The “Tipping” Point of High Turndown Ratios: How much of a good thing is too much?
Boilers used for domestic water and space heating are one of the highest energy consumers in residential and commercial facilities. After the global energy crisis in the 1970s, higher fuel prices prompted the search for solutions to reduce energy consumption. Back then, conventional steam boilers were the only option available. However, these boilers were very inefficient (due in part to the unrecovered latent heat losses from the fuel combustion process) which eventually led to the advanced development of high efficiency condensing boilers.
Ideally, condensing boilers would operate as designed – condensing as expected and delivering the highest efficiency possible. However, in reality, external variables create unwanted problems, affecting the boiler’s efficiency. To ensure that condensing boilers are operating at maximum efficiency, effective controls and operation become crucial. The load on a boiler constantly fluctuates depending on the weather conditions, occupancy rate, internal heat gains, and other variables. Therefore, effective boiler control can significantly affect energy use that may have resulted in system inefficiencies, increased utility costs, unit downtime and costly unscheduled maintenance.
In the past, boilers were designed with only one mode of operation – on/off burner control. As innovation was added into boiler designs, manufacturers began offering units with multiple firing rates followed by units that could modulate seamlessly between a fixed low and high fire rate based on requirements. This reduces the number of on/off cycles resulting in increased efficiency. On start-up and shutdown, the boiler goes through a pre-purge and post-purge cycle that would result in heat losses affecting the boiler efficiency – the extent of these losses depend on boiler construction, type of application, and design of the system. How far the modulation goes from full fire rate is determined by the turndown ratio.
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