I will start out with a disclaimer, my setup is very untypical for a wood burning setup and just because I dont have to clean my chimney yearly its highly likely most folks do. Nevertheless with the right chimney design and operation, chimneys may not need to be cleaned yearly although I do inspect mine yearly with a mirror. I have an older Burnham Wood/Coal boiler with 550 gallons of storage. Its hooked to 8x10 rectangular tile flue in a standard concrete block chimney. There is an adajcent oil boiler flue, but the boiler is a cold start so its cold most of the time. The boiler is in the basement and runs through 2 heated floors and a 6' unheated attic to a high point on the house. The outdoor portion sticks up about 4 feet from the ridge pole. I have never run coal and the shaker grate was long gone when I got the boiler so a friend made me a grate. All air comes up from the bottom of the fire and the grate sits down at the base of a refractory lined triangular pit. There are some steel smoke shelves that redirect the flames prior to going through a bank of vertical tubes with the remnants of turbulators. Its not super efficient compared to newer designs. I run the boiler full tilt with 1.5 to 2 year old dry wood until the storage is up to temp. There is a motorized air damper if the boiler overheats but the goal is to never have it shut when the boiler is hot. As the storage peaks out I can dump heat into a domestic hot water tank or into the house. I use a minisplit in shoulder seasons so generally only run it when it gets consistently cold. The bottom location of the grate and the air source below it means I get no coals and if I dont feed it quick enough when the wood burns out I might have to relight it. The ash is fine dust with no coals and it dumps into a lower air chamber so it doesn't build up on the grate. I would like to retrofit it with heated secondary air but the design prevents me from doing it. I run about 2.5 to 3.5 cords a year. Its not just the boiler as I had a papa bear clone for several years and a VC Defiant after that before the boiler. In total since 1988 I have cleaned the chimney three times and generally it was out of guilt more than need. I usually did small loads with the stoves during shoulder season and rarely used the dampers to cut back on load. My reasons why I think I dont need yearly cleaning while most do; the big things are an interior chimney and inefficient stoves and boiler. I expect the stack temp stays warm enough that any condensables stay as superheated vapor while in the stack. The other reason is I have always tried to burn hot fires. The stoves and boiler are in a partially insulated basement and I can usually get away with hot fast fires. I rarely if ever banked a stove for overnight as the house is well insulated and even in cold weather, it stays warm enough all night so that I really dont need an overnight fire. Obviously with the boiler and storage no need for overnight burns. The one downside is I have to relight the boiler every time I use it. That means I need to stock up on newspaper and kindling.