In article ,
jim k writes:
I want to add a boost function to the rather basic immersion
heater switching of my heating system.

Currently there's a simple switch with neon.

What do I need to look for when choosing?

There are immersion heater boost switches, such as:
https://www.tlc-direct.co.uk/Main_In...s_2/index.html

There are also ones with a timeswitch built in, if
you want it to switch on at same time every day.

Amy other gotchas?

Many of these eventually burn out their contacts,
hopefully after many years of use. Position it
somewhere where it isn't going to set light to a
pile of towels or whatever you keep in the airing
cupboard, and make sure you make really good screw
terminal connections.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]