How Does a Nordic Bath Work?

The principle: heat from fire, water and a little patience

The operation of a Nordic bath can be summed up in one sentence: a heat source warms the water in a large wooden tub, and physics does the rest. No jets, no bubbles, no pressurization. The main advantage of a bath is its silence, somewhat like a bathtub, but with much more water. With a wood-fired model, you don't "turn on" the bath, you prepare it, like you would prepare a campfire. Simple operation often remains the best.

Three elements are enough to understand everything. The tub, Nordic baths made of Red Cedar wood at Størvatt, holds several tons of water (1.5 to 2.8 depending on models) for 2 to 15 bathers. The heat source transfers its energy to the water. And finally, heat is distributed throughout the volume by convection. Filtration, cover or lighting are only options around this trio. 

Convection: the invisible motor

The operation of a submerged furnace relies on an ancient physical phenomenon: hot water, being less dense, rises, and cold water descends to take its place:

1. The fire heats the walls of the furnace, completely submerged in water.

2. The water in contact warms up and becomes lighter.

3. It rises to the surface.

4. Cold water from the bottom descends toward the furnace to take its place.

5. The cycle repeats until the entire tub reaches the target temperature.

A detail you discover with use: as long as you don't stir, the water at the top can be several degrees warmer than the water at the bottom. A paddle passed before each session harmonizes the temperature throughout the tub. 

Wood, electricity or hybrid: three ways to heat your Nordic bath

The heating method changes how you experience the bath. At Størvatt, three configurations exist, and a heat pump is offered on certain models. 

With a COP of 3.5 to 4, a heat pump consumes approximately four times less than a standard resistance heater. This matters if you maintain hot water all year round.

From ignition to bathing

Daily operation is a ritual. Starting from water at 10 °C, it takes between 1.5 and 3 hours to reach 38 °C. Duration depends on outside temperature, wood quality, and your presence to refuel. To put in perspective: heating 1,000 liters from 10 to 38 °C requires more than 30 kWh. This is why a few reflexes matter more than power: 

• Light from the top, with the driest wood possible, to limit smoke at the chimney outlet.

• Start with softwood (poplar, birch), which produces flames quickly.

• Switch to hardwood, like oak, once the target is reached: its lasting embers maintain temperature.

• Play with the furnace cover: when closed it smothers the fire, when open it relaunches heating, when ajar it maintains temperature.

• Cover the tub with the insulating cover, from filling and after each bath.

Target 38 °C, a simple reference point: since the body is at 37 °C, water feels cool below this and frankly hot above it. The ceiling, however, is not debatable: 40 °C. A floating thermometer will help you get the right gestures, and one or two logs within reach maintain heat without leaving the bath. If the water is too hot, let the garden hose run for a moment to evacuate impurities, then submerge it in the tub. If water overflows when you enter, this presents no danger. Mastering this operation takes two or three sessions, after which you can tell by eye and the sound of the fire where you stand.

Water: the other half of operation

Heating is only half the issue: hot water promotes bacteria and algae, so it must be treated. The operation of the filter and disinfectant duo determines water lifespan:

• Without treatment or filtration: the duration of a weekend, no more.

• Portable skimmer and active oxygen: 10 to 15 days depending on use.

• Permanent filtration: 1 to 3 months.

Filtration is an option on wood-heated models. The classic mistake is believing the filter is enough: it retains particles, but kills no bacteria. Before each session, a test strip (pool type) allows you to check pH, alkalinity, stabilizer and disinfectant. And the most effective action remains free: a soapy shower before entering.

In a new tub, water often turns red. Don't worry: these are Red Cedar tannins, which fade after a few months.

A Nordic bath, not a spa

The operation of a spa is quite different: jets, bubbles, sometimes light therapy, with heating, filtration and water pressurization systematically associated with the tub. A spa must remain filled all year, with minimal filtration. A bath, however, is a basin of heated water, without jets, which can remain empty under certain conditions. 

All year round: heat waves, freezing and hoops

The bath's operation changes little with seasons, but a few precautions matter. Many of our clients occupy their second home one or two months per year: the rest of the time, the bath can be emptied. During heat waves, simply leave a little water at the bottom, otherwise the wood dries and takes several days to regain its water-tightness.

On the freezing side, water can freeze completely without damage to the tub or furnace. Only the technical unit and PVC parts fear the cold: purge them if absent for extended periods. Finally, after a long period empty, tighten the hoops when restarting, then unscrew the same number of turns once the wood has expanded.

Safety and health

Remember that heat is vasodilatory. We strongly advise against bathing for pregnant women, young children, and people suffering from cardio-respiratory conditions, and we advise against alcohol during the session. Plan for 15 to 30 minutes, and exit at the first signs: dizziness, palpitations, nausea.