Perhaps this excerpt from an article I wrote about heating systems for boats may summarize some of the many contributions on the subject. My article was published in "Sail" in Sept 1977 and "Motorboat" in October 1977. “One caution that should be common knowledge which always bears repeating: never use an unvented combustion-type heater in a closed cabin. The fire uses oxygen, the same as people do. People go to sleep with such a heater on. The heater uses oxygen. The oxygen level goes down. The heater can’t get enough oxygen. The combustion process puts out carbon monoxide. The people begin asphyxiating. Their central nervous system tells them to breathe more deeply. They do. They breathe in huge gulps of carbon monoxide. Then they die. The proper solution to this fatal possibility is a properly vented system. “Many fuels produce water vapor as a combustion byproduct, and an unvented heater aggravates the dampness aboard a boat. On the other hand, a properly vented combustion heater dries the air. As air is exhausted up the chimney, fresh air is drawn into the cabin. As this air is warmed, its relative humidity decreases. This drying effect is actually typical of any heater which causes fresh air to enter the cabin even when the outside air is damp. “If cool, damp outside air is warmed up, the water vapor is dispersed in a larger volume of air and the air feels drier. Look at tables 16 and 17 in Bowditch. If the outside temperature is 50 degrees and the relative humidity 100 percent (saturation, visible mist), warming the air to 74 degrees will drop the relative humidity to a comfortable 43 percent. A boat cabin probably won’t get that dry, as the outside air will mix with the humid inside air and breathing and cooking will add moisture to the air, but the difference will be noticeable and pleasant. Solid fuel stoves will make a greater difference because they use more air but they also use more fuel, given their chimney loss. However, the lower cost of wood or coal makes that greater fuel consumption acceptable. “Unvented heating systems, such as some electric systems, will cause an initial drying of the air as the cold damp cabin is warmed, but when the temperature stabilizes and breathing and cooking add moisture, the humidity rises. Then when the temperature is lowered at night for sleeping, the cooling air can’t hold as much moisture and the humidity reaches the saturation point. Then water condenses on ports, overheads and the inside of the hull. The dripping calls for dehumidifying.” John R. Butler theoldcat@cox.net