1) The closely spaced tee’s separating the boiler loop from the secondary loop do not isolate the differential pressure from the boiler pump. In short, the boiler pump was in series with the zone pump.
2) The boiler is a non-condensing boiler and the (typical) low-return water temperatures from the radiant floor heat would cause the boiler to condense. Sustained flue gas condensation would over-time cause the heat-exchanger in the boiler to fail and the vent could degrade and allow CO to fill the structure. The LTV (Low Temp Valve) will ensure the water temperature entering the boiler will be 140°F or higher.
3) The boiler will produce temperatures above that of low temperature radiant floor heat requirements. The use of a 3rd party mixing valve to provide outdoor reset will allow the proper mixing. (One of several methods of OD reset mixing)