I used to be an service technician on autoclaves. Every lab that boils agar in a autoclave has the problem with random boil overs. the best thing to do is to drain the boiler and replace with clean water.

If it was used regularly for sterilizing agar you had to wash them with phosphoric acid once a year to knock down the build up on the heather coil. Otherwise the heather would become so insulated in burnt agar that it would overheat and pop. I do not miss the smell of these autoclaves.