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post #21 of 42 Old 12-06-2019
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Re: Forced air heater duct design

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Originally Posted by gtstricky View Post
https://law.resource.org/pub/us/cfr/....A-07.1973.pdf

See 5.111
Combustion air must be fresh but heated air can be recirculated.
Without sarcasm I have no idea what that means. Same thing with Main Sails post above. I read it, I decode the words, but I have no useful or practical interpretation of what they are driving at.

Reminds me of electrical codes; there are the code books and then there are the companion books, much thicker which purport to “explain” the code.

Hell when I took my Electrical PE there was a section of the test simply related to being able to read and decipher the code book. And the passing rate for that “class” was 23% IIRC.

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post #22 of 42 Old 12-06-2019
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Re: Forced air heater duct design

The critical take away is that exhaust air must be exited outside the hull and this should not end up inside the cabin. Same thing for your auxiliary. Motoring downwind when there is still a slight net downwind component smells awful as exhaust air is blown forward into the cockpit and maybe into the cabin. All combusted air must be exited from the boat and not allowed to mingle with the conditioned spaces.

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Re: Forced air heater duct design

Sander,

Agree with what you posted above, that makes sense.

It’s the quote ABYC Stuff I have trouble with.

Sometime, too often, I think these standards agencies are really pretty bad. Confusing and over generalize. There is not a lot of overlap between a 18’ center console with twin 200hp outboards and my 1985 44’ steel cutter.

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post #24 of 42 Old 12-07-2019
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Re: Forced air heater duct design

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Originally Posted by SchockT View Post
Diesel combustion produces little to no carbon monoxide, so co poisoning is not a concern. Drawing outside air instead of return air would greatly increase the chances of drawing exhaust into the boat, be it from your own heater, or other boats heaters or generators if you are in a crowded anchrage or at a dock.
I disagree. CO takes up to two weeks to clear the blood stream. Low level contamination over two weeks could be a significant health risk.

Drawing outside air for combustion reduces the risk of CO .... IF the intake and exhaust are designed in accordance with ABYC Standards which specify a minimum distance between.

Re-combusting recirculated air can do nothing other than slowly increase a potential CO component.
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Re: Forced air heater duct design

Makes sense
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post #26 of 42 Old 12-07-2019 Thread Starter
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Re: Forced air heater duct design

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Originally Posted by boatpoker View Post
I disagree. CO takes up to two weeks to clear the blood stream. Low level contamination over two weeks could be a significant health risk.



Drawing outside air for combustion reduces the risk of CO .... IF the intake and exhaust are designed in accordance with ABYC Standards which specify a minimum distance between.



Re-combusting recirculated air can do nothing other than slowly increase a potential CO component.
I don't think you are entirely understanding how these heaters work. There is no combustion taking place in the conditioned air at all, let alone "recombusting". Air for combustion is being drawn into the combustion chamber, mixed with fuel, burned, and the combustion gas is exhausted outside the boat. At NO TIME does the combustion gas come into contact with the air from the conditioned space. Heat is transferred to the conditioned air via a heat exchanger, therefore it is impossible for products of combustion to enter the living space barring a catastrophic heat exchanger failure.

My point is that there IS a possibility of combustion gasses entering the living space of the boat if you are drawing outside air into your boat, and there is NO possibility of that happening if you are recirculating the air in the living space.

Of course there are other benefits of bringing more fresh air into the boat, such as humidity control, and maintaining indoor air quality by offsetting the build up of co2 within the enclosed space, however that co2 is coming from respiration of the occupants, not combustion.

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Re: Forced air heater duct design

Quote:
Originally Posted by SchockT View Post
I don't think you are entirely understanding how these heaters work. There is no combustion taking place in the conditioned air at all, let alone "recombusting". Air for combustion is being drawn into the combustion chamber, mixed with fuel, burned, and the combustion gas is exhausted outside the boat. At NO TIME does the combustion gas come into contact with the air from the conditioned space. Heat is transferred to the conditioned air via a heat exchanger, therefore it is impossible for products of combustion to enter the living space barring a catastrophic heat exchanger failure.

My point is that there IS a possibility of combustion gasses entering the living space of the boat if you are drawing outside air into your boat, and there is NO possibility of that happening if you are recirculating the air in the living space.

Of course there are other benefits of bringing more fresh air into the boat, such as humidity control, and maintaining indoor air quality by offsetting the build up of co2 within the enclosed space, however that co2 is coming from respiration of the occupants, not combustion.

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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.

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Last edited by boatpoker; 12-08-2019 at 01:04 AM.
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post #28 of 42 Old 12-08-2019 Thread Starter
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Re: Forced air heater duct design

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Originally Posted by boatpoker View Post
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



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Last edited by SchockT; 12-08-2019 at 04:38 AM.
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post #29 of 42 Old 12-08-2019
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Re: Forced air heater duct design

For those who have installed one of these or similar ShockT's question is simple. You CAN recirculate cabin air which will mean the cabin heats up sooner to "design" temp and uses less fuel to maintain it. The only issue would be "short circuiting" if the input air grille was close to an outlet grille.

So the ideal location for the recirculate return air grille would be low down in the cabin where the coolest air is and far from any outlet grilles. The return air is a negative pressure and draws the air to it. This will assist in circulating the warm conditioned air and help maintain even temp throughout the conditioned space.

++++

Many vehicles allow you to recirculate air, or drawn only outside air or mix the two. The air is heated on a heat exchanger called a radiator!

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post #30 of 42 Old 12-08-2019
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Re: Forced air heater duct design

Be careful describing your system as being "impossible" to introduce combustion air into the cabin. If there is a crack or other failure of the manifold, that may be very possible. I'm very familiar with other fuel combustion heaters that are required to be pressure tested every so often, to confirm this is not the case.

The premise that diesel exhaust has little to no CO is the reason for all the pushback. It has enough to accumulate over time and be deadly. You should have a CO monitor for sure and failure is possible.

Otherwise, recirculate all the fresh cabin air you like.


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