While annual maintenance varies by manufacturer, in most cases the minimum requirement is a visual inspection. Inspecting the boiler and surrounding area for leaks or signs of corrosion. Verify the water feeder is operational and boiler pressure is within range. Check for unusual odors or prolong gas smell during combustion. Verify combustion, and check efficiency with a combustion analyzer as needed. Test power switches and safety controls like low water cut-off and high limit. Check thermostat(s) for low battery and proper function. Inspect ignitor and flame sensor, clean and or replace as needed and lastly if it is a condensing appliance then it may have a condensate neutralizer which should be removed, replaced or refilled as needed.