The heater is thermostat controlled and triggered when the temperature of the incoming air drops bellow comfort level, the range options are between 10 - 20 degrees and the setting is activated by the user (controller is in the diffuser).
The heater should be considered as a means of tempering the incoming air not as supplementary home heating as it would be cheaper to compensate for incoming cool air by temporarily increasing the heating thermostat.
The basic PIV was designed for houses which have a high ceiling at the top of the stairwell the incoming air would be blended with warm air that rises to ceiling height through convection, the heater version was introduced for properties such as bungalows that may have the same air volume but do not have the ceiling height.
You are correct, it is possiblethat during a very wet period we may get periods of high external humidityparticularly if it is wet and warm as the air can retain water vapour andthe lack of air movement (breeze) makes the air clammy and uncomfortable.However, it would be during a time like this that windows and doors are morelikely to be opened to provide additional air change, the continuous air movement prevents stagnant airpockets and creates a drying effect.
Temporary periods of high humidity outside are not a cause for concern as the RH is relative to temperature and even if introduced into the home the higher internal temperature will reduce the RH, even rainy periods during winter do not generally reach very high levels of humidity because theair is less able to hold moisture as vapour and during very cold periodshumidity can drop to 25% - 35%, evaporation of rainwater after a fall iscreated less by heat evaporation but is usually caused through increased airmovement due to high winds (same effect as drying clothes on a line) and it isduring these periods when we want to keep all of the windows and doors shut,but continue (as needs require) to create water vapour, internal humiditylevels can be kept stable through strategic use of heating, insulation andcontrolled air changes.
I have tried to keep this answersimple but also hope it answers your question.