I know, there are variables, but approximately what should be the difference in temperature between return air and register air? I have a brand new system and it is running almost constantly on hot days. The installer came back and checked the charge but that was in late January at it was barely 70 degrees outside. Should I insist they come back again? My difference in temp from return to register is 15 degrees. I measured the register air at a register halfway down the trunk line in order to get the average when taking heat gain from the attic into account. Thanks in advance.