I don't know about your specific situation and the 3% claim for a 10 degree decrease, but it is true that the lower you can run the water temp, the cooler the boiler will run, theoretically resulting in cooler exhaust gas temperatures, which means less heat goes up the "chimney", meaning more heat stays inside the house.

The circulator may run more often, but it's better than the boiler ramping all the way up to 185° if the house can maintain temperature with the 140° water running more. A boiler that runs more is going to be more efficient because it will spend less time re-warming the water that cooled down while waiting for the next call for heat, which causes it to have to flame up longer to reach the water temperature set point again.

If your boiler is cast iron, 140° is a safe minimum.

Generally a boiler is set at 160 LOW and 180 High on the aquastat, with 150 being the Low for cast iron radiators.

Thank you. That is very interesting. The burner is a Burnham Series 2A gas boiler. It seems the circulating pump would use up a significant amount of electricity but maybe not. I appreciate the comments; more welcome.

You are going to have to do some research and/or experimenting.

Heat usually transfers best with larger temperature differentials. As an example: An ice cube will melt much quicker with ambient at 100 degrees, as opposed to ambient being 35 degrees. Heating is the same deal .. heat transfer. Of course, some will transfer up the chimney, too. The boiler will always be controlled within an envelope, so that it's extremes will always be within 20 degrees (or whatever), of each other.

A Taco 007, which is a common circulator, uses a fraction of 1 amp of electricity, maybe about as much as a 50 watt light bulb when it's running.

Your water temperature should be the lowest possible to heat your home on a cold day. Some boiler systems use an outdoor reset that can vary boiler water temperature as outdoor temperature changes. If you use an outdoor reset and have a conventional, non condensing, boiler, you should install a thermostatic bypass to prevent the boiler from condensing with low water temps.

While i think my boiler is set to 160 degrees I can tell you if its set to a lower temperture the house will never get warm. i set the temp to around 160 to 165. if you set it to high you risk the high temp release valve releasing water on the floor. So for my system the like a mechanic tunning a carburator you listen with your ears. When i hear my baseboard make a crinkling sound i know its set rightt. So if you dont hear your basebord in your rooms making noise then itts to low. The fins in the base bord will make a crinking sound as it heats up. Hopes this helps.

Frank