What is the reduction in BTU delivered to radiators attached to a hot water boiler as you reduce the temperature of a boiler from 180 degrees to 170, 160, 150, 140, 130, or 120 degrees. Is it linear? For example, if a boiler delivers 500 BTU per foot of radiator at 180 degrees today and you reduced the temperature to 120 degrees, would I reduce the BTU per foot of radiator to 500 x 120 / 180 or 333 BTU? Are boilers designed to run efficiently at one temperature (this is not a mod-con boiler) and then lose efficiency if you turn down the temperature?
The scenario is an old house that is not well insulated. I am planning to seal the house much tighter, add a lot of insulation, and replace many windows/doors. I am trying to anticipate what potential adjustments might be necessary on the boiler to accommodate these changes.