Why combustion air from outside?

[cal40john]

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.

John

[Badsanta]

You need to BREATH, the more CO and the less oxegen you have the worse it is for you. Many prople die each year in there homes, campers and boats from faulty heater systems. I would not use any of them with out a CO alarm, smoke dector. Dickerson recomends a ballanced draft system. It's in the install manual. this is to help eliminate back drafting. also if you seal up your boat and burn your inside air, at some point you will use up oxygen, and I think you need that.

[Stillraining]

John...IMHO...you just listed all the reasons

Why do manufactures now have to stamp into a gun barrel that.. "This firearm may cause injury or death"?

Its because people do, do dumb things

Unless my installation is a Dickerson which inherently by design incorporates the mandate..I will not go out of my way at great lengths to do it either...and will just provide good ventilation.

On another note...I have always wanted to plumb my home wood stoves air intake to draw from outdoors as like you said it would help maintain a positive interior pressure not a negative one therefor not drawing cold damp air in through any and all available cracks and creveses...your boat will be the same and in the smaller area the draft will be even greater...so it is something to think about..I have never done it with the wood stove at home but I think about it being a nice feature if I did all the time.

By The way Mobile homes are required to have this done...At Least in Washington