coolant smell

Can't figure out this coolant smell through the AC/Heater. Recently replaced O rings on hard pipe to engine and temp sensor that were leaking a month ago. Now when I turn on AC I smell coolant for a few seconds and if I turn the temp up (ac down) I smell coolant for a few minutes. Haven't found any coolant on the floorboard and coolant level hasn't changed. Appears that I have a heater coil leak, but can't find evidence. Any suggestions?