Xringer,
I just browsed the thread so if I'm repeating something someone said I apologize.
TRIACs and SCR need trigger diodes usually bilateral. Once the current is flowing you need to apply a voltage to the gate to shut off the TRIAC. Just removing the gate voltage won't do it.
I use zero crossing detectors. What they do is sense when the AC wave is transitioning from one half cycle to the other and switch at the zero point. This reduces load stress.
I've included a small circuit that I have used to show driving a TRIAC with a zero crossing IC.
MOSFETs can be tricky. 'N' type work best on the low side, driving an 'N' type mosfet on the high side of a circuit requires a charge pump to make sure the MOSFET turns on completely.
There is nothing wrong with relays, sometimes they are my preferred option. Ideally what you want to do is turn the relay on at the zero crossing point (dry) this stresses the relay contacts the least.
I'm not sure why your heater doesn't have a temperature limiting circuit built in, maybe I missed something in one of your posts.
Rob
