June 19, 2025, 9:04 am | Read time: 3 minutes
Using a simple frozen water bottle to combat summer heat: The so-called bottle trick is said to help cool indoor spaces without air conditioning. This DIY tip is currently being discussed on numerous social media platforms—but how effective is the method really?
When temperatures rise and air conditioners are either too expensive or unavailable, many turn to alternative methods of cooling a room. One such method: A PET bottle filled with water is frozen and then placed in the room. The melting ice is supposed to lower the ambient temperature. But does the bottle trick really work to cool the apartment?
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How the Bottle Trick Works
The process is quite simple. Fill an empty PET bottle with a volume of 1.5 to 2 liters about two-thirds full with water and seal it tightly. Then, place it in the freezer for several hours or overnight. Once the water is completely frozen, take the bottle out of the freezer and place it at a high point in the room, such as on a cabinet or shelf.
An additional effect can be achieved by placing the bottle in front of a fan. The resulting evaporative cooling can better distribute throughout the room. However, it’s important to place the bottle on a mat to prevent condensation from damaging furniture.
Cooling Effect Through Evaporation, but with Limits
The principle behind the trick is based on what’s called latent heat: When the ice in the bottle melts, energy is drawn from the surroundings, slightly cooling the room. Unlike damp sheets, which are also used for cooling, the humidity does not significantly increase. This can create a more pleasant room climate.
In theory, a frozen 1.5-liter bottle can achieve a minimal temperature reduction in a room of 15 to 20 square meters—provided the room is well-insulated, there are no people or running devices inside, and no additional heat enters. In practice, such ideal conditions are rarely met. The cooling performance is therefore limited.
Short-Term Solution with Limited Benefit
The effect of the trick is not only spatially limited, but it also doesn’t last long. Once the ice in the bottle has melted, the cooling effect ends. Especially in large rooms, the effect is barely noticeable. Additionally, there is the energy expenditure required to freeze the water and use a fan. The efficiency of this method is therefore overall low.
As a short-term measure for smaller rooms, the frozen bottle can still provide relief. It is also recommended to use several bottles alternately to maintain the effect longer.
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Not a Replacement for Air Conditioners, but a Small Helper
The bottle trick offers a simple and inexpensive way to cool an apartment at least temporarily during summer heat. It does not replace an air conditioner but can contribute to improving the room climate when combined with other measures—such as darkening rooms or ventilating at night. Users should not expect miracles but should be aware of the physical limitations of this method.