When the door closes, the warm air that came in cools and contracts, and because the room is sealed a vacuum builds inside. According to Danfoss, if the pressure is not equalised the door is hard to open and in extreme cases the walls and ceiling can be damaged. A small difference still catches the door: 100 Pa across a 1 × 2 m door leaf is about 200 N, close to twenty kilograms of force. The pressure relief valve removes the difference by letting air pass into and out of the room.
| Symptom | Likely cause |
|---|---|
| Door is hard to open after closing | Inside pressure has dropped; valve missing, blocked or frozen |
| Door sticks after defrost ends | Air that warmed and expanded during defrost is cooling again |
| Constant air noise at the gasket, panel bowing inward | The difference is trying to pass through the gasket and panel |
In rooms below 0 °C the valve is heated, because the humid air passing through it freezes and an unheated valve then works as if it were not there. At the monthly check look for ice at the valve mouth, confirm the heating circuit works and that the outer opening is not covered. Forcing the door tears the gasket, and fixing the valve open lets humid air into the room all the time. If the door is hard to open, record the symptom, the time and whether a defrost was running, and pass it to the cold room service; do not take the valve apart.