I hope somebody else does, because I sure haven't figured it out yet. I'm going to post a picture and tell you what I think I know. Then hopefully you other EKO owners can verify it or dispute it. Primary Air Inlets: Control the flow of air into the firebox. What are the consequences of letting too much air in? Too little? Is there a recommended setting? Secondary Air Tubes Deliver secondary combustion air to the nozzle or nozzles What are the consequences of letting too much air in? Too little? The recommended setting is 3.5 turns for each tube Blower Air Inlet Slider Controls the total amount of air available to the boiler What are the consequences of letting too much air in? Too little? The recommended setting (according to the dealer) is about 1/4-inch in warmer weather, expanding to an inch or more when it gets colder What I don't have a good feel for is how these settings work in harmony, and how you would go about finetuning the three different settings to try to dial in the combustion process. Do you do it when the boiler is hot or cold? Can one setting negate another one? I'm am also dimly aware that the air settings help determine, to some extent the stack temp, and therefore the efficiency of the heat transfer. And obviously the proper mix is important to get the cleanest burn. I learned the other day that the secondary air tubes have a series of holes drilled into them to deliver the right air mix to the nozzle. I'm picturing something like a natural gas burner. Cozy Heat Dave tells me you can see the holes with a flashlight, but I haven't looked in there yet (shy, I guess). Here's two photos that will hopefully get us all on the same page when trying to figure out which gizmo we're discussing. If I've got the name or function wrong, please speak up. Note that I glued a couple of modified (shortened) wire nuts to my secondary air adjustment screws to make them easier to adjust. It's not perfect, but it's better than fooling around with a little screwdriver. Of course, now you can't get a crescent wrench in there to set the retaining nut. I use an 8mm open-end wrench now. A couple of 8mm wingnuts would be just about perfect.