My A/C has not been working well. I went to the attic to check it
and found the hydronic coil warm (estimated around 110 oF). From
the water heater, I closed both the inlet and outlet valves from
and to the hydronic unit on the attic.

I know almost nothing about hydronic heater. My question is: is it
safe to close the valves? Would closing the valves hurt anything?

Is the pump running, is it still heating, turn it All Off.