I am gonna post a different sort of answer. I am most comfortable with large burners. We do a lot of RTU's and Chillers as well. While I feel comfortable and confident working on these, my skills set is strongest with burners. LP, Natural, Oil (#1-#6), to solid fuel it doesn't matter. I can work on a stoker grate pulverized coal boiler at a paper mill or a 3 section Weil McLain burning #2 oil with the same level of confidence. We are all a product of our experience. We are comfortable around what we work on the most.