance inside the boiler and an appropriately set ABV to allow the water to circulate around the boiler fast enough.
OR - an LLH, which would achieve similar, and allow a smaller pump on the primary circuit (with an additional pump on the secondary side).
