I have a 6unit apartment building with 6 zones and 6 booster pumps... one for each unit. The 4 top units have baseboard heat and the 2 lower level units have radiant heat. These 2 lower units have the red Bell and Gossett pumps in the left of the pics. It looks like there is a mixing valve in line that comes up from the return and mixes with the hot supply line. I noticed that I had both the mixing valve and the hot supply line valve completely open all the way.

Is there a recommended way to adjust these two valves for optimal heating without worrying about cracking the lines in the concrete? I was thinking with both valves open all the way I am feeding too much return water in there so I closed off the return valves slightly? Any input would be appreciated!

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Copper lines will not crack from normal residential water pressures. Don't worry about that. Only from freezing!

Most hydronic systems are designed for full flow so closing return valve will pinch off flow and increase pump energy use and decrease radiant heat capacity to conditioned space. Find out how the space thermostat controls the heat to the space however.
1. Does it modulate the mixing valve? Common method (Some mixing valves are only two position anyway).
2. Does it cycle the pump?

P.S. Note your system appears to have iron pipe to copper connections without needed isolation fittings. Expect electrolysis corrosion there.