A burner with 10 to 1 turndown can fire from full input down to one tenth of it while staying stable and clean. Higher turndown lets a boiler match light loads without shutting off, cutting short cycling and purge losses. On-off burners have no turndown, high-low burners have two steps, and modulating gas burners and mod-cons commonly reach 5 to 1 up to 10 to 1.
Building load in April is 8 percent of design, with 4 to 1 turndown the boiler can't get that low and short cycles all day.
Turndown figures from a brochure are at ideal conditions; real stable turndown is often lower once the burner is tuned for low CO.
Also heard as
- turndown ratio
Related terms
Modulation
Varying a burner's firing rate continuously between low fire and high fire to match the load.
Short cycling
A burner starting and stopping far too often, with short run times that waste fuel and wear parts.
Burner
The assembly that mixes fuel with air and holds a stable flame to heat the boiler.
Low-fire start
The rule that a burner must light off at its minimum firing position, proven by a switch, before increasing rate.