Recently had a 15/21kw Grant Vortex external oil boiler installed but it seems to be struggling to cope when both the HW and CH are on, despite it being the same power as the old boiler (which was 70000btu and coped fine). The boiler manual seems to say that it's output is factory set at 18kw, and to get the full 21kw requires a higher oil supply pressure. Would raising the tank help, or do I need a larger bore supply pipe? (The tank outlet is approx 50cm higher than the boiler burner, 10mm o/d supply pipe, approx 17m from tank to boiler.)