I’m contemplating a control change that would shut down the fan sooner when th fire has essentially burned out, and turn on the circ to scavenge the leftover heat. One of my concerns is that I might then be creating a condensing environment.
The orgin of the moisture seems to be what the stove manufacturers and people I know who have seem think can be avoided, and the reason why normal boilers go much longer with no effects.
Let me make some statments that maybe counter intuitive, but I think true. Correct them if you know I am wrong.
because I am certain I am the guy who gets his steel plate ate through in three weeks.
1. If you shut down the fan “early”, you can only trap moisture. Cold winter air has less absolute humidity then the dryest deserts.
1.5 absolute humitiy is what effects chemical reactions.
2. If your wood is in optimal burning dryness, any trapped wood fumes would with out doubt have more moisture than can be absorbed by the winter cold.
That is. you burn your wood, the wood is 80 percent wood 20 water. You burn 8 lbs of wood during this burn. You have 2 lb of water in your combust chamber.
Water doesn’t burn, so the water must be released. At the end of the burn you kick off you fan. you trap some relativly high amounts of moisture.
3. you can’t burn wood, without making “potentially” caustic checmicals-- have to add water.
I’m hoping that this is a non-issue for boilers operated the way that I operate mine.
Very possible. Also don’t forget it could be the fuel too.
I think it goes back into how it is operated. It is unfair to say that the moisture is trapped, because what if the mosture leaves early in the process- nearly completely in the first hr of combust. It seems to me that if we are looking at very hot temps that could be the case. It makes another good arguement for storage vs idling.
I figure it could have to do with green wood or a the species(because of the amount of pot. oxide) of wood as well. On this forum most of the gassification guru’s seem to know how to get the fuel to optimum and are not burning 50% moisture in their wood; Hotter fires with less moistuer.
The explanations that I have. I stole most of this from people who know more than me about this.
By the way, none of the steel plate sees anything like 1000 degrees. It’s backed up by 180 degree water. It does have gases that are even hotter than 1000 degrees on the combustion side, but the surface of the steel doesn’t get anywhere near that hot.
I am going to chalk it up to dramatic liscense for the story. LOL. Your right, I was making a generazation about all gassers. Some don’t have any exposed steel.
I think this effect also has effects on parts that aren’t steel like ceramics and all the funny material types that go into these gassers. Certainly steel is the most vunerable material and it shouldn’t reach those types of temps under normal circumstances.
The water content certainly is a major factor in any woodburner. As you note, 20% is still a lot of water that needs to be exhausted. I was reminded of this during our recent below zero snap, when I saw the steam plume coming out the stack after firing the boiler up in the morning with very dry wood. But based on that, I’d say that most of the moisture is expelled at the beginning of the cycle. Doubt there’s much moisture left in the firebox at the end.
As I suggested in another thread, another reason for the premature failure of wood-fired appliances, compared to their fossil fuel counterparts, is that fact that you’re dropping more than 100 pounds of raw wood into the combustion chamber every day--usually a couple of times a day--as well a changing the atmospheric conditions in the firebox every time you open the loading door. How many times can that backplate get banged with a big chunk of wood before the welds crack or the plate itself cracks? I’m very careful, but occasionally even I will “ring the bell” when loading wood. Ouch! Ever seen a teenage boy load a boiler? Keep them the hell out of the boiler room, is all I have to say on that.
I have also heard that with combination units sharing the same firebox, you can get sulfuric acid from the sulfur in the oil and/or coal and the water in the wood. Maybe not in a few years when the sulfur content of domestic heating oil drops dramatically, but it’s something for combi owners to think about--particularly if they have masonry chimneys. And then there’s always low return water temp corrosion which, I believe, is a bigger problem in gasifiers than in conventional wood-fired boiler designs.
In a desperate attempt to steer this back on-topic, I would suggest that the best boiler for summer DHW is the smallest one that you can get, combined with enough storage to last for many days.
