Spring: clean up after the season
Book the sweep now rather than in autumn. Creosote left through a humid summer is more corrosive and smells worse, and spring appointments are easier to get.
Empty the ash cleanout, check the damper closes and seals, and look at the masonry for any spalling that appeared over the winter.
Summer: do the work that needs warm weather
Crown repair, repointing and waterproofing all need above-freezing temperatures to cure, so this is the window for masonry work.
It is also the season chimney odours are most noticeable, and the negative pressure that carries them indoors is easiest to diagnose while the air conditioning is running.
Autumn: get ready to burn
Book gas appliance servicing in late summer or early autumn, before the first cold snap creates the queue. Check that the wood you plan to burn is genuinely dry.
Test carbon monoxide and smoke alarms, check their expiry dates, and confirm the cap and mesh are clear.
Winter: watch and listen
Through the season, watch for glass sooting on gas units, wood that burns faster than it should, and smoke spillage on light-up. Each points to something specific and none improves on its own.
Keep the vent termination clear of drifting snow on direct-vent appliances, which is a genuine cause of mid-winter shutdowns.