What size brush?

What size and kind of brush or brushes do you recommend to clean out my 520 boiler? I was thinking that I should brush it out a couple times a year on warm days when the fire pot isn't blazing hot. I could remove the baffles with some welding gloves and brush the soot off the heat exchanger surfaces and not miss a beat, or is it more involved?

The proper brush is horseshoe shaped, steel. A bristle brush will remove the flyash deposits, but won't remove the harder deposits. Or some other suitable steel brush.