A typical household uses 60,000 BTU or so of hot water per day. If you can store 420,000 BTU of usable heat, then you can get by with one fire per week. With an 80,000 BTU boiler, that fire would only need to be about 5 or 6 hours long.
I’d heat the storage directly, not through an oil boiler, and I’d use larger / lower temp storage to preheat the ice cold incoming water before introducing it into my smaller higher-temp tank.
With solar hot water panels, you might need fires only in the fall when the sun angles get low.
I actually would like to do two storage tanks . . . one out in the building with the GW (to avoid summer standby losses heating the residence) and one on the basement next to the oil boiler (to capitalize on the standby losses the rest of the year). But that would seem to require ANOTHER 90’ of Thermapex and more excavation. . . .
For DHW, remote heat storage is almost useless. The only thing you can do with it is use it to heat the actual DHW tank through the use of a circulator and a heat exchanger. You can’t use the heat directly, and you can’t use it to preheat the cold water before it enters the DHW tank. Even in doing ‘batch heating’, you lose the heat in the lines between tanks every time.
The problem is that DHW use is intermittent, and usually isn’t enough to even flush the cold water out of the lines between the DHW tank and the remote storage.
My outside storage tank and my DHW tank are literally less than 24” apart to minimize this problem.
Thanks All. And Thanks Nofossil for getting back on track lol. My question was wich type of gasifier would perform better in summertime. I bit the bullet today and bought a Greenfire. It is basically built like a Seton/Greenwood. Now the fun starts. I gotta rip the old boiler out and put this baby in this weekend. After that I will spend many more hours studying storage. I hope to build a tank and install a solar collector this year. Thanks all for your wealth of knowledge. I am glad to find a group of people just as sick as I am when it comes to harnessing fire!!
I have spent some time looking into this. It had almost stopped me from getting a gasser.
To put it in perspective on a scale of 1 to 10. road salt or the “saltiest parts of the ocean” is a 1.6. This is about an 6.
Or about the equivalent of 20 percent mixture of Hydrochloric water solutions.
And, I know the potassium hydroxide compound isn’t rare in fire’s ash, because they have used it in numerous civilizations across the world with all types of wood.
I have never read anywhere it could be quantified as a net amount of potash you could expect from an amount of wood.
By acids and corrosion you mean for where
I said caustic. It actually an strong base—“like an acid”.
I am sort of surprised they don’t have some sort of procedure, additive, spray or whatever to address this. I am sure the stainless would go a long way towards an answer. On the other hand thick plate goes a long way towards solving corrosion in my experience with rust on machines, cars ect. Thick steel and cast iron like my 1949 Cat dozer can just sit there exposed for decades while thin sheet metal just goes to hell in a few years if the paint is nicked. Of course its not a direct comparison but still pretty applicable considering how nasty road salt is.
Obviously part of the solution is remove all of the ash—sorry to state the obvious. That removes the danger, but it is impratical to have to be present at cool down. Or to clean out the thing every time it gets cold. I was told if you going to leave ash in the stove cold,then open the loading door to prevent a moisture seal.
Did the oil help?? I will say again sealed combustion chambers in the winter atmosphere w/ intermittent use are what I was warned against; Not long term summer downtime
I made a call and here is how it was explained. You have a piece of metal and this is how it spends one day in the winter. starts out at 12pm at 0 F then heats up to 1000F by 2pm, is covered with caustic ash(unknown amount)-- cools to 40F at 12 am moisture held air tight against the steel while mixing with ash.
I think now the rapid/extreme heating and cooling may play a role in this as well
Just to pick up where nofossil left off, there’s really no ash to speak of on the walls of my gasifier. There is some sitting on the refractory mass and the nozzles, and in the ash pit, of course, but most of the exposed steel backed up by water has a coating of creosote, not ash. Seems to me that you’d see any damage first on the stovepipe connecting the boiler to the chimney. There you get plenty of fly ash. Or on the turbulators, which are basically always coated with fly ash and exposed to any moisture exiting the stove during operation. Presumably, these parts would rot out long before the pressure vessel became compromised from the same forces.
