The heater is the heart of a sauna, like Erkki Helamaa states in his eponymous book. If the warm stone heart is not working properly, then neither is the sauna. A sauna heater heats up the hot room and the bathers, but also turns the water that is thrown onto the hot stones into steam.
Heaters are roughly grouped according to their heat output and heating method (RT 91-10475), to name a few elements. Typical energy sources include wood and electricity, but special heaters that use LPG, natural gas or fuel oil are also available.
Wood-burning heaters can either let the smoke in (heat travels inwards) or out through a chimney (heat travels outwards). Furthermore, heaters can either store the heat from a single heating for the duration of an entire bathing session, or they can be heated continuously throughout a session.
A heat-storing heater is heated up before bathing, and the heat stored in a large amount of stones will be sufficient to last for the whole bathing session. In other words, the stones act as a heat storage. Heating a sauna with a heat-storing heater usually takes a few hours.
Chimneyless sauna heaters
The stones of a chimneyless sauna heater, i.e. a heater that lets the smoke into the hot room, are heated by the flames, hot air and burning gases from the combustion chamber below. During the heating, the smoke initially escapes through an open door, and later a hatch near the ceiling or a funnel through the roof. The hot smoke that lingers in the sauna heats up the walls and ceiling, which together with the heater itself will radiate constant warmth to the bathers. A chimneyless smoke sauna takes a few hours to heat, sometimes several. A warm chimneyless sauna has a faint scent of smoke, resin and tar. The activated charcoal that accumulates on the surfaces cleans the air efficiently. A so-called ruins heater (rauniokiuas) can be constructed in a traditional way using stones but no mortar, or from bricks or slate, to form a dome over an arched combustion chamber. The RT building code guideline card (RT 91-10493) contains the instructions on how to build a smoke heater from brick and mortar. Nowadays, heaters for chimneyless saunas are also available commercially.
A couple of centuries ago, a chimney was added to the sauna, allowing the smoke to be let out. Since then, sauna heaters have gone through several evolutionary stages. As the wood is burning, the resulting hot air and fire gases rise and warm up the heater’s stones. The lowest and largest stones warm up first, allowing heat to pass from stone to stone through convection as well. The smoke escapes through a flue on the side or at the top of the heater. The position of the flue damper, located in the chimney, can be altered to adjust the draught in the combustion chamber and close the flue once the heating is complete. The amount of stones depends on the heater’s output and model, and varies from a few dozen to several hundred kilos. Larger amounts will take several hours to heat up. Heat-storing heaters have a closed design. Its brick or metal outer shell and the hatch or lid of the stone compartment is insulated to retain heat in the stones. The RT building code guideline card (RT 91-10475) contains instructions on how to build a heat-storing heater from bricks and mortar. Commercially made heat-storing sauna heaters are also available.
Continuously heated wood-burning heaters
Continuously heated heaters are also heated before the bathing begins, but the heating may be continued during the bathing as well, so that heat is provided for as long as wood is being burned. In order for a continuously heated sauna heater to work as intended, the stones must be separated from the combustion chamber. Different designs use differing solutions to achieve this. The idea is that the flames and smoke must not touch the stones, and cannot therefore pass through the layer of stones, unlike in heat-storing heaters. Instead, they circulate through channels, travelling between the stones and cavities inside the outer shell. The stones are partly heated by the hot air through convection, partly as conduction from stone to stone. That is why the stones must simultaneously have sufficiently many gaps and form enough heat bridges. Continuously heated heaters that store heat are also available. Such heaters typically only require a single load of firewood, and they function similarly to an ordinary continuously heated heater at the start of bathing sessions, but once the flames die down, the stones remain hot for a long time like in a heat-storing heater.
Electric heaters can either be continuously heated or they can store heat. The former option is the most common type, making it the most typical form of electric heater. Continuously heated heaters are open heaters. This means that circulating air rises from bottom to top between the hot heating elements and becomes warmer. The hot air then heats up the stones, but heat is also transferred through conduction outwards from the stones nearest to the heating elements. The difference between heat-storing heaters and continuously heated heaters is that the former are insulated and have a closed design. During the heating, heat is stored in the stones and released to the hot room after the hatch has been opened. Electric heaters consist of a body and an outer shell, heating elements, a stone compartment with stones and a link box. The heating elements are located either between the stones or around them. The stone compartment is typically made from stainless steel. The heater’s operation is controlled by a control centre, a timer, a thermostat and a temperature limiter. The thermostat will cut off the current when the heater reaches the pre-determined temperature to prevent it from overheating. Various types of new special heaters that use electricity are constantly introduced to the market. One example is a flat continuously heated heater constructed using stone elements made from soapstone or granite. The heating elements are located behind the mass of stone, so that when water is thrown onto the stones it will not come into contact with the hot heating elements.
Source: Erkki Helamaa, Kiuas, saunan sydän, Rakennustieto Oy, 1999.
The information on this page was compiled by Raili Vihavainen.