Choosing which is the right wood burning sauna heater, flue kit and heat shielding for your specific sauna project can be difficult and it's easy to make a mistake if you don't know what you're looking for. Below are some of our frequently asked questions on which wood burning sauna heater to buy:


Which model wood burning sauna heater do I need?


The first thing to bear in mind when deciding which sauna heater to go for is the size of your sauna. The internal volume of your sauna will dictate which sized wood burning sauna heater you will require, as well as whether you will need heat shielding or not. For each of our heaters, the heater size to sauna size guidelines are included within the product specification table on each product page's 'Product Specification' tab. This is typically as follows (but varies model to model so please check the individual product):



Aito 16

Aito 16 Water Tank

Aito 20

Aito 20 Water Tank

Aito 20 Tunnel

Aito 24

Capacity of the sauna








It is important to note that 'actual' volume and 'computational' volume of your sauna may differ. The 'actual' volume is simply length x width x height, the 'computational' volume also takes in to consideration factors such as how well insulated the sauna is and whether there is a lot of glass used in the design. Saunas with a large amount of glass and/or little insulation will require a more powerful heater. Lack of insulation, such as a barrel sauna can result in as much as 50% extra computational volume. With glass, a rule of thumb is ever square metre of glass adds a cubic metre of computational volume. If you have any question regarding volume calculations, simply contact us and we'll be happy to advise.

If your saunas computational volume is borderline, in most cases recommend rounding up and using a larger heater. It is always easier to slow a burn or fuel a wood burning sauna heater less, than to have a fully stoked heater that doesn't get to the desired temperature.

We strongly suggest you consult a suitably qualified plumber or HETAS engineer for the installation and flue connection for these heaters. It is worthwhile having an initial consultation to discuss the route of the flue and ensure that you adhere to flue and fire safety regulations.


What kind of flue do I need for my wood burning sauna heater?


We recommend using either the Kota Steel Flue Kit 15 or Kota Steel Flue Kit 24 as both are designed for use in saunas and provide everything you need to install in either a single story (in the case of the 15) or two-story building (in the case of the 24). This includes all the gaskets and flanges to ensure the flue is safely installed through the roof spaces and provide a watertight seal against the roof itself.

Whilst other flue kits can be used in conjunction with Narvi, Aito and Kota wood burning sauna heaters, we recommend the Kota Steel Flue Kit's as it is designed to fit all three brands without the need for adaption. The flue must be at least T600 specification, suitable for continuous flue gas temperatures up to 600°C, for all Narvi, Aito and Kota wood burning sauna heaters. Lower-rated flue kits MUST NOT be used.

We recommend your flue kit is installed by a HETAS certified plumber or heating engineer with experience installing chimneys.


Do I need heat shielding for my wood burning sauna heater, and if so, how much and where to have it?


Heat shielding is required where the wood burning sauna heater's minimum safety distances to combustible material cannot be met. For example, if the benching or wood panelling is closer to the heater than the minimum safety distances, heat shielding would be required. Similarly if the heater is on a wooden floor, a heat shielding insulation base would be required. We recommend where this is the case, that you use a Kota Heat Shielding Kit and installation base.

The free-standing twin layered steel design is perfect for ensuring the heat is not transferred to the panelling behind, and being freestanding, allows you access to ensure there is no scorching - something not possible with wall mounted heat shielding. It is important to be able to check this periodically to ensure that no scorching has occurred and allow you to prevent sauna fire.

The need for heat shielding can be removed where non-combustible materials are used on the walls and floor around the heater. We recommend that the wall itself around the heater is made from a material like breeze-blocks that can be tiled or clad. Tiling or cladding a wooden wall could result in scorching and fire occurring behind it that would not be visible and thus a fire risk. We recommend that tiles in the sauna be made of stone or porcelain to prevent cracking at heat.


What about the wood burning sauna heaters with water tanks or tunnel models, how do they work?


A tunnel sauna heater allows the wood burning sauna heater to be fueled from outside the sauna, in an anteroom or adjacent room. This allows your wood burning sauna heater to double up as a wood burning stove in another room, with the flames pleasantly visible. The tunnel is easily adjustable to fit a range of wall types and thicknesses. With the tunnel model heaters, there is no need to bring log fuel into the sauna itself, helping to keep the sauna cleaner. However, this in turn means the pleasant light from the sauna fire would not be visible in the sauna.

Water tank heaters allow the user to heat a large amount of water in the sauna cabin. The purpose of this is typically for bathing and washing within the sauna. Sauna heaters with in-built water heaters are particularly useful in off-grid cabins or where there is no access to hot running water, such as an outdoor/garden sauna cabin. Whilst not all of our heaters have the option to have a water tank in-built, there are other options to provide hot water boilers in off-grid cabins, these include flue collar water boilers and sauna cauldrons (these are particularly popular in Finland). The water in the heaters is extremely hot (boiling hot!). Care should be taken when getting water from the hot water boiler and this water should be mixed with cold freshwater before use for bathing.


What kind of sauna stones/coals should I use on my sauna heater?


We recommend using Diabase (aka Dolerite), Olivine Diabase, Gabbro or Olivine. Ceramic sauna stones can be used, they are more durable but the heat is not as soft and pleasant. At Finnmark we only sell grade A Finnish Olivine Diabase for most of our wood-fired sauna heaters. For heat storing sauna heaters such as the Aito Kiuas AK47-110 range, we use Olivine and Ceramic stones.

You can also use cover stones, these are purely aesthetic and add to the style of the sauna, particularly used for modern sauna designs where contrast is required.


How big a stone capacity should I have for my sauna heater?


We at Finnmark believe sauna should be done in the traditional Finnish way. Following this doctrine, the Finns say that the more stone the sauna heater has, the softer and more pleasant the löyly (sauna steam/sauna atmosphere). From our experience this is very much true, therefore, the answer to this question is: The more stones, the better!


How many kilograms of sauna stones do I need for my wood burning sauna heater?

Each heater is different and the amount is included under the "product specification" section on the product page. For more information on how many sauna stones each of our sauna heaters takes, see the sauna stones product page.


Can I pour water over the stones on my wood burning sauna heater?


With all Narvi, Aito and Kota wood burning sauna heaters, not only can you throw water on the stones, it's actively encouraged! These sauna heaters are designed to withstand generous ladles of water on to the sauna stones during use. This is what creates the unbeatable löyly characteristic of all Narvi, Aito and Kota heaters. Just make sure you have correctly arranged your sauna stones to ensure the majority of the water evaporates from the stones and minimal water reaches the firebox below.


Why should buy a Narvi, Aito or Kota wood burning sauna heater?


Once you learn more about our sauna heater brands you'll be asking "Why would I buy anything else!".

Learn more here!


Any other questions?


If you have any other questions, please do get in touch through the Contact Finnmark page. You can also send us an email or call us on +44 (0) 1768 489273 (UK North) or +44 (0) 20 8050 2895 (UK South). Don’t delay, get in touch today!

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