Humidistat installed...won't turn off

[Harry Bloomfield]

As I recently wrote to someone else, one of the problems of humidistat control is that, if the idea is to reduce condensation, there will probably already be quite a lot of it (condensation) by the time the humidistat senses the high humidity and switches the fan on (unless one has the humidistat set so low that the fan was on most of the time - which may be what you are currently experiencing!)

The work best, if used to keep the fan running, if the humidity is too high, rather than used to trigger the fan to run..

The sensors are inside the fan case, which when the fan is not running, will be in a comparatively dry atmosphere. However, with the fan already running, the sensor will be subjected to a much more representative level of humidity. Once the humidity falls, the running fan, also help dry the sensor, to switch the fan off.