mnhoff is correct, Annual maint consists of a new nozzle and fuel filter. If it sat for a while the nozzle is probably clogged, time to replace. Can you do it? Yes you can, Go slow be careful follow the directions that you can find in the Aquahot company website. Be careful not to pinc any wires when putting the burner head back in, and you will be ok. Normal service about $150 if you can find a GOOD tech, you may want a watch a service once.
After service it should burn hot, clean and odor free.
You will love it. Neglect is the worse enemy of the system