I recently helped upgrade an old (±20years) 4ton boiler (heavy fuel oil) with some monitoring instrumentation. I decided to have a look at the data and I'm not sure if these kind of oscillations are normal for an older boiler? I haven't really worked with boilers before.
Ignore the sudden drop (a topic for a different question), I'm referring to the oscillating behavior in the steady region. It seems to be rather...excessive? but follows a relatively consistent profile which leads me to believe its just a really barebones control system.