Especially in cases of a home which is heated by a non-circulating method like baseboard or in-floor heat, there is the need for mechanical air circulation to the exterior. Many homes of this design are also equiped with Humidistats which control the operation of the main bathroom fan.
In many cases, I find that owners will turn off this Humidistat due to the noise of the fan. These controls must be set at 50% at all times. The device will automatically turn on the fan to exhause interior air when humidity readings are beyond the set level, then turn off when within range.
If the home is unoccuoied for a period of time, be sure to maintain a low level of heat to prevent humidity build up.
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