This seems too easy....is underfiring an oil boiler gaining efficiency? Appears so, since the stack temp is lower and the ratio of boiler surface area to flame quantity is higher. A customer account I serviced about a week ago claims an engineer had him grossly underfire his 5 section Burnham V7 oil boiler, and it appears to be heating the 7000 sq. ft house just fine, and he says it only burns 800 gallons/yr, hot water included. Nozzle size was .85 gph but with 140 psi fuel pressure, so actual firing rate is 1.05 gph. Stack temp was 350F. The boiler is rated for 1.65 gph. Hmmmmm.