Hi,
I planning on descaling the DHW heat exchanger on my combi. It's an Ariston Microgenus 27 and the rate of flow has gradually declined since it's been installed (over three years ago). The descaling looks like an easy enough job, and removing the heat exchanger itself is dead easy, but before I make a complete fool of myself I've never really understood exactly how the combi does the DHW. Do I need to drain the boiler completely (and all the rads) or is just turning off the cold water feed to the combi enough?