Turndown Ratio is the ratio of the maximum output of the boiler to the minimum output at which the boiler can be operated efficiently. For example, a boiler which has a maximum output of 30 Horsepower and minimum efficient output of 6 horsepower will have a turndown ratio of 5.
The higher the turndown ratio, the more flexibility one gets in the operation.
If the turndown ratio is too low, the boiler may need to operate at a higher than desired output, switch off for a while and then switch on. This increases the cycling frequency.
Boilers operated on gas will have a turndown ratio of around 10-15. Oil fired boilers can have a high turndown ratio. The turndown ratio can be around 20.
Coal can also have a high turndown ratio.
Electrically operated boilers have the lowest turndown ratio. This is because, it is difficult to control the electrical input to a heater.