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Old 07-12-2005, 13:29   #1
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A common rule of thumb... incorrect?

Ok, now that I have had a woodstove running continuously for the past week or so and have had plenty of time to think while loading logs in... (maybe too much time? ha ha) I have come up with a little theory regarding its classification as an "open flame" device, as it pertains to CO poisoning and using up oxygen in the boat causing soffocation.

Let's start with assumptions:

1) When the fire is lit, hot air/smoke from combustion flows up the chimney, creating a negative pressure inside the stove.

2) To compensate, air can flow into the stove through the main vent and through a few crevices.

3) Unless the draw up the chimney is not working, and you have smoke coming out of the stove, it is taking the air/smoke from inside the stove and pusing it up the chimney while replacing this air/smoke with new air it sucks in through the vents and through any crevices.

My theory is this:

If your stove is operating well (as in has a good draw, a good fire, and is sucking air in well through the vents) it is impossible to get CO posisoning from it. I'll even go a little farther to say that opening a vent or hatch to let fresh air in to replace the "oxygen the stove consumes" is a myth. It's a myth because any air that goes in the vents or crevices of the stove goes right up the chimney. There is no old air that has its oxygen consumed inside the stove that makes its way back out to the boat, due to the negative pressure inside the stove created by the draw of the chimney. Which brings me to the final point... just about every boat has some place that air ends up coming in anyway. In my case, I imagine it's a combination of the dorade vents with socks stuffed in them, the hawspipe in the anchor locker, and the slats in the companionway door. If I close everything up tight, I still have a very powerful draw.

So, I think the "crack a hatch" while you burn a woodstove rule is a myth. I mean if you were in a 100% sealed boat (no cracks, no possible way for any air to enter, glued shut everywhere, etc...), you would have no draw up the chimney since the entire boat's pressure would equalize with the pressure inside the stove and you would not be able to really light a fire, except one that would fill the room with smoke.

So... another myth I think is the myth of being able to get CO poisoning without noticing it from a wood stove. In order to get CO in the boat, you have to get smoke in the boat along with it.

Anyone have any input on this? I know it's way out there, and not a very useful topic, but everything I read kept saying to crack a hatch when you run a stove. I say a better rule is: If you have a draw without a hatch cracked, you already have sufficient air entrance not to worry about it.

Just to back up the theory, I ran the stove for 4 days straight with no hatch cracked (24 hrs a day) this week. I have a CO detector, so it wasn't Russian Roulette. No CO was emitted.
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Old 07-12-2005, 14:05   #2
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Sean - I believe you are correct ! 1) You may have too much time. This could be an early symptom of cabin fever. If you feel the urge to run around the decks in your underware - seek medical attention. 2) As long as the fire is drawing, there is IMO no risk of CO poisoning. Same deal as a house. Most have enough cracks to allow enough make-up air in to replace air going up the flue. Some super insulated homes are so tight they need to crack a window to have a fire. There IS a real risk of downdrafts causing a CO problem when you first start the fire and especially when the fire burns low. As the fire gets smaller, it is more likely air can be driven down the chimney, backing up the fire. You could smell it, if you were awake. Remember, too, the hot embers continue to consume oxygen, but do not give off much odor. Also, any obstruction could restrict discharge. Low fire, very heavy snow, strong winds......For those times, you need the detectors.

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Old 07-12-2005, 14:09   #3
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Hillarious! Yes... I think it is cabin fever and spending way too many hours putting together a website this week instead of doing boat work. ha ha ha

Somehow, I had thought that changing careers from the computer industry to the charter industry would allow me to escape programming and website layout.
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Old 07-12-2005, 14:31   #4
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Looking forward to the web site. capt lar
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Old 07-12-2005, 14:45   #5
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When the stove is burning the heat in the firebox is creating a positive pressure - heat is expanding the air. Imagine a small leak between the firebox and the chimney where CO can get out into the interior of the boat...
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Old 07-12-2005, 15:15   #6
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When the stove is burning the heat in the firebox is creating a positive pressure - heat is expanding the air. Imagine a small leak between the firebox and the chimney where CO can get out into the interior of the boat...
I'm sorry... not trying to start a debate, but physically, this statement can't be correct. If there were a positive pressure built up inside the fire box, smoke would come out of the stove's vents whenever they were opened. Only negative pressure is possible since air is most definitely sucked into those vents (at a good clip in my stove - great draw).

No ill will intended, but I wanted to be clear since this is somewhat of a hazardous topic.
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Old 07-12-2005, 15:41   #7
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Stove

Initially when lighting it is possible to have outside air blow down the chimney and blow smoke in to the boat. Once the fire is lit and drawing you should be able to open the stove door and not have anything come in the boat because the draft is going the other way. A momentary downdraft might change that but the draft down and in, would have to exceed the draft out, ie a very windy day. It is possible to run out of air if not enough is comming in. You need a bigger fire and a more air tight structure. Large stoves like the one in my basement have a built in dedicated pipe for combustion air. That air is comming from outside. It is about five inches for the largest wood stove available. It is too heavy and too big to go on your boat. It can consume large amounts of wood while heating a large house. In our small old house we ran out of air inside. When you breath you realise there is not enough substinance to the air. Kinda like American beer.
I ran a four inch pipe from outside and under the wall and up through the floor next to the stove, with an adjustable vent thingy on it. That was more than enough to provide air for the stove. On my 28 foot boat I have two vents for the motor and one vent forward plus one on the cabin top. Plus the vents in the hatch boards. That has proven to be more than enough for a kerosene heater running all night. So I do not worry about running my Origo heater or the stove with the hatch boards closed. In short, I agree with you so far. The practical test is always the best.
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Old 07-12-2005, 18:49   #8
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The purpose of an ouside air duct on a woodstove is not primarily for cubmustion air - most houses and boats have enough air leaks to supply that. The outside air duct is to supplt he stove with air from outside, instead of using inside air that has already been heated, which then gets replaced by cold air through the air leaks. It's a heating effifiency thing.
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Old 07-12-2005, 19:12   #9
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Air

