With help from contributors to this forum, I've had an HTP Phoenix mod-con boiler water heater installed but am now a bit confused about what settings make sense. The unit heats both DHW and a hydronic heating system. I've set the thermostat on the boiler to 130 degrees and the factory setting on the differential is 7 degrees. But I've noticed that the upper tank temperature has dropped as low as 113 degrees and the boiler is still not running at full tilt. The fan speed at that temperature was only 490 RPM while I've seen it run as high as 700 when it's going full bore.

Is it reasonable that there should be a drop of 17 degrees from the setpoint yet the boiler is still not running at top speed to bring the temp back into range? I assume this may have something to do with balancing precise temperature maintenance with energy efficiency. But since 120 degrees is recommended as the minimum temperature for sterilizing dishes, it appears that I may have to either lower the differential or raise the boiler's thermostat to 140-145 to ensure 120 as a minimum. There's a mixing valve on the DHW to prevent scalding but having the higher thermostat setting will compromise my energy efficiency.

Any thoughts?