The reason for the powerful heaters on most reach-in cooler and freezer doors is to prevent fog or condensation on them. When the glass in those doors becomes fogged, the products behind are less visible to the customer, so either the store loses sales or customers open more doors for longer times to better see their choices, thus leading to increased air infiltration and wasted energy. When condensation forms on the doorframes, it is unsightly and can drip onto the floor, causing a safety hazard. The good news is, constant heat is not required to prevent condensation on all doors at all times.
Freezer doors, being colder than walk-in cooler doors, are more prone to condensation and need more heat more often. Both are more likely to “sweat” during summer weather, when the air in the store is more humid, especially when the store is not air-conditioned. This is because AC both cools and dries the air, making condensation less likely.
Most reach-in doors have electric heaters that warm the glass and the doorframes enough that their exterior temperature is above the dew point of the air in the store, and condensation is thereby prevented. Many pass-through doors without glass have heaters only around their perimeter. Usually, door heaters are on 100% of the time, regardless of the need for heat. When the air is dry enough for moisture not to condense, but heat is applied anyway, it is a total waste of energy. Actually, it is more than a total waste. Much of the heat reenters the cooler or freezer, adding to the cooling load of the compressor, and causing it to run unnecessarily. Whenever the electrical energy used to operate the door heaters can be reduced, a corresponding “compressor bonus” savings of perhaps an additional 30% (for walk-in coolers) to 50% (for walk-in freezers) will also be achieved.
The relative humidity (the percentage of the total moisture a given sample of air could hold at a given temperature) and the dew point (the temperature that air would need to be for condensation to begin) of the air in the store are constantly changing. For much of the year, the air is dry enough for door heaters to be completely off, with no condensation problem at all. Sometimes, a storeowner will manually switch off the circuit breaker that energizes the door heaters for an entire winter and switch them back on in the spring, thereby avoiding much unnecessary energy waste. However, remembering when to make these changes can be difficult, and the door heaters will often remain on unnecessarily, or off when they should be on.
Relative Humidity Bins
There are ways of controlling door heaters more automatically, however. One way is to install a simple humidistat that measures the store’s relative humidity and operates a relay to energize the door heaters whenever a user-adjustable setting has been reached. This keeps the door heaters completely off when they are not needed at all, perhaps 10-40% of the time, but will still lead to 100% heat being applied even when a small fraction of that is all that is sometimes necessary. Here is a graph showing the profile of the relative humidity of the air inside a Massachusetts liquor store. You can see that the time when some door heat is needed, the shaded area starting at 50%, is really a not much of the year.
A more efficient method than simply turning the door heat completely on when the relative humidity reaches a certain setting is to use an electronic controller, such as a Freeaire® Cooler Controllertm. It has a sensor that measures the actual relative humidity, temperature and dew point of the store air. That way, the heat applied to the doors can be proportional to the need for that heat. Using “pulse width modulation” (PWM), the heaters are switched on for a varying number of seconds each minute. More humidity leads to longer pulses of door heat. Efficiency is increased because only the minimum amount of energy is used to prevent the problem. Instead of 100% of the door heat being applied, 10%, 30% or whatever is necessary to prevent condensation, can be brought to bear.
Look at this chart from that same liquor store using a Freeaire® system showing actual door heater energy use (orange line) over a year’s time through varying levels of relative humidity inside the store (blue line). Door heat is never completely on, but rather only on as much as necessary. The annual door heat savings are a whopping 88%. And, don’t forget the previously mentioned 30-50% “compressor bonus” that comes with that door heat savings.
Another way to prevent condensation on cooler doors is to install “zero-energy doors” that are so well constructed (with up to three layers of heat-reflective, gas-filled or low-e glass and thermal breaks in the frames) that they don’t need any heat at all. This will save energy in the long run, but can be a significant increase in the up-front investment. Freezer doors are available with enhanced insulating features to make them more energy efficient, but still require heaters.