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Need direction installing 3 way mixing valve, radiant heating system

I have 6 zones. Five of the 6 are working fine. In what I am calling zone 3, the temp heading to the zone, after the mixing value, and just above the pump, stays at 70 degrees, and the return temp that goes back into the mixing valve, cold side, is approx the same temp.

The pipe just below the 3 way mixing valve is warm, the pipe above (return) is cool, and the last pipe that should be the mixed water is cool. In other zones this last pipe, after mixing, is warm. This last pipe is the pipe that feeds into the pump.

The mixing valves are taco 5000 3 ways. They are union fit.

On each zone I have 3 shut off valves, by the pump, 2 before it and 1 immediately after. Where the mixing valve is I have one valve beneath the mixing valve, on the hot water side...and another up above the mixing valve, on the cold water return side.

So it would seem my mixing valve is not mixing in the hot water side.

Two questions:

1. If I turn off the power to the system, and close off these valves, is it as simple as unscrewing the union nuts, taking out the old valve, and putting in the new valve
2. If I do that, I know now I have air in the system...how is the system "filled" with water and purged of air....I'm assuming its a closed system, but maybe its not and as I bleed out air (where??) and then water is automatically filled in??

Here are pics of my system


Any help is appreciated. I did try to call for service, but the best I can find is 4 weeks out and getting cold here in maine. Thank you.