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post #11 of 42 Old 12-03-2019
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Re: Forced air heater duct design

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Diesel combustion produces little to no carbon monoxide, so co poisoning is not a concern......
Diesel carbon monoxide content is a fraction of gasoline, but it's not zero. The concept suggested by the OP would continually multiply the emission, by recirculating the contaminated air. Potentially adding more and more. It would just be a matter of time, which a heater running 24/7 would have.

Does your heater actually expose the air to the diesel flame, or would it require a breach in a manifold of some kind. I personally wouldn't trust either, but if directly exposed, it's certain to accumulate CO over time, if you recirculate.


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post #12 of 42 Old 12-03-2019
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Re: Forced air heater duct design

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Originally Posted by olson34 View Post
A surveyor has told me that current ABYC "recommendations" call for outside intake air, even tho it might seem unintuitive.
(i have been scoping out how to install a small forced air furnace in our boat.)

Note that the exhaust and intake should not be close to each other, and further note that any assumptions about the skill and knowledge of the original furnace installer are often erroneous.

Unfortunately 'tis the way of the world.....
What sort of heaters is this for? Obviously the combustion air should not come from inside the cabin.... certainly if it is not replaced as this could make the interior air unsafe to breathe.

You would not want a sealed interior space to live it for the same reason... breathing consumes oxygen and adds C02. I suspect you don't need much area for this fresh air ventilation.

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post #13 of 42 Old 12-03-2019
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Re: Forced air heater duct design

Quote:
Originally Posted by olson34 View Post
A surveyor has told me that current ABYC "recommendations" call for outside intake air, even tho it might seem unintuitive.
(i have been scoping out how to install a small forced air furnace in our boat.)

Note that the exhaust and intake should not be close to each other, and further note that any assumptions about the skill and knowledge of the original furnace installer are often erroneous.

Unfortunately 'tis the way of the world.....
ABYC requires outside combustion air, but my understanding is that recirculation of cabin air was normal. Remember, of course, that some proportion of outside air is required to control humidity. Often as not, companionway drafts supply that. The installer may have misunderstood the intent.
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post #14 of 42 Old 12-03-2019 Thread Starter
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Re: Forced air heater duct design

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Originally Posted by Minnewaska View Post
Diesel carbon monoxide content is a fraction of gasoline, but it's not zero. The concept suggested by the OP would continually multiply the emission, by recirculating the contaminated air. Potentially adding more and more. It would just be a matter of time, which a heater running 24/7 would have.



Does your heater actually expose the air to the diesel flame, or would it require a breach in a manifold of some kind. I personally wouldn't trust either, but if directly exposed, it's certain to accumulate CO over time, if you recirculate.
I am not sure why you would assume that there is contaminated air, or why you think it would accumulate?

Modern forced air heaters such as the Webasto are indirect fired, meaning the combustion chamber is isolated from the air stream, and then the heat is transferred to the conditioned air via heat exchanger. The combustion air and exhaust are completely separate. Unless the heat exchanger cracks or has rust holes there is no way for combustion gas to enter the living space.

If one is concerned about combustion gasses entering the boat, then drawing 100% outside air increases the chances of that happening.

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Re: Forced air heater duct design

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Originally Posted by SchockT View Post
I am not sure why you would assume that there is contaminated air, or why you think it would accumulate?

Modern forced air heaters such as the Webasto are indirect fired, meaning the combustion chamber is isolated from the air stream, and then the heat is transferred to the conditioned air via heat exchanger. The combustion air and exhaust are completely separate. Unless the heat exchanger cracks or has rust holes there is no way for combustion gas to enter the living space.

If one is concerned about combustion gasses entering the boat, then drawing 100% outside air increases the chances of that happening.

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I didnít assume. If anything, I asked the question in the second paragraph you quoted. No where in your OP did you state what heater you were using or that it was even a modern design.

Iím not familiar with your heater, but am familiar with exchange heaters in general (I own other versions) and in no case is the risk of a manifold breach all that remote. It happens. The best designs will create a positive pressure on the cabin air side, so in the event of a breach, itís more likely to draft air into the burn chamber than the opposite. Still, no guarantee and not identifying the problem for a lengthy period could be deadly. A good CO detector is wise. I assume you have one.

In the end, Iíve never heard of ABYC standards being for naught.


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Re: Forced air heater duct design

It would be helpful to mention a model number or the type of heater although a diesel fired forced air ducts supplied heat may be adequate.

I've had several Espars and they require a separate combustion air intake. The exhaust air should be conntect to a transom vent and the duct gets supper hot. Mine are stainless steel.

The unit pulls air one side and it goes over a heat exchanger where the glow plug creates a flame to heat the input air. Conceptually it's like any forced air heating system.

The input air in a home recirculates the conditioned air. But of course you need to have fresh air so CO and CO2 don't increase to unsafe levels.

A boat is pretty leaky as far as air is concerned. I think it's probably OK to re heat the interior air... and more efficient to do so.

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Re: Forced air heater duct design

Quote:
Originally Posted by olson34 View Post
A surveyor has told me that current ABYC "recommendations" call for outside intake air, even tho it might seem unintuitive.
(i have been scoping out how to install a small forced air furnace in our boat.)

Note that the exhaust and intake should not be close to each other, and further note that any assumptions about the skill and knowledge of the original furnace installer are often erroneous.

Unfortunately 'tis the way of the world.....
This is for combustion air, not furnace/heated air. The ABYC calls for "room sealed combustion" and Webasto & Espar both have a combustion air intake that is separate from the heated air that's heating the boat.
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Re: Forced air heater duct design

I Have a couple of Espar D-4 units, one on each boat. They both draw combustion and conditioned air from the environment, same source. The “exhaust” air goes out the chimney, the heated/conditioned air goes out the duct into the cabin.

FWIW I’ve never seen any requirement or installation material suggesting that combustion air should be pulled from outside.

Think about a wood stove for a minute. The combustion air comes from the cabin and goes out the chimney. The conditioned air circulates around the hot stove, which is just a poor heat exchanger, and circulates back to mix with the cabin air.

Same thing.
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post #19 of 42 Old 12-03-2019 Thread Starter
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Re: Forced air heater duct design

My heater is a Webasto Air Top 5500, but whether we are talking about Webasto, Espar, Planar, they all operate on the same principles.

I honestly don't know how common heat exchanger failures are, but I would think that if a failure were to occur one would be able to smell diesel exhaust. Regardless, it would make very little difference whether the unit was configured with outside air or return air. How much combustion gas leaks into the airstream is going to depend on a lot of factors; location and size of the crack, fan speed, air velocity, and be pressure difference between combustion chamber and the surrounding area.

A co detector is never a bad idea when you have hydrocarbon burning appliances, but I think it would be more for the propane oven and stove. With the low amounts of co a cracked diesel heat exchanger would produce a standard house co monitor may not even be sensitive enough to pick it up.

I work on parkade exhaust systems that use commercial grade CO monitors to activate exhaust systems. If our plumbers have diesel powered hydro vac trucks in the parkade cleaning sumps with their diesel engines running at high idle they can easily fill the parkade with exhaust without activating the fan system. We have to do a manual override to ventilate the parkade.

I am curious exactly how much CO heaters like mine produce, so I am going to grab a combustion analyzer from work and test it. I will post the results if anyone is interested.

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post #20 of 42 Old 12-05-2019
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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.
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