Hi guys, ive got a problem with a walk in freezer . Evaperator freezing up every 3-4 days. Heater ok, refrigerant ok, controller new, exponsion valve new, its a small box 3 metre square. It starts icing up from the top right hand side, near the exponsion valve. Valve iced up as well. Note heater is at the bottom of coil on drip tray. Ice build up on back of coil so looking on coil when walk in freezer the front is clear of ice. Then it work its way to the front of coil. By this time temp drop from -18 -10 and below. controller set up as standard. Drip time defrost cycle ect. Any ideas would be appreciated thanks guys.