I've been trying to reduce the flow temperature of our 30kW condensing system boiler to improve it's efficiency. The lowest I can practically operate it and have HW at 55oC, is with a flow temperature of 65oC. Any smaller difference results in extended running and short cycling.   As this chart shows for the start of today, the cylinder temperature sensor governing the HW demand fell just below the overnight minimum setback temperature of 40oC and very briefly fired up the boiler at 4: