Why cold surfaces sweat
Air holds water vapour. When moist air touches a surface colder than its dew point, the vapour condenses into droplets. The outer glass and frame of a refrigerated door are cooler than the shop air, so on a humid day they fog up. On a freezer door, the frame and gasket area can be cold enough for frost or ice to form, which can stop the door sealing.
Coastal cities and monsoon months push the dew point up, so the problem is worse in Mumbai in July than in Delhi in December.
Where anti-sweat heaters are fitted
- Door frames and mullions, where metal parts conduct cold to the outer surface.
- Around the gasket seat on freezers, to stop ice forming and the door sticking.
- On or in the glass itself on some freezer and low-temperature doors, using a transparent conductive coating.
- Drain lines in some freezers, to stop defrost water freezing.
The double energy cost
An anti-sweat heater that runs continuously uses energy around the clock. Part of its heat leaks into the cabinet, so the compressor must work to pump it back out. For freezer doors, where heaters are larger and cabinet temperatures lower, the combined effect is significant. That is why the efficiency of a glass-door cabinet depends heavily on how its glass and frame are designed, not only on its compressor.
How designers reduce heater energy
| Approach | What it does |
|---|---|
| Double or triple glazing | Keeps the outer pane closer to room temperature, so it sweats less |
| Low-emissivity coatings | Reduce radiant heat transfer through the glass |
| Inert gas fill between panes | Lowers heat conduction across the glass unit |
| Thermally broken frames | Stop metal frames conducting cold to the outside |
| Humidity-based heater control | Runs heaters only when shop humidity needs it |
| Pulse or duty-cycle control | Cycles heaters on and off instead of running them flat out |
Controls that switch heaters intelligently
Simple cabinets run heaters at full power all the time. Better controllers measure room humidity or temperature and vary heater power to match. On dry winter days the heater may hardly run; on a humid monsoon evening it works harder. Some controllers also reduce heater power when the store is closed and a night blind or lights-off mode is active.
Ask suppliers how anti-sweat heating is controlled and how much power the heaters draw. If the answer is 'always on', expect higher bills.
A quick way to see the heater's share of the load is to compare the heater wattage on the wiring diagram with the cabinet's measured daily kWh. A heater of 20 to 40 W running all day adds roughly half a unit to one unit of electricity per day, before you count the extra compressor work.
Practical advice for owners
- Do not disconnect anti-sweat heaters to save power; ice on freezer gaskets causes air leaks and much bigger losses.
- Keep doors closing fully; a door left ajar lets humid air in and overloads both heaters and coil.
- Avoid pointing air-conditioning outlets or fans at glass doors.
- If doors fog constantly, have a technician check heater function, gaskets and controller settings.
Frequently asked questions
- Why does my visi cooler glass fog up only in the monsoon?
- Monsoon air carries more moisture, raising the dew point. The outer glass is then colder than the dew point and water condenses. Better glazing and properly working frame heaters reduce this.
- Can anti-sweat heaters fail without me noticing?
- Yes. A failed frame heater on a chiller may only show as fogging on humid days. On a freezer, look for ice building around the gasket or a door that is hard to open. Both are signs to call a technician.
- Do chilled glass-door coolers need heaters as much as freezers?
- Usually less. Chiller glass runs warmer than freezer glass, so many chilled coolers rely mostly on double glazing with small frame heaters, while freezer doors usually need more heating.
