The art of peddling Boilers has shifted from the realms of Logic and Thermodynamics to sheer magic and out-of-context truths. It is becoming imperative for Engineers like myself, who get involved with selecting Boilers maybe four or five times a year, to educate ourselves about the basic facts of boiler efficiency. In this series of posts we will take a closer look at the work-horse of our industry – the common commercial gas fired hot water boiler in the 2 million BTU input range. The posts will attempt to investigate the realistic level of efficiencies that might be expected from these boilers. We will assume that no special “heat recovery” contraptions are in use.
One of the key concepts that needs to be clearly understood when discussing boiler efficiency is the concept of the Dew Point Temperature of the Flue Gases. (Or more specifically the Dew Point of the water vapor in the flue gas.) If you are going to design a HW Boiler installation, you must be aware of what the Flue Gas Dew Point is going to be. In fact, flue gas dew point becomes even more important if your boiler is of the Non-Condensing type. Ignoring this critical piece of design information can be very costly.
Now let us examine the second category of energy loss – the loss of sensible heat of hot flue gasses being vented to outside. In Equation 6 above we have 18.5 lbs. of dry hot flue gas (products of combustion) being vented out of the stack. The specific heat of dry products of combustion is approximately 0.24 BTU/lb. °F.