Heating System Replacement

[DickR]

Originally Posted by Charlie T

The reading the tech is taking using a probe in the stack is simply a combustion efficiency number that is based upon gas readings in the stack after it has left the boiler. This only is a measurement of how well the burner is turning fuel into heat energy, it has nothing to do with how efficiently the boiler is collecting that heat energy and transferring it to the water in your boiler/ heating system......

Actually, measuring the stack temperature is a very good way of measuring the efficiency of recovering combustion energy into boiler water or air circulated to heat the house. With no useful recovery at all of the heat of combustion, the stack temperature would be the flame temperature. Every BTU of heat recovered from the flue gas lowers its temperature. Since the heating value of the fuel and heat capacity of the flue gas mixture are well known, it's very easy to correlate efficiency of heat recovery with flue gas temperature. Providing more heat transfer surface and a sufficiently low temperature of the fluid (water or air), the flue gas can be cooled to the point where combustion water is partially condensed, which adds a large amount of heat to the recovery, at roughly 1,000 BTU/lb.