triac burn out problem
The triac will be dissipating in the neighborhood of 10W at the specified current (read your triac datasheet to get the exact value, but it won't be much less). 

So you need to get rid of 10W for the heating cycle. You can pretty much ignore heat capacity and assume steady state. Suppose you want the junction to get no hotter than 110°C, with an ambient of 40°C maximum (make sure your oven is not heating the triac itself), so the rise is 70°C. Thermal resistance junction to case is 0.9°C/W so the case rise must be less than about 60°C, and the heatsink performance should be less than 6°C/W, which is a fairly modest requirement. A bigger heatsink will give better reliability.