Kiran, I would use phase angle control only in combination with loads which can easily withstand 100% duty cycle without being in danger to be damaged, like your heater. Lamps, for instance, I would dimm by phase angle control, heaters, if they can withstand a 100% duty cylce for more than several seconds.

In applications where the load cannot withstand 100% duty cycle I would use a different technique to reduce the power, because phase angle control needs an idealy working zero cross detector which unfortunately is a thing that does not actually exist. There are so many instances when a zero cross detector can fail that I would not trust a phase angle control to limit the power of a load that cannot withstand 100% duty cycle.

Are you sure, that the heater is 230V rated at all and not 110V?? A heater, that cannot withstand the full mains voltage for some seconds or so but needs control over milliseconds periods seems to me to be a really bad design. Maybe the heater is the problem and it is just unhappy coincidence that it failed with your controller?

Kai Klaas