Redstone energy conduits (among other things) can cause a chunk to be temporarily loaded even without any other chunkloaders in the area. Don't ask me why. If you have a conduit connecting your boiler to stuff, this might be keeping it loaded.
If you don't want to chunkload your fuel production (I applaud you for saving your server!), make sure to have enough buffer in your biofuel tank. Alternatively, probably a safer option, build your boiler setup so that the boiler and all of the fuel production is within a single chunk - so it either is all loaded, or none of it is.
Although they aren't clearly labelled as such, portals and portal spawners may also act as chunk loaders.
Are you sure the boilers are running, and not just "losing track of where they were" once the server comes back online? Are other systems in the same chunks also staying online? Planting a tree before logout might be the simplest way to tell if you're not sure.
I'm not talking about you, I'm talking about the boilers. Do OTHER things in their chunk keep running too? I'm thinking maybe the chunk IS unloading but at some point the server bugs out and changes the state of the boilers.100% sure I'm not losing track. I log off full heat full capacity going in, I sleep 8 hrs ish, log back on with no one having logged on in between, and it's cold.
I'm not talking about you, I'm talking about the boilers. Do OTHER things in their chunk keep running too? I'm thinking maybe the chunk IS unloading but at some point the server bugs out and changes the state of the boilers.