Many factors affect boiler efficiency including combustion and boiler load. Boiler efficiency is a bell curve shape with maximum efficiency of 78-81% in the 50-75% load range depending on boiler age, etc. Over 75% for extended periods you are over firing the boiler which throws heat up the stack.

For loads under about 50% a different set of numbers come into play. Since a boiler is a large hunk of metal, radiation losses from the boiler are significant. For boilers radiation is a percentage of the maximum capacity of a boiler and usually about 4%.
For our example, assume you are operating a 25,000 lb/hr boiler at low loads for extended periods of time. For a 25,000 lb/hr boiler, radiation losses would be 4% of 25,000 lb/hr or 1000 lb/hr. The radiation number does not change if the boiler is running. Now the bad math -
At 12,500 lb/hr the radiation is still 1000 lb/hr or 8% radiation loss
At 6,250 lb/hr the radiation is still 1000 lb/hr or 16% radiation loss
At 3,000 lb/hr the radiation is still 1000 lb/hr or 33% radiation loss
You can do the math. If the combustion efficiency is 81%, then at 3,000 lb/hr subtract 33% for radiation losses for a total efficiency of 48%! As you lower the load on a boiler, the efficiency drops in a big way because once the boiler is in operation the radiation load does not change. Put another way, the most efficient boiler is a boiler not in operation.
If your load changes as shown in this example, then operating a single large boiler or boilers is costing you big money because of high fuel prices. The solution is to go to smaller multiple boilers with an automatic PLC based control system to turn on or off boilers as required to meet your steam demand. With so many possible variables to consider, the best way to run a fuel savings analysis is to construct an Excel spreadsheet for your specific operation and run the numbers. Different scenario's can be modeled to determine fuel savings. You can expect reductions of 25-50% if the load variations are significant. This kind of savings can pay back new boilers in 6-18 months.
Also in most cases, multiple smaller boilers can be kept under the 10 million BTU limit which avoids any air permits as well as all the issues with record keeping of fuel consumption for your plant.
Interested in having a set of numbers run for your plant? Give a call so we can discuss your current operations.