Hello; The relative humidity is very often low during the winter months compared to the warm months. (Think of the static shocks you get during cold weather.) The problem of excess moisture in a home during the winter has a lot to do with the house being closed up. Houses can be very tight now due to better insulation and seals around windows and doors.
A heating system with a hot fire will have some way to exaust the fumes to the outside. The flue draft will draw some air out of the house and thus pull in some of the drier outside air in addition to the drying effect of the hot fire itself.
I have not lived in a house with a hot fire and that is direct vented, so cannot say for sure about that type of heat. A direct vent type heat will pull the air to feed the combustion from the outside and not from the inside of the house. This keeps heated air from the inside of the house being pulled out the flue. The house can be sealed tight. There will be radiant heat from the hot fire and that should dry out the inside of the house to some extent.
A house with a heat pump, like mine, does not draft inside air to the outside at all. If manage to seal the house tight, the air will mainly be recirculated thru the heat system with very little exchange of the outside air. (Some exchange when doors are opened and closed, bathroom vent fans and kitchen vent fans, dryer vents and some minimal exchange around the door and window seals.) This is where a moisture source can be a problem in the cold months. My tanks evaporate more water in the winter and that moisture will stay largely inside. If I manage to set up the remaining six tanks that are now empty, the problem will be even worse.
I am looking into a coal/wood burning heater to use as auxillary heat and for emergency heat during a power outage.
