On Wednesday, 8 June 2016 20:14:48 UTC+1, F wrote:
I'm looking to put our immersion heater on a timer. It's protected by a
16 amp fuse so I assume the timer needs to be rated the same. Many of
the timers I've found so far seem to be 13 amp, even though some claim
16 amp in the headline description.

Domestic immersion heaters are 3 kW maximum so a good quality 13 amp timer will be adequate.

The immersion heater must have a thermostat and all new ones should have a secondary safety cutout with a manual reset. The thermostat will be inside the cover of the immersion and usually slides into a pocket so it can be replaced without draining down the cylinder.

Owain