If the temp drop because the input voltage is lower, your temp sensor should tell you to increase the firing angle.
Thanks for your reply Erik,
I am having a temp sensor and certainly I will be setting the firing angle as per the feedback from this sensor.
In my opinion rather than acting after the temperature has lowered or raised why not actively monitor the voltage continuously in order to keep current going and thus the heat going into the system constant. Isn't it better to use both the methods,
1. Temp Sensor Feedback.
2. Voltage Feedback.

Thanks & Regards,
Prahlad Purohit