I have a customer who owns a bar with a walk in cooler with R-409a, the evap was freezing up, I checked the system out, pressures and temps look fine, no defrost as its a cooler for beer and some food for the kitchen, medium temp 38-42. The first time there I saw a few bubbles in the sight glass on start up with a warm box customer shut it off earlier to thaw the coil for me. I blew out the condenser in the basement, checked the evap coil, checked the charge, all looked good the glass cleared shortly after start up, all looked fine with the charge, txv seems to be controlling superheat ok, has a danfoss txv. The top 1/4 of the coil froze up the next day and the box didnt come down to temp stopped at around 50. I ordered and replaced the txv adjusted the super heat, all looks ok except my high pressure is 180. The basement is HOT, 85-90 for sure and thats without the condenser running. So far as I know the coil has not froze up again, the customer said it would freeze up once in a while before but they would just shut it off thaw it out and it would be fine. My thinking is that with this condenser and the other two for the keg cooler and and the reach in cooler behind the bar all running down there that it is warm enough around the condenser to cause high enough discharge pressure so even though the txv is closed down as much as it can the higher head pressure is still forcing more refrigerant through the txv than needs be which explains why it sometimes freezes up and usually only during hot weather. Does this make any sense to anyone else, or am I way off? If I am correct would it be sensible to install a defrost heater and defrost stat? Or is there another solution?? A water cooled condenser in the basement would have made moere sense to me but I didnt install the system just responsible to keep the antique stuff running here! Always a challenge which I love.