The turn-down ratio is explained by ASHRAE as: on larger boilers, a modulating burner will be installed that can adjust the output from 100% down to some minimum output. The burner modulation range is called the “turn-down ratio”, which is the ratio between full “on” and the lowest continuous operation. A burner can operate from 100% output down to 10% output which has a 10:1 turn-down ratio. A boiler with a modulating burner increases in efficiency as the output drops. Efficiency increases because of the increase in the ratio of the heat-exchanger surface area to heat-input as the output, or firing rate, is reduced.