There are many misconceptions around achieving highly efficient operation from hydronic condensing boiler systems. The reality is, various factors will influence the operational efficiency of boiler plants. In this article, system considerations, boiler applications, and combustion control functionality are discussed for realizing energy savings.
The main factor system designers, end users, and operators can directly influence that will affect boiler efficiency is the return water temperature. Lower return water temperatures will result in more condensing operation. Since condensing occurs when latent heat is extracted from the flue gases, whenever condensing is happening, boiler plants operate at higher combustion efficiencies. One way to address return water temperatures is by adjusting supply water temperatures — if supply temperatures are lower, that will naturally result in lower return water temperatures. In systems requiring higher supply water temperatures on design days/hours, outdoor temperature reset can be utilized for the remainder of the operating hours. Outdoor result functionality includes a temperature sensor installed outdoors that lets the boiler system know as outdoor temperatures rise. Since heating loads generally vary inversely with outdoor temperature, the boiler plant responds to these higher outdoor temperatures by lowering the heating water set point, thereby reducing the return water temperature and increasing efficiency. Another way of reducing return water temperatures is by designing and selecting systems and coils with a larger difference between the entering and leaving water temperature (∆T). In addition, this approach results in lower flow rates, reduced pumping requirements, and potentially smaller pipe diameters, resulting in lower operating and capital costs.