Radiant heat boiler hookup

I have an under floor mount radiant heat system that I put in 10years ago. The initial hookup was fed from the return water and tempered at the mixing valve by the hot side of the boiler. There was a circulator pulling on the radiant return. I ran it at 120 out and it returned at 100. It ran fine. I just had a new boiler installed by a friend and he put the circulator on the hot feed pushing the water to the system. The mixing valve is right after that and is suppose to pull cooler water from the return side to temper it down. When I turn the mixing valve to supply more cool water, it whistles and I can’t get the feed down lower than 150. I tell him it’s backwards from what I had it when it worked well but he says that’s how they hook them up now. Either the mixer is bad or what else could it be?
Any thoughts?
Thanks.