- Boiler turndown is the ratio between a boiler’s maximum and minimum output. Depending on the burner’s design, it may have a turndown ratio between 4:1 and 10:1 or even higher.
- A 4:1 turndown means the boiler’s minimum operating load is 25% of the boiler’s full capacity (100% capacity divided by 4).
- A 10:1 turndown means the minimum operating load is 10% of the full load capacity (100% capacity divided by 10).
Why is Boiler Turndown Ratio Important?
- Turndown ratio is important for boilers that are required to operate at a wide variation of capacities. A boiler with a higher turndown ratio will usually handle fluctuating loads more efficiently than one with a low turndown.
- A boiler’s burner will modulate or “turn down” as the demand for hot water or steam decreases, in an attempt to meet only the required load. The turndown ratio tells you the minimum output the boiler can handle before turning off and then cycling on and off frequently.
- If the demand goes below the burner’s minimum turndown, the boiler will cycle off until there’s more demand. This results in the system cycling on, running for a brief period and then cycling off again. Every time this happens, the boiler must go through a pre-purge, a firing interval and then a post-purge.
- For example, a 500-horsepower boiler with 10:1 turndown will modulate down to 50 horsepower before turning off and then cycling on and off. At a 4:1 turndown, the same capacity boiler would only modulate down to 125 horsepower. If the application has a minimum load of 50 horsepower, the 10:1 boiler could accommodate that without shutting off, but the 4:1 boiler would have to use multiple short cycles to maintain the load.
How Turndown Ratio Affects Boiler Efficiency
For applications with a low load demand, a low turndown ratio will generally result in lower boiler efficiency and frequent on-off cycling. When the system has to cycle on and off frequently, it can also cause a number of costly issues:
Selecting the Right Turndown Ratio
Correct boiler sizing and turndown ratio go hand in hand. In many cases, facilities may opt for a single over-sized boiler to accommodate the maximum load for the application and any expansion in the future. However, if load demand fluctuates widely, a high-capacity system may not be able to handle the minimum load without frequent cycling. On the other hand, it wouldn’t make sense to choose a boiler with a high turndown if it’s not sized to meet the highest load demand.