Your last point is excellent, especially if you have a modulating/condensing boiler. (Do you have plastic exhaust piping and a small plastic line that drips wat

Your last point is excellent, especially if you have a modulating/condensing boiler. (Do you have plastic exhaust piping and a small plastic line that drips water from time to time when the boiler is in operation? You do.) The lower you can make the return water temperature, the more efficient your condensing boiler will run. The way to do that is to lower the leaving water temperature (and accept/relish the corresponding longer run times). With my outdoor reset properly tuned, my Nest thermostat will run 22+ hours of every day and sometimes 24 out of 24. The outdoor reset control is the primary temperature control on the house and the thermostat serves more as a limiter than as a control. This has the effect of using the lowest possible water temperatures (most efficient), which ends up being the most comfortable for the occupants because there's very little overheat/shutoff/overcool/turn-on cycling.