Actually, under normal circumstances, with the increase of excess air the temperature of boiler exhaust gas increases. Reducing the excess air to the minimum achievable to ensure complete combustion will reduce the exhaust gas temperature. This of course assumes that the boiler has been properly designed for the expected load conditions - over-firing a boiler will always result in increased stack gas temperatures. There always needs to be some excess air to insure complete combustion - how much typically depends on the type of fuel.
Another factor people often fail to consider is that combustion is a mass based reaction - it takes x lbs of O2 with y lbs of fuel to achieve complete combustion. Air density (and the mass per unit volume of it's gas components) changes significantly with temperature, so using cfm only gives you an approximation. Commercial/industrial boilers will use an O2 trim system to compensate for these variations. In situ O2 measurement is the most cost effective approach for small to medium sized boilers. Larger boilers will often add CO measurement to monitor unburned products of combustion to allow optimum control of the process.
Interesting that a small amount of excess air flow would cause higher exhaust temperatures. That's good news for me as the combustion chamber on my burner is air cooled, and cooler metal will lead to a longer life of the burner.
One of the benefits of using DC electric motors to spin a centrifugal air compressor (the leaf blower) is that the motor's rpm is mass flow dependent. When given a fixed voltage and current, the rpm of the small DC motor on the leaf blower, (used to supply air flow into the burner) will vary depending on the load; for an air blower, that load is the air's mass. Low density air on a hot day will allow the motor to spin faster and move a larger volume of air, while high density air on a cold day will cause the motor's rpm to slow down, moving a smaller volume of air. The work being done by the motor is dependent on the mass of the air, not the volume, so a DC motor will move the same mass of air regardless of the air's temperature related density.
The AC motors used on large industrial blowers are typically designed to run at a fixed rpm, regardless of the load, resulting in the higher mass flow on cold days vs hot days, that you've pointed out.
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