Walk-in refer frosting over.

Hi all. I have a small BBQ restaurant in an old building with an old walk-in. And I know a little of the basics about refrigeration, but not much. About 7-8 months ago, the unit on the ceiling started icing over so the guy I was having service it installed a timer that was set to run a defrost cycle (you can see the grey box on the left wall) and that worked for a while. A couple weeks ago, the ice started again, and now it’s getting bad. The temp started rising a couple weeks ago, and I figured it’s been a while and just needed to be topped off with Freon. That did fix the temperature, but now it’s starting to creep back up, and it’s sitting solid right at 40°, which is making me very nervous. What causes this, and is there any thing I can do myself to treat this? Please let me know if you need any more info. Thanks in advance. submitted by /u/SmokeAndMeatBBQ [link] [comments]