Packaged boilers installed in multiple-boiler cascade systems have become a predominant trend in the commercial boiler market, Lou Vorsteveld, senior director of engineering for Laars Heating Systems, writes in Plumbing & Mechanical. These packaged boilers arrive at the jobsite preassembled and factory tested for proper operation, while the contractor installs the near-boiler piping, the vent system, and the electrical connections and configures the controls for proper multiple-boiler operation. A major reason for this setup’s continuing popularity? It allows the boilers to run at a minimum firing rate to enhance the overall efficiency of the boiler system, Vorsteveld writes.

“With parallel cascading, the first boiler is fired to 30% to 40% before the second and third boilers are fired,” he says. “The lower the firing rate, the higher the efficiency. The flue gases are kept in the boiler longer and more energy is extracted from them. A high turndown ratio and parallel cascading go hand in hand.”

Vorsteveld adds that the goal in designing a high-efficiency boiler installation is to reduce the temperature of the return water to as low as possible. In the article, he recommends a couple of techniques for engineers and mechanical contractors, including keeping low-temperature zones separate from high-temperature zones, utilizing outdoor reset, and increasing the use of oxygen sensors to limit excess oxygen within the combustion process and enhance the overall efficiency of the boiler.

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