Question on boiler protection valves?

Danfoss Thermostatic protection valve versus a Honeywell V 135 used with a T100 R thermostatic control. Question is, if the boiler water is not up to the set point of the mixing valve, will either of these valves allow a small amount of colder return water from the zones past and back into the boiler or do they totally cut off any return water. A friend is installing a Portage & Main Optimizer 250, and at present he is using the Honeywell V135 with a T100 R thermostatic control. It seems to not be letting any return water by until the boiler water gets to 160 F.