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Post by owkaye on Nov 28, 2016 8:09:31 GMT -8
Everything I read about rocket mass heaters suggest that if enough heat is extracted, the final exhaust temperature will be very low. In fact, I have read that when too much heat is extracted the natural draft fails!
So what would be wrong with designing a 'fan-draft' system that does not rely on natural draft, but instead relies upon a small exhaust fan to create a constant negative pressure on the flue/exhaust pipes?
If a small fan were installed at the end of the exhaust pipe where it exits the building, the exhaust air temperature would be as low as possible, and therefore the fan should have long life because it will not overheat. Maybe a simple and inexpensive computer fan would work!?
If I build such a system, it seems that I can have the exhaust exit the building at or near ground level, rather than building a tall chimney.
According to Peter van den Berg, a batch box rocket mass heater normally runs for only an hour, more or less. The electric power required to run a small fan for an hour is practically negligible. Therefore it seems that using a small fan at the exhaust might be a cheap and fast way to terminate a very efficient rocket mass heater system.
A fan on the exhaust will make cold starts easier too, since there will be negative pressure on the entire system the moment the fan is switched 'on'. This should eliminate any potential problem with smoke backing up and entering the building, correct?
What drawbacks of such a 'fan-draft' system have I failed to consider?
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Post by smartliketruck on Nov 28, 2016 10:09:29 GMT -8
If your fan slows down or stops for any reason you'll have CO coming into your living area. Not only will you have the regular CO levels of the exhaust, as the fire starves itself the levels will compound exponentially. Even with intake and exhaust and fuel loading sealed away from the indoors I doubt you will find any heating system anywhere that requires external power to exhaust itself.
Even mild CO poisoning is a very serious scary ordeal!
The risk you take by setting a system up as such far outweigh by a huge margin the minuscule amount of heat you'd extract from 80° exhaust.
If a buildocrat caught you with such a system they'd not only roast you on a spit, they'd have more ammunition for the continued torturous measures taken on the rest of us.
Again, the amount of heat you'd gain by purposely riding over the cusp of natural exhaustion is minuscule.
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Post by satamax on Nov 28, 2016 11:11:28 GMT -8
Been there, done that, prety much. I was using it to prime the system, and to fight against southern wind, as my exhaust was mid way up a southern wall.
Used a pabst fridge fan, of about 70 watts.
If you can do without, do without. It could save your family if you don't have to rely on electricity in the midst of a winter storm.
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Post by smartliketruck on Nov 28, 2016 11:29:46 GMT -8
Sure, as a stop gap measure for priming a non-critical system to give yourself some head scratching time, but planning to use it as a full time required part of the system is a bad idea.
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Post by drooster on Nov 28, 2016 13:19:59 GMT -8
If a small fan were installed at the end of the exhaust pipe where it exits the building As Satamax has tested a similar system successfully, there's your answer. (The answer is yes.)
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Post by owkaye on Nov 29, 2016 11:09:00 GMT -8
Thanks for the warnings about CO poisoning! I'm thinking to construct a large new greenhouse some day, with a continuous cob bench around all 4 sides. The only exterior wall space without this thermal storage bench would be at the entry door. A very long bench like this could extract and store the maximum heat from every burn, but only if the draft is maintained during the entire burn ... I have a feeling that maintaining an effective natural draft over such a long horizontal distance of buried pipe might not be possible, even with a tall smokestack/chimney, because the exhaust temperature may simply go too low. In this case a small exhaust fan should easily keep the fire burning efficiently until all the fuel is consumed. And since all pipes terminate outside, the risk of CO poisoning for people inside the greenhouse is virtually eliminated. Taking this concept one step further, a similar system might provide not only a safety benefit but also more comfort in some residential homes. Here's why: When the primary and secondary air supply come from outside the house, cold air will no longer be sucked into the home to replace the combustion air that's moving through the system and being expelled outside. All air simply moves through the system from one outside location to another. Naturally it might be more inconvenient to go outside every time you want to burn a batch of wood in a system like this. But you won't have to haul wood inside the home any more either, or clean up wood bits and pieces that invariably end up on the carpets or floors as a result of burning wood from inside the home. There won't be any smoke or soot inside the house should the fire backdraft, either. There are positives and negatives to every decision. I'm just entertaining a little bit of "outside the box" thinking here. Thank you for your thoughts!
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Post by patamos on Nov 29, 2016 22:19:52 GMT -8
It may not always work. I tried it on a 6" j-feed where i was attempting to lengthen the flue run a bit further than the system could normally handle. I found it troublesome getting the combustion to optimize. Blowing blue smoke when the fan was going too fast relative to the burn rate. And chugging when the burn rate wanted to go faster. Quite surprising really. All in all, i'd recommend letting the ever changing magnitude of the fire determine the rate at which it wants to draw…
Another idea with your green house plan is to build a mass north wall (insulated on the outside). Build the heater with a bit more fast heat delivery that can be absorbed by the wall...
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kpl
New Member
Posts: 47
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Post by kpl on Nov 29, 2016 23:25:20 GMT -8
Can a long bench be routed so that chimney would be located near the burn chamber? If chimney is directly heated thru a thin common wall or so, that should help to increase the draft, even if exhaust gases are not warm enough to sustain a good draft by themselves.
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Post by owkaye on Nov 30, 2016 6:39:57 GMT -8
I have 20-30 old steel bathtubs that can be used as the bench tunnel. I'm thinking to cut their ends off and butt them together end-to-end to create a continuous tunnel around the greenhouse inside perimeter, then encase the tunnel in concrete (or cob). This will give us the maximum raised flat warm heated space to sprout and grow plants while keeping the center aisle clear.
We have recently discussed putting a greenhouse like this on our "back porch" so that when we exit the house at the rear we have already entered the greenhouse. This makes it very convenient to attend to our plants. It also allows us to use excess greenhouse heat to warm our home in winter simply by leaving the door open and running a circulation fan in the doorway.
We like your "mass north wall" idea especially if we build a free standing greenhouse. But I think we prefer the "long heated bench" concept if we attach the greenhouse to the back of our main house. No decisions yet, but your ideas are very welcome.
I have one question about condensation in the bench tunnel: Is it a problem to set the upside down bathtubs on flat leveled ground and just let the condensation soak into the ground? Or might we need some kind of drain to remove the water?
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Post by satