With i first cold ones and the heating is turned on like every November, an old “trick” is coming back into circulation to keep our domestic environments warm for longer: having one or more bottles filled with water next to radiators to have a warm room for longer, thus allowing us to turn off the home heating sooner and save something on the bill. The idea is that the water – thanks to its high thermal capacity – will absorb part of the heat emitted by the radiator and then gradually release it to the environment, even when the heating is turned off.
In principle the “trick” It makes sense from a physical point of view. However, it is worth underlining that, by implementing this practice, the result will drastically depend on the specific conditions in which one finds oneself, because the physics of heat exchange is really complicated and it depends on an impressive number of factors.
The “water bottle trick”: how it works
The explanation is completely similar to the summer “trick” of ice in front of the fan, with the difference that in that case you want to remove heat from the air, while here the aim is to distribute the heat transfer over a longer period of time. THEWater has a great capacity to retain heatin the sense that it is necessary to provide it with a lot of heat to raise its temperature and, on the contrary, it must give off a lot of heat to decrease its temperature. In technical jargon it is said that water has a high specific heator a high heat capacity.
To be precise, 1 kg of water at room temperature must yield approximately 4180 joules (about 1 kcal) of energy to see its temperature drop by 1 °C. For comparison, air at room temperature only needs to give up a little more than 1000 joules (depending on its humidity) to cool by 1 °C. The consequence is that water cooling lasts much longer than that of the air.
This is the same principle according to which coastal cities or cities close to large bodies of water tend to have milder winters: the seas gradually release the heat accumulated during the summer even in the cold months. The “trick” of the water bottle next to the radiator reproduces the same phenomenon of thermoregulation of water on a much smaller scale.
In any case, we reiterate that putting water next to the radiator does not increase the temperature of the room: Water absorbs heat that would otherwise have gone into the air. The advantage lies exclusively in the “slowness” of the water to release the accumulated heat, thus acting as a small heat source even with the thermostat off, thus allowing the heating to be turned off a little earlier.
How to practice the “bottle trick”
So that the effect is perceptible a sufficient mass of water is requiredfor example using multiple bottles or a single large bottle – even plastic – filled completely. Just for comparison, a 50m room3 contains approximately 60 kg of air; with the same temperature variation, approximately 15 liters of water are needed to accumulate the same quantity of heat that can be absorbed by this mass of air.
If you prefer to increase the humidity of the air you can use open bottles, otherwise closed bottles will release heat keeping the air dry. From a thermal point of view, humid air very slightly increases the specific heat of water (therefore making it slightly “slower” to accumulate and disperse heat) but above all increases the so-called “perceived temperature” – even if it is more correct to talk about index of discomfort – because it slows down the skin’s evaporation process.
In any case, to find out if or how much it works in your home the only way is to try!
