Where Does Sauna Heat Actually Go? Flued vs Flueless Energy Balance

Every sauna heater loses energy somewhere. The question is how much and where. An open fireplace sends 80–90% of its heat up the chimney; a well-maintained wood stove loses 20–40%; a sauna stove, according to independent testing, loses 28–43% to chimney exhaust. In contrast, a flueless gas sauna retains virtually all combustion heat—including latent energy in water vapour—inside the room. This difference in energy balance is why a flueless 8.1 kW heater outperforms a flued 10 kW stove, and why you should understand where your heat goes before choosing a heater type.