Been working on a walkin freezer, this is a 404a system, the problem is the coil has to much ice build up and running to warm I had problem with defrost control got that fixed coil problem and got that fixed. I put my gauges on today and put small charge to it I was running 150 head and 10* coil with 20psig. cooler was coming down, went back 6 hours later and running 160 head 20* coil 15psig with the cooler running38* inside I still have to much ice buildup I have it defrosting every 6 hours for 40 min. temp. control is set at 10* any ideas would be greatful. I work on hvac more than refrigeration. Thanks. Went up this morning and unit running good, down to 10* inside cooler but still have to much ice build up on evaperator do I need to go to 4 hours defrost or could there be something else going on?