Stoves With Side Water Tanks: What They Are For

What a side water tank is

A side water tank is a metal reservoir that hangs on or attaches to the side of a wood-burning sauna stove. The stove body gets hot when the fire is running, and that heat transfers into the tank, gradually raising the temperature of the water inside. The tank is usually filled by hand with a ladle or a hose, and it has an open top or a small lid so you can dip in a scoop.

Not every wood stove comes with a tank as standard. Many models offer it as an optional extra, and some stoves are sold with the tank already welded or bolted on. If you are choosing a stove now, it is worth checking whether the model you like has a tank option or at least a flat side where a tank could be mounted. For a closer look at stove types, see what is a sauna stove and what is a kiuas.

What the warm water is used for

The main purpose of the tank is to give you warm water right inside the sauna, without running a pipe or carrying a bucket from the house. In Finnish sauna practice, bathers often rinse off with warm water after sitting in the heat. A small wash with warm water cleans the skin and feels refreshing between rounds. The tank makes that easy.

The same warm water can be used to wet a vihta or a towel, to top up a bucket for pouring over the stones, or to rinse the benches if you clean them after a session. It is also handy for mixing with a bit of cold water so you can pour it gently over your head or shoulders without scalding. For more on water use in the sauna, see how to pour water on stones and scented water for ladle.

How the tank affects the sauna environment

The tank does more than store water for washing. As the stove heats the water, some of it evaporates into the air, adding a little extra humidity to the room. In a traditional sauna the air is normally quite dry, with typical humidity below 20 percent, according to Cleveland Clinic. The gentle evaporation from a tank can soften that dry heat slightly, which some bathers prefer.

That said, the tank is not a substitute for pouring water on the stones. The steam from throwing water on hot stones is what gives the sauna its löyly, the burst of moist heat that bathers feel as a wave. A side tank simply adds a steady, low level of moisture over the whole session. If you enjoy a wetter feel, you will still want to ladle water onto the stones. You can read more about the difference in dry sauna vs wet sauna.

Cleaning and upkeep

A side tank is one of the few parts of a wood stove that needs regular attention. Because it holds water, it can collect minerals from the water itself, especially if your tap water is hard. Over time, these minerals form a thin scale on the inside of the tank. The same kind of buildup is what steam generator makers warn about when they recommend regular flushing, as in Amerec's guidance to flush the tank monthly or more often depending on local water conditions.

To keep your tank clean, empty it after every sauna session. Wipe the inside with a cloth and leave the lid off so it can dry completely. Once a month, give it a rinse with clean water and scrub gently with a soft brush to loosen any scale. Avoid soap or detergent, since residues can leave a taste or smell in the water. If you notice a white film building up, a rinse with a mild vinegar solution can help, followed by a thorough rinse with plain water.

Check the tank mount from time to time. The bracket or hook that holds the tank to the stove should stay tight and free of corrosion. A loose tank is a hazard because hot water can spill. Look for signs of rust, especially around the seams or the bottom edge. If rust appears, replace the tank before it leaks.

Staying safe with hot water

The water in a side tank can get very hot, close to boiling if the stove is running at full strength and the water has been sitting a long time. Never reach into the tank with your bare hand. Use a long-handled ladle or a dipper to take water out. When you fill the tank, start with cool or lukewarm water so you know what you are working with before it heats up.

Keep children away from the stove and the tank, and make sure the tank is mounted firmly so it cannot tip. If the tank has a lid, keep it on when the stove is not in use so nothing falls in. If you spill water on the stove, it will evaporate quickly and may throw off some steam, so step back and give the water time to settle.

The same caution applies if you use the warm water to rinse off. Mix it with cold water first, or pour it into a bowl and test the temperature with your wrist. For more on managing heat and humidity safely, see sauna humidity and temperature terms and what to wear in traditional sauna.

Choosing a stove with a tank

If you are shopping for a wood-burning stove, a side tank is a nice feature to have, but it is not essential for everyone. Think about how you plan to use the sauna. If you like to rinse off between rounds or wash yourself after a session, a tank will make that much easier. If you rarely use warm water in the sauna and you shower at home afterward, the tank may just add a small cleaning task.

Consider the size of the tank too, though the capacity is rarely listed in the product specs. A larger tank holds more water, which is useful for a family sauna, but it also takes longer to heat and adds more weight to the stove. A smaller tank is lighter and heats quickly, and you can always refill it with a ladle. For more on matching a stove to your space, see how to size a sauna stove and wall-mounted vs floor-standing heater.

If you are set on a traditional wood-burning sauna experience, a stove with a side tank fits right into the Finnish way of doing things. If you are still deciding between wood and electric, read wood-fired vs electric decision before you commit.

What to pick for your use

Questions

Will the water in a side tank boil?

It can, if the stove is running hot and the water is left in the tank for a long time. The water will simmer and release steam, and it may eventually reach a full boil. That is normal, but take care when you open the tank or dip a ladle in, because the steam and water can burn.

Can I use tap water in the tank?

Yes, tap water is fine. If your water is very hard, you may see more mineral scale buildup on the inside of the tank. That is not dangerous, but you will need to clean the tank more often to keep it looking tidy.

Do I need to drain the tank after every use?

It is a good habit to empty the tank after each sauna session. If you leave water in it, the next session will just reheat stale water, and over time minerals can settle on the bottom. Emptying it and letting it dry also helps prevent rust in the seams.

Can I use the tank water to pour on the stones?

Yes, you can. The water in the tank is just warm tap water, and pouring it on the stones creates steam the same way cold water does. Warm water will steam a little faster, but the effect on the room is the same. Just be careful not to overdo it: too much water can cool the stones.

Do electric sauna heaters have side tanks?

No, side water tanks are a wood stove feature. Electric heaters do not have a hot metal side that would heat a tank, and they are usually designed to be dry. If you want warm water for washing with an electric heater, you will need a separate source, such as a kettle or a thermos.

Does a side tank make the stove heat the room more slowly?

The tank draws some heat away from the stove body, but the effect is small. You need extra energy to warm the water, so the room may heat a little more slowly at the start, especially if you fill the tank with cold water. Once the water is hot, the tank actually stores heat and releases it slowly into the room.