You're build your own waste oil heater, and having trouble keeping it to a steady temperature using a needle valve or tap. It wants to increase in temperature without you even touching anything, and sometimes even turns red into a Chernobyl nuclear reactor.
Why does it happen? Well it's simple, as oil warms up, oil flows faster into your heater, so if you walk away from it, your heater can get out of control very quickly because you can't see the increased fuel entering the fire. This took us a few years to work that out.
The only way around this common problem with waste oil heating, is to use a timer and a pump to control that oil flow.