My house is about 5 years old. In my loft are two plastic tanks - a massive round one and a tiny box one. We have a big cylinder in the airing cupboard, which heats the hot water on a set timer (not on demand). We have a control for the hot water, and another for the central heating.
I'm trying to identify if one of them holds water ready to go into the cylinder to heat the central heating/water, so I can put a descaler in there.
What are the two tanks for?