It's a well-established fact that setting up combustion systems for too much excess air (or dilution air) wastes fuel. You can use this relationship to calculate just how much you can save by decreasing excess air:

% Fuel Savings = 100 x [1- (Available Heat, High XS Air/Available Heat, Low XS Air)]

One of the cardinal rules of heat transfer, whether it's by convection, radiation, conduction or any combination of these, is that the more load surface you expose to the heat source, the faster you'll be able to heat it.

Dick Bennett points out the evil effect of heat exhaust losses.