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Design Tips to Avoid Boiler Short-Cycling
ASHRAE Journal, July 2018
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Boiler short-cycling occurs when a boiler quickly satisfies the load and reaches the cycle off limit setting in the boiler controls and cycles off, or the high-temperature limit safety. This becomes a problem when there is heating demand and the boiler will quickly need to come back online. This problem has grown more prevalent due to the modern light–mass boiler design—to reduce costs and improve efficiency, modern boilers have less stored water volume and lower mass heat transfer area due to enhanced tube designs. This contrasts with the old fire-tube designs that had large water volume and old water-tube designs with high mass of straight tubes.
The design decisions made in selecting and sizing heating boilers and their components as well as the control sequence have a direct impact on the performance of the overall heating hot water plant. This month, I provide some design tips in boiler selection and the design of heating hot water systems to avoid boiler short-cycling. These problems can impede the system’s ability to perform reliably and efficiently.
Short-cycling is not good for any type of gas-fired boiler system. It causes excessive wear and tear on the boiler and substantially decreases efficiency. In many cases, the boiler cannot reliably deliver heat to the loads. The mechanical problem comes from the effects of rapid cycling on boiler components.