The old burner likely achieved ~100% combustion (not sure what you mean by the burning being efficient). As to the other part, yes, less heat is sent up the ch

The old burner likely achieved ~100% combustion (not sure what you mean by the burning being efficient). As to the other part, yes, less heat is sent up the chimney in modern designs. My boiler is getting pretty old but was sold as 80% efficient. As far as I can tell, the near drop in replacement would be 84 or 85%. Neither is a modern condensing boiler (which I'd have to figure out a new exhaust to switch and probably add active outside air exchange; tempting with the way the current one sucks all the humidity out of the house).