Why are heater installation
guides recommending that you use outside air for the combustion air?
I can see one reason why for natural draft
heaters. I have personal knowledge of at least 3 boats with Dickinsons installed where above a certain windspeed the dodger
creates enough low pressure in the cabin
that the heater back drafts into the cabin
. One owner plumbed a combustion air intake to the fan on his Dickinson
and got about another 5 knots of windspeed before it backdrafted.
It's the forced air heaters that I've heard lots of questionable , to me, reasons.
Someone said that you don't want to create a negative pressure in the cabin to possibly suck in exhaust
fumes. First the cabin shouldn't be sealed that tight. Second, the cabin air intakes shouldn't be anywhere near the heater exhaust
. Unless you're trying to prevent a problem due to the unlikely conditions of sealed cabin and a leaky exhaust pipe so you're pulling the exhaust out of the leaky exhaust pipe into the cabin.
Another one is that the boat is so well sealed up that the oxygen is depleted which can also cause the problem poor combustion resulting in CO production. Again, why is the cabin that well sealed?
Unless those I've asked have missed something important, it seems that insisting on using outside air for combustion is to prevent rare tragedies by people that are using poor operating practices.
The counter current
system that Wallas has seems a good way to keep the hot exhaust isolated from the cabin reducing or removing the need to insulate the exhaust by having the combustion feed air come to the heater in the outside of the concentric tubes. Someone tried to tell me that it prewarms the combustion air, somehow increasing efficiency. This seems trivial to me compared to the heat generated, so I would think makes a trivial difference.