:( I have bought a direct cylinder and my system is indirect. My question is can I still use this cylinder with my system? I can't return it because I have damaged the insulation fitting the immersion heater.
If yours is an indirectly heated system, it means that the heat to warm the water in the cylinder comes from an 'external source' ie the boiler pumps hot water through the pipe coil inside the hot water cylinder and this in turn warms the water which you draw off to use domestically. Theoretically you can heat the cylinder using the immersion heater but this would be an extremely expensive way- it's cheaper to connect the coil and use the boiler to heat your domestic water indirectly. If this description is a bit naff, let me know and I'll try to describe it a different way.
If you have an indirect system you cannot fit a direct cylinder.
If an electric immersion is the only way the water in the tank can be heated and the tank has only two plumbing connections, this is not an indirect system and therefore a direct cylinder can be fitted.