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
[EMAIL PROTECTED]
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