0.5 ach is a good number! ((0.5ach)*(500,000*30'))/60 = 125,000 cfm.
125,000cfm*1.08*20 = 2,700,000 btuh assuming you are using a 20 degree delta T. This transalates to 270 gpm additional water flow to pick up just infiltration, assuming you are circulating water to terminal units and that it...