There are several factors that go into efficiency. There is the efficiency of the burner which is measured by the difference between the temperature input and the temperature being
lost up the flue. Electric heat is 100% efficient when it is
used by the consumer because there are no exhaust flue losses. The losses occur at the power plants. There is also
the efficiency of the burner that is measured in CO2 readings. Although we want to reduce our carbon dioxide emissions,
low CO2 readings indicate that the burner is not converting
the fuel to energy efficiently and therefore producing more contaminates such as carbon monoxide, sulfur dioxide and various forms of nitrous oxides.

There is also system efficiency which is a matter of how well the heating system delivers the heat to the heated space
and the domestic hot water. Although it is hard to measure, common sense tells us that the lower the temperature of the boiler and the less it runs the more efficient the system.