Vaillant S53 Problem

[westyvip]

Just installed a Vaillant Ecotec Plus 428 on a pretty large system about 20 rads. Im happy that iv sized the boiler correctly just struggling to get over the S53 circualtion problem. Boiler is in garage and cylinder is upstairs in loft. I have fitted 2 heating zones (upstairs/downstairs) and relocated all zone valves and pump into loft at cylinder. Problem im having is that boiler wont fire on full rate and is just idling around 40/50 degrees due to too large temperature difference betweem flow and return. I have utilised existing 15/60 pump so am wondering if this may be problem and need a new pump fitting? Any other suggestions as to what i could do to get boiler firing......maybe fit a small bypass type circuit closer to boiler to trick it into thinking return temp is higher?

 

[diamondgas]

Wouldn't you think it would boost to max rate because of the temp diff not stay idling? What difference and temps you reading F&R? Any air in pump/system? If you're getting to great a temp diff then you've got circulation problems IMO.... Personally I don't like stuff in lofts especially pumps, all that extra work it has to do will be a strain on it I'd imagine .. drag water all the way to the roof just to push it all the way back down again .... Nightmare to balance I'd imagine? Ho-hum