A few weeks ago there was a fairly long discussion of the subject. Let me know if you can't find the thread.
What is the purpose of D3 in your circuit? You've drawn it as a diac, but diacs are normally used in phase shift dimmers and speed controllers, in conjunction with a variable resistor and a capacitor. The idea is that at a certain voltage, the diac breaks down and discharges the capacitor into the gate of the triac. I can't see that it serves any useful purpose in the circuit you have drawn.
The rest of the circuit looks OK to me. As the load is resistive, you could use a zero crossing opto triac such as MOC3042 instead of a random fire one, which would create less mains borne interference. But unless the heating element takes a lot of power, it's probably not necessary.
Yes, D3 is clearly redundant, since thats an optical diac in the coupler.Is there any reason you opted to build your own versus use a Solid State Relay?You can get a ready to use 10 AMP SSR from MPJA http://www.mpja.com/, for example, for as little as $7.95.
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