But there’s no welding a leaky boiler, tru.dat.
To restate what I said earlier, my understanding is that low temp return water corrosion is the big worry, and that’s when the boiler is operating. If condensation during long periods of inactivity (say all summer) is a concern, it seems to me that the lightbulb idea is a good one, as well as maybe draining the boiler and disconnecting the chimney connection would be prudent as well. I can virtually guarantee you that no special precautions (including low temp water protection) were taken with my previous wood-fired boiler, and it was still going strong after 25 years when I got rid of it. It was a steel plate, wood-fired boiler full of ashes pretty much at all times.
Finally, why would an idle boiler in the winter be more susceptible to corrosion than one that is idle in the summer. I know I typically fire my boilers up around September or October and basically run them steady until the following spring.
Eric, I am afraid that I either mistyped. Or, your mistaken in what you read. Because I didn’t mean to give you the impression that this caustic chemical is created imediatedly after combustion. I meant to say this ash/creasote combining with water is supposedly bad stuff. Like if you boiler condensates during cold weather with ash in it.
I assume all boiler’s have an ample supply of ash and creo in winter—not nessarily so in summer. In winter it is easy to condensate flue gases at the end of a burn. Or from the air maybe if it is in your house—almost like a sweating window.
Not eay to condensate with a stove in the summer. Unless you refridgerate your stove.
I think Dead Btu’s should grab some of his Goo and give it a litmus test. I think it could put this to bed.
Can you tell me what type of low return temp corrsion is your primary worry. Is it in the summer?
No, the low temp return water corrosion I’m talking about happens when your return water goes below about 120 degrees. What happens then is that you get condensation on the back wall of the firebox and the combustion products eat away at the steel. Over time, it gets paper then. Then, one day you toss a chunk in too hard and it cracks. Time for a new boiler. Apparently it’s a big concern with wood gasifiers, but also a potential problem with any boiler--wood or fossil fuel. Most people put mixing valves down there to keep the return water temps up. I have a 007 wired to my controller that pumps supply water into the return when the boiler is below a certain temp (140, I believe).
I’m pretty sure that goo coming out of the backs of those Seton-style boilers is liquid creosote. It sure looks like it to me.
First both you and I agree that we are talking about a new phenomena. High return temps are necessary-- you see it in all literature
I know your correct that mixing valves are constantly touted as necessary, but I don’t understand why < 120 water creates condensation. What is the principle that corrodes in this example.
First both you and I agree that we are talking about a new phenomena. High return temps are necessary-- you see it in all literature
I know your correct that mixing valves are constantly touted as necessary, but I don’t understand why < 120 water creates condensation. What is the principle that corrodes in this example.
Water+creosote= caustic.
Let me take a crack at this....
Wood is about 15% water by weight, even when it’s dry. Combustion combines any hydrogen that’s in the wood with oxygen to create water vapor. Thus, flue gas has a good amount of water vapor in it. There are other trace (and not so trace) elements which are soluble in water, or which react with water, to form corrosive compounds. If the temperature of the water jacket is cold enough, water vapor will condense out of the flue gas onto the cold metal. Combined with other compounds, this can cause corrosion. I would expect this to be most severe in spots where the water is cold and where the flue gases have had a chance to cool down or where there’s not much flow - the bottom back corners of my boiler for instance.
I’ve seen other boilers (non-gasification) that have rusted through in similar spots.
Yah, your right I just looked it up. 131.3 is the dew point inside the boiler with 18.6 percent humidity fuel. @ sea level.
dew point of 265 with 50 percent water wood. Jeez.
I really don’t know how dangerous A dew point above boiling could be. Any insight?
Is this right???
