That's all down to the coil and how efficient (Kw rating) that is at transferring the heat from the system water into the water column. Slowing down the flow can allow a less efficient/lower output coil longer to transfer that heat therefore the return will be cooler for longer but the boiler can reach its high limit faster but it should be able to modulate down rather than cycle as it does when the return reaches max. The other alternative is have a cylinder that has an ultra fast heat up time, that is typically achieved by a large coil with a much higher output rating that would be able to handle much more energy (KW) that the boiler can output.
Do you know the rating of the current cylinder coil?
Do you know the rating of the current cylinder coil?
