Radiation loss of a boiler enclosure is 1% at full load and 4% at quarter load. Operating boiler closer to full load improves efficiency.
The boiler being hotter than its environment, some heat will be transferred to the surroundings. Damaged or poorly installed insulation will greatly increase the potential heat losses.
A reasonably well-insulated shell or water-tube boiler looses between 1% to 4% of its load. This may not appear to be a large amount, but it must be remembered that this is 1% to 4% of the boiler's full-load rating, and this loss will remain constant, even if the boiler is not exporting steam to the plant, and is simply on stand-by.
This indicates that to operate more efficiently, a boiler plant should be operated towards its maximum capacity. This, in turn, may require close co-operation between the boiler house personnel and the production departments.