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The "inspection window" would be for controlling secondary (above the coal bed) air, not really a peep hole for viewing the fire. Allowing some secondary air will help burn the coal more cleanly, help extract more heat from the fuel and limit soot build-up.
As far as controlling air flow through and above the coal bed, you want just enough air to efficiently burn the fuel with the least amount of smoke without losing too much heat up the chimney. You should be able to acheive this by trying different air settings over a period of time.
Jimbitmen wrote:for draft control in the chimney, is it better to use an adjustable internal damper (to save room heat) , or an adjustable draft control door. I have both.
When you say "adjustable draft control door" are you talking about the primary draft control in the ash pit door or is this a "check damper" at the flue outlet of the boiler?
An "adjustable internal damper" or smoke pipe damper is commonly used but I don't like to use them with bituminous coal because soot can quickly build up on them adversly affecting the draft.
The best thing to do would be to get a manometer (draft gauge) to see how strong the chimney draft is so you can choose the type of damper you need to best control the draft to this boiler.
If you're not familiar with a manometer or how to use one, do a search on this forum and you'll find all the information you need