Which of the following is NOT a main component of a hot water cylinder?

The heat exchanger is typically not considered a main component of a standard domestic hot water cylinder. In many systems, particularly those that use a direct method for heating water, the main components include the thermostat, which regulates the water temperature; the expansion relief valve, which ensures safety by managing excessive pressure; and the immersion heater, which directly heats the water if there is no other heat source available.

A heat exchanger, on the other hand, is usually found in more complex systems, such as those that incorporate central heating, where it transfers heat between two fluids. In these cases, the hot water cylinder can be indirectly heated by a boiler or another heat source, and a heat exchanger facilitates this process. Therefore, while heat exchangers can be part of certain setups involving hot water systems, they are not integral to the basic design of a hot water cylinder itself. This is why they are identified correctly in this context as not being a main component of a hot water cylinder.