The circuit is very odd. It MIGHT not drawn right, but let's go on the assumption it is. I don't like it. For one, you effectively have paralleled Triacs, but you do get a voltage divider between the heater anf N. It's odd that the heater is part of the divider.
In contrast, take a look at this
https://www.electro-tech-online.com/custompdfs/2011/05/fk423e-1.pdf circuit, I'd replace it with this one. For one, there is a capacitace divider and there is the potential to have an RC snubber. Both are good things. Metal oxide resistors are a good type to use, but they will destroy themselves completely and act like a fuse.
The lack of a snubber in your circuit is bad. It allows dv/dt of the mains to turn on the triac independent of the drive signal.
Note some of the suggested driver sheets in the datasheet:
http://www.datasheetcatalog.org/datasheet/fairchild/MOC3023-M.pdf
I have a microwave oven I need to attack. No schematic. Those small resistors blew up and I am expecting that the opto did too.
A heater that opens can cause the triac to close because dv.dt is exceeded. A surge will do the same thing. A short can to. The voltage collapsed too fast.
At work we had special SCR units operating into transformers. They used to replace $30 fuses every once in a while. After I set them up to use current limit properly, used overrated (25 A) SCR's instead of 10A and added a $0.25 3AG glass type fuse cost savings went through the roof. Later, with much fuss from management, I eliminated them all together and changed to a DC power supply. This gave metering, short circuit protection, 120/240 operation in a 1RU rack rather than a 2U rack.
There were lots of configurations, but a 4U rack held two controllers, then a 2U powersupply, then another 2U power supply and another 4U for 2 controllers. In the final set-ip two evaporative sources had top and bottom tantalum heaters and usually the top heater was there to prevent clogs. It wasn't temperature controlled.
So. moral. An oversized triac with a fuse sized to the load makes a very reliabe cost-effective solution.