Make sure that the safety distance requirements are met.
Check the heater stones. If the stones are stacked too tightly, compacted over time or the wrong kind, air circulation in the heater may be blocked, which can cause overheating in the structures (especially when an electric heater is used).
It is perfectly normal for the wooden surfaces of the sauna to darken over time. The darkening may be accelerated by
– sunlight
– heat from the heater
– protective agents on the walls (protective agents have a poor heat resistance level)
– fine particles coming off the heater stones and lifted by the air flow
Wood-heated saunas: smoke that enters the sauna when adding firewood, for example