It' easy to add a sensor that goes in the flue to shut-off the boiler's pump. Several strategies, with the most simple being a snap disc thermostat and then moving through several options to a PID with suppression (automatic reset).Snap Disc... he would have spent the 1k$ for the lambda control and the second temperature probe that goes in the water storage and the third sensor that goes in the flue to shut-off the boiler's pump.
Also: Flue Temp Control
Adding a temp probe for water storage also is not difficult and can be used with a differential temperature controller to cycle the boiler loading circulator on/off to move residual boiler heat to storage. I haven't added this to my Tarm Solo Plus 40 boiler of 2007 vintage, but it is something I'm considering. Making additions to boiler controls can be daunting, buying a boiler with an integrated control is simpler and may offer more programming options, but doing this oneself can be very satisfying, inexpensive and may offer more flexibility than a pre-programmed controller.
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