Re: relay modules?

Mike,

Second is something to switch mains safely - a full 13A to run a 3kW
heater.

You could take a peek at so-called "solid state relais".  And as your heater
is a resistive load, it should be no problem to control it with even a
simple one (no zero-pass detection needed, as is needed for inductive
loads).

One thing to check is what your heater pulls in the first seconds or so (up
until its fully heated).  That current might well be more than the 13A that
it draws after those first few seconds.

The last thing to check is (ofcourse) the voltage those solid-state relays
"switch on" on.  The Pi's 3.3 volt is on the low end, so you will likely
need something to connect it to a higher voltage.  A simple open-collector
transistor should work though.

Regards,
Rudy Wieser