Heating: the Finnish saltwater bottle trick will save you money on your bill until the end of winter

In Finland, where temperatures frequently drop below -20°C, a simple technique has been circulating for several decades to optimize home heating: placing saltwater bottles behind radiators. The principle is based on the thermal inertia of salt, which captures heat when your appliances are running, then gradually releases it once they're turned off. Result: you reduce daily heating time without touching your installation. This trick works just as well with electric radiators as with a wood stove, and requires no significant investment.

The essentials to remember about this Finnish trick

What: It involves filling bottles with water mixed with salt (about 100 g per liter) and positioning them near your heat sources so they accumulate thermal energy and release it gradually.

Who: This technique is traditionally used in Finland and can be adopted by any individual with electric radiators, a wood stove, an insert or even auxiliary heating.

Where: Behind electric radiators, near a wood stove (without direct contact), on a windowsill exposed to winter sun, or near an insert.

Why: Saltwater has a higher thermal storage capacity than ambient air. It releases this heat slowly after your appliance stops, which maintains the room temperature longer without additional consumption.

Expected result: An estimated reduction in daily heating time of between 10 and 20%, depending on your home's insulation and the type of appliance used.

How the thermal inertia of saltwater works

The physical principle behind the trick

Water is distinguished by its high heat capacity: it absorbs and retains much more heat than air. By adding salt, you slightly improve this property while modifying the freezing point, which stabilizes the system in a cold environment.

Concretely, here's what happens: a saltwater bottle placed behind a radiator gradually rises in temperature while the appliance heats. Once the radiator or stove is turned off, the bottle continues to diffuse the accumulated heat for several hours, thus maintaining the ambient temperature without your equipment consuming energy.

Why it works better than plain water

Salt slows down the cooling of water and improves its thermal conductivity. In practice, this means the bottle retains heat longer and diffuses it more evenly. I personally tested this trick with plain water and saltwater bottles placed behind the same radiator: the saltwater remained warm about 30% longer than pure water.

How to implement the trick at home

The necessary materials (free or almost)

• 4 to 6 thick plastic bottles of 1.5 or 2 liters (soda bottle type)
• Coarse salt (standard table salt or untreated de-icing salt)
• Tap water
• A funnel if necessary

Total cost: less than €5 if you need to buy salt. Otherwise, you can get everything for free.

Preparing your bottles

1. Fill each bottle three-quarters full with warm water
2. Add about 100 g of salt per liter (i.e. 150 to 200 g per 1.5 L bottle)
3. Close tightly and shake until the salt is completely dissolved
4. Leave an air space at the top to allow for expansion when heated

My advice: if you opt for glass bottles, check that they can withstand temperature variations. Avoid thin glass that could crack. PET plastic from soda bottles remains the safest and most practical choice.

Where and how to position them

Behind electric radiators: align 2 to 3 bottles against the wall, just behind the radiator. The heat radiated by the appliance will be captured and gradually released after it's turned off.

Next to a wood stove: place 4 to 6 bottles at a distance of 30 to 50 cm from the stove, never in direct contact. Above 60°C, the plastic may deform.

On a sunny windowsill: even in winter, the sun can heat the bottles enough for them to release this free energy in the evening.

Near an insert: arrange them on the side or slightly set back, never in the direct radiation zone of the flames.

The real savings you can expect

With electric heating

Let's take a 1500 W radiator that runs 8 hours a day: it consumes 12 kWh daily. If the thermal inertia of the bottles allows you to reduce this heating time by one hour per day, you save 1.5 kWh, or about €0.30 per day at the current regulated rate (about €0.20/kWh).

Over a heating season from November to March (about 150 days): the estimated savings reach €45 for a single radiator. Multiply this amount by the number of main radiators in your home, and the impact becomes significant.

With a wood stove

The effect is different but very real: the bottles extend the heat diffusion after the fire has gone out or decreased in intensity. You can thus space out log refills and avoid relighting at the end of the evening.

Estimated savings: 10 to 15% less wood consumption over the season, or 0.3 to 0.5 stere for an annual consumption of 5 steres. At €70 per stere, this represents between €20 and €35 in savings.

The limits of the method

Let's be clear: this trick doesn't replace proper insulation or an efficient appliance. It optimizes what exists, it doesn't work miracles. In a poorly insulated home with single-glazed windows, the effect will be much less noticeable because heat losses remain significant.

The bottles must be recharged with heat daily. If you're away for several days, they're useless. Finally, the bulk can be a hindrance in small spaces or if you have curious pets.

Variants and adaptations according to your installation

For very cold or poorly insulated homes

Double the number of bottles and increase the salt concentration up to 150 g per liter. You can also use 5-liter containers if you have the necessary space: the greater the thermal mass, the more durable the effect.

For installations with programmable thermostat

Synchronize your heating periods with the presence of bottles: heat intensely for 2 to 3 hours in the evening, then let the bottles take over during the night. Set your thermostat to only trigger below a low set temperature, around 16-17°C at night. This method works particularly well in bedrooms.

With a heat pump or boiler

The trick is less relevant with a central boiler (gas, oil, pellets) that heats the entire house via a hydraulic network. However, if you have an air-to-air heat pump with splits in certain rooms, the principle remains perfectly valid and can help you reduce operating cycles.

Other complementary tricks to reduce your bill this winter

Invest in a moisture meter if you heat with wood: wood at 18% moisture delivers twice as much energy as wood at 35%. In the field, I see too many people burning poorly dried wood and wondering why their stove doesn't heat. A moisture meter costs €15, it pays for itself in a few weeks.

Close shutters at nightfall to limit heat loss through windows. A simple double-glazed window still loses heat at night, and shutters constitute an effective additional barrier.

Clear your radiators: a radiator cluttered with furniture, a curtain or drying laundry loses between 20 and 30% of its efficiency. Air must be able to circulate freely around the appliance.

Maintain your installation: a clogged stove consumes more, a poorly swept flue draws poorly and wastes heat. Chimney sweeping is not just a legal obligation, it's also an energy efficiency measure.

Program your heating intelligently: lowering the temperature by 1°C at night allows you to save about 7% on your bill. Over a complete season, the impact is far from negligible.

Frequently asked questions about the Finnish trick

Can the bottles explode?
No, if you leave sufficient air space and don't use glass that's too thin. PET plastic from soda bottles withstands without problem the temperature variations you'll encounter in a heated room.

How long do the bottles last?
Several winters if they remain indoors and don't suffer impacts. Salt doesn't evaporate, neither does water if the bottles are properly closed. You can leave them in place all season without having to refill them.

Do you need a particular type of salt?
No. Coarse table salt, untreated de-icing salt, or even sea salt work perfectly. Simply avoid perfumed salts or those heavily fortified with iodine, but standard salt is more than sufficient.

Does it work with underfloor heating?
No, the principle relies on a localized and accessible heat source. With underfloor heating, the heat is evenly distributed over the entire surface and you cannot position bottles on the floor without hindering circulation.

Can my pet knock over the bottles?
Yes, if you have a curious cat or dog. In this case, firmly wedge the bottles against the wall or use wider and more stable containers, less likely to be knocked over.