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.
Boilers must run a purge cycle at the start of each on-cycle to assure that there is no accumulation of explosive gases in the fire box. By Code, enough air must be blown through the boiler to perform a 4x air exchange of the fire box. This cold air also removes heat from the boiler. Flames tend to be the most unstable and radiation heat transfer is at its lowest at start-up. Therefore, boilers are the least efficient at the start of the on-cycle. Reducing the number of on-cycles should therefore increase boiler efficiency over a longer period of operating time. Boiler cycling also effects emissions. See Boiler Combustion
An important consideration is the boiler efficiency at each operating range. Old boilers have a simple low and high fire setting. The air-fuel ratio is set by fixed position dampers and linkages. The ability to control boilers with simple mechanical linkages over a wide range can be difficult. Often the result are settings that cause the boiler to use much too much excess air. (NOTE: too little air/oxygen leads to more dangerous results such as CO and soot formation and even ‘back-firing’ and explosions from unburnt fuel; therefore, it is safer to ‘err on the high air’ side, but there is an efficiency penalty for using too much air. See Oxygen Control.)
New boiler controls use separate controls on the fuel and air supplies. The best systems also use some sort of continuous closed loop feedback system, such as a flue gas oxygen sensor, to continuously adjust the air and gas controls to obtain the best combustion efficiency.
Taking all of this into consideration, a boiler with good controls should be the most efficient at stead-state conditions. If the load is highly variable, then a boiler that is also highly variable should be the most efficient fit. This would be a boiler with a high turndown ratio and infinitely variable controls over its entire range.
Johnston Boiler Technical Paper in support of 4:1 over 10:1 This paper presents a case where it is more efficient to run a 4:1 in an on-off mode rather than run a 10:1 boiler at 10% fire. The paper assumes that the 4:1 boiler can be shut off for long periods of time, and the 10:1 boiler uses large amounts of excess air at low fire. This paper demonstrates the difficulty in applying ‘one-size-fits-all’ solutions.