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.