We have kept our B34 on the water in Liverpool marina, during the winter, since 2002. We bought our dehumidifier from Argos and it has a control knob for how much moisture it will remove and a seperate switch for the two speed fan.
After the summer season, when the boat is kept cruising or on her mooring, without the use of a dehumidifier, the top draw at the aft end of the table just starts to catch, a sure sign that the boat is getting a bit on the damp side. Charts and bedding also take on a damp cold feel.
I start the winter by setting the dehumidifier on maximum and the fan on high. The container fills up in about a week and when full it automatically switches the unit off. This process is repeated and the amount of water gathered in a week reduces after about 6 weeks. At this stage i put the fan onto low and turn down the unit to about 60% of maximum capacity.
The unit does not work when the temperature is too low, but i have found that there are very few occasions on which this happens so i would not bother paying a lot of money for a unit that can operate at low temperature.
If you cannot visit the boat on a weekly basis you will need to connect the water outlet into the sink.
We make sure that all cabin doors are lashed open and we stand cushions on their edges and make sure all vents are closed.
We leave all bedding, charts etc on board throughout the winter, even some sailing clothes and to date the only problem we have had is that a few items of heavy cotton sailing gear have developed mould spots. For some reason the same items just do not dry, yet items immediatly next to them are as dry as a bone.
The dehumidifier has been the most important single item in keeping our boat in as new condition and we would not dream of leaving her without one during the winter. We have never bothered with the heating system other than to keep us warm, whilst working on the boat in very cold conditions.
It is possible to dry the boat out too much and the sign to watch for is unvarnished timber appearing at the edge of inset panels in the joinery or any position where the interior furniture is likely to move.
I have also tried using two small 12 volt units during wet summer cruises, and whilst these worked to some extent, they did not do as good a job as the heating system combined with good ventilation. I also tried them during the winter with one in each of the sleeping cabins whilst the main dehumidifier was in use in the saloon. The result was that the 12 volt units collected next to nothing whilst the mains unit did its normal job.