Sweeping and inspecting are two pieces of work
They are sold together often enough that people assume they are one thing. They are not. The inspection is the assessment: the level, the access, the camera where the level calls for one, and the written findings. The sweep is the removal of what the inspection measured. Booking a sweep without an inspection means the deposit comes out and the cracked tile behind it stays in, and booking an inspection every year without ever sweeping means the report will eventually tell you the deposit is over the threshold.
Why a chimney smells after rain
Damp meets deposit. Creosote and soot on the flue wall are dry and mostly odorless until water reaches them, and then the smell comes down into the room, usually in warm damp weather when the chimney is drawing backwards. Sweeping removes the material the smell is coming from. If the smell returns after a sweep, water is getting in, and the next question is the cap, the crown or the flashing.
The animals
An open flue is a sheltered vertical shaft, which is what a bird or a squirrel is looking for. A nest is a blockage, and a blockage in a flue that also vents a furnace or a water heater is a carbon monoxide path. A chimney that has been out of use for years is where a nest turns up, and the fix is a cap fitted after the flue has been cleared. That is a smaller job than anything the nest can lead to.
Gas and oil appliances
A flue serving a gas or oil appliance does not build creosote, so nothing is being brushed out of it in the way a wood burning flue is. What it does collect is scale from a deteriorating liner, condensation damage and debris from the masonry itself, and it is still inside the annual inspection scope. The reason to look is that the flue is the exhaust path for combustion gas in a house people sleep in.
When to book it
Late summer into early fall, before the first cold night. Trade publications put the busy season at late August through November, when households are getting ready for the heating season. A call in July gets a date in July. A call in the second week of November, after the first weekend anyone lit a fire, joins a queue.
What a sweep uncovers
The brush reaches surfaces the camera photographs and the eye does not. A joint that has opened, a tile that has cracked, a smoke chamber that was never parged, a damper seized in place. None of those is a cleaning job, and all of them are written into the findings with a photograph, not mentioned on the doorstep on the way out. A finding that turns into a repair estimate is a separate decision made after you have read it.