Delta is commonly used to refer to a difference between two points. So in this case, a delta of 110 degrees means whatever your target temperature is, the radiator should be 110 degrees away from that temperature. Trying to reach 70° means a temp of 180 at the radiator when heating, or -40° when cooling. OP was pointing out that -40° obviously isn’t a feasible temperature for a water-based radiator, so they simply aren’t great for cooling.