Boiler turndown is a ratio of capacity at full fire to its lowest firing point before shut-down. Old boilers may have only two firing positions, low and high. Newer boilers fire over a wider range of capacities. Depending on the controls, there may be fixed setting points or fully variable settings. If a 1 million BTU boiler can fire as low as 100,000 BTUs, then it has a 10:1 turndown ratio. Turndown ratios are important for boilers that must operate over a wide range of capacities/ demands. In general, from an efficiency standpoint it is best to have the boiler sized to match the load. Boilers that have a wider turn-down ratio are therefore typically more efficient at meeting variable loads. However, there are exceptions, such as when it's possible to shut the boiler off for long periods of time, rather than run or cycle the boiler at very low fire conditions.