I believe I do understand. At no time should combustion air be drawn from anywhere but the outside of the vessel as per ABYC Standards. I know many disagree with these standards but in this case I believe they make absolute sense.
This is known as "room sealed combustion".
CO is not the only concern. Drawing combustion air from inside the vessel also reduces oxygen. Some fossil fuel heaters have oxygen depletion sensors but not all.
What I am talking about has nothing to do with drawing combustion air from inside the living space. I am talking about recirculating warm air from the occupied space rather than heating outside air and dumping it into the space. The combustion air intake and combustion exhaust are 2 separate ducts that are completely unrelated to the supply and return ducts.
The combustion process can have absolutely no effect on the air in the occupied space aside from raising the temperature. It cannot deplete the oxygen in the space. It cannot introduce ANY contaminants of any kind into the conditioned space. It is all separated by the heat exchanger.
With regards to combustion air, I agree that it is good practice to draw that combustion air from outside, however functionally it is perfectly acceptable to draw air from within the compartment in which the heater is installed provided the compartment is "loose" enough that air can get in to replace what has been drawn out by the combustion fan. It is NOT a good idea to draw return air out of the same compartment that you are drawing combustion air from because the suction of the much larger supply fan can interfere with the free flow of combustion air through the combustion chamber. Having said that, even if you WERE drawing combustion air from the living space it could have no effect on the air quality of the space. Again, the combustion process is isolated from the living space by the heat exchanger. All of the products of combustion are discharged outside the boat.
Perhaps my terminology is confusing things:
Supply air=the hot air discharged into the living space
Return air(aka cold air)=air drawn into the heater from either outside or the living space
Combustion air=air drawn into the combustion chamber exclusively for the purpose of burning the fuel
Combustion exhaust=the burned air/fuel mixture exhausted outside
Sent from my SM-G960W using Tapatalk