Our 1920s vintage house ran out of air when consuming inside air for combustion, so the air we piped in was for combustion. Same thing with the big stove, primary use of the air is for combustion with the secondary need being heat conservation and efficiency. Some of the homes are very airtight when built to R40 ?? standards. The main furnace uses outside air plus inside air. We also have a pipe to provide outside air in case we use too much inside air. However the smart heating guys have managed to balance it nicely.
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Old 07-12-2005, 21:08   #10
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The expanding gasses of combustion fill more volume for its mass that the surrounding colder, denser air in the firebox, and so rises. The Charlie Noble (flume, chiminey, pick your nomenclature) directs the hot gasses up and out of the firebox, preventing a significant build-up of positive pressure (it's an open system).

Those escaping hot gasses do create a negative pressure in the firebox. Cooler, ambient saloon air is drawn in through the vents to replace it, supplying oxygen for continued combustion.

Much of the heat released by the combustion radiates outward, heating the stove, and ultimately, the saloon air.

Sean, what are you using for a wood supply? That stove can get pricey unless you have a cheap source.
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Old 07-12-2005, 22:14   #11
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Michael, there is another big difference between how homes were insulated in the 20's vs now. In that era homes were oftem insulated with newspaper, and all drafts were sealed with blankets. This prevented air from coming into the home very effectively. Modern homes are insulated with fiberglass insulation that allows air to pass through, but actually holds heat, warming the air that passes through. Although modern homes are sealed much more effectively than they were 80 years ago, some air still gets in. THe volume of air in a boat is substantially less than that of a house, and accordingly, can be depleted of oxygen much more quickly by the burning of fuel. (such as a wood stove) As nothing is pulled into the boat if it is sealed, in order to deplete the boat of oxygen, the stove would have to actually create a vacuum. Not gonna happen. The real danger is leaks. A true air tight stove will not leak, and therefore can not introduce CO into the cabin. Should the burning fuel use sufficent air to decrease the oxygen level in the cabin, the fire will smother very quickly. The assessment that the smoke will begin to back up into the boat as the fire smothers, and the pressure differential changes is very accurate, but a smoke alarm will detect this well before it is a danger. Wood stoves have a very safe record on boats. In fact, the majority of fires are a result of 3 things. 1)poor installations 2)combustible items being left too close to the stove and 3)embers escaping through screens, or poorly designed intake vents. All are operater error.
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Old 07-12-2005, 22:25   #12
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Quote:
CaptainJeff once whispered in the wind:
The expanding gasses of combustion fill more volume for its mass that the surrounding colder, denser air in the firebox, and so rises. The Charlie Noble (flume, chiminey, pick your nomenclature) directs the hot gasses up and out of the firebox, preventing a significant build-up of positive pressure (it's an open system).

Those escaping hot gasses do create a negative pressure in the firebox. Cooler, ambient saloon air is drawn in through the vents to replace it, supplying oxygen for continued combustion.

Much of the heat released by the combustion radiates outward, heating the stove, and ultimately, the saloon air.

Sean, what are you using for a wood supply? That stove can get pricey unless you have a cheap source.
Thanks for some more technical terms behind my general idea. I agree 100%. Also, I'm using free seasoned hardwood for a source (mostly oak), so it's only a matter of the price for some chainsaw fuel once in a while, and the extra snack I need after splitting.
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Old 07-12-2005, 22:26   #13
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Thank you, Kai Nui.

Thanks, Kai Nui!

You really put what I was trying to say into a more clear, succinct paragraph. Right on the money!

Thanks.

Attesting to what you say about wood stoves aboard boats, the Little Cod was used for hudreds of years in the North Atlantic (and I mean WAY north - think where the perfect storm movie was to have taken place). It's a proven design.

You know, I have run into a lot of people who are scared to death of wood stoves. I think a lot of the parinoia surrounding heating with wood stems from some marketing seed the oil companies planted at one time. I mean think about it. Why would they want you to use something that is free and renewable when they can charge you $400/mo for home heating oil, $3.00 a gallon for diesel, etc..? Of course everything they put out would discourage people like me from paying $0/mo.
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Old 07-12-2005, 22:30   #14
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I have my moments
I do hope you are keeping track. I really want to know how long a cord of wood will last on a boat
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Old 07-12-2005, 22:33   #15
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I have my moments
I do hope you are keeping track. I really want to know how long a cord of wood will last on a boat
Re-read the above... I added a little propaganda. ha ha ha

It is very hard for me to keep track of exact cordage used by this stove. The problem is we can't keep a cord on the boat, so we go cut wood about once every two weeks. We fill a small, economy car (Hyundai Accent) trunk with unspit logs, then bring them back to the marina and split them. A single trunk full of oak hardwood lasts us approx 2 weeks with the stove going 24/7. No idea what that translates into from a cordage perspective. If you have any ideas on how to accurately take those facts and come up with a good fuel usage measurement, let me know. I'd be happy to do it for you.
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