should an immersion water heater have an on/off switch

[carlajean]

our builder has fitted an immersion heater, but it can only be turned on or off via the fuse board. it's on an Economy7 timer. We're used to having immersions that can be controlled seperately. Is there any legal requirement for this?

Not necessarily, but omitting a local isolator in a house is very strange, and the cost is negligible.

It is likely that the instructions for the immersion heater, or the cylinder assembly, state the requirement for switching. BS7671 Regulation 134.1.1 states that "The installation of electrical equipment shall take account of manufacturers’ instructions."
If the cylinder is a white one, it will have comprehensive instructions. They are likely to contain wording such as "A suitably rated double pole isolating switch for supply disconnection must be incorporated in the fixed wiring in accordance with current wiring rules." A separate immersion heater is likely to say "This immersion heater must be wired through a double pole isolating switch with a contact separation of at least 3mm in both poles."

It is possible to quibble about where the isolator should be. See Regulation 537.2.2.6. So if the isolator is not next to the cylinder, making its purpose obvious, you would have to put a notice next to the cylinder, and another next to the isolator, saying what and where it was. This would be more effort than fitting a switch local to the cylinder. Turning it off at the MCB would not comply because an MCB is not a DP isolator.

The switch must not be within 600mm of a bath or shower.

thank you very much pigletjohn. very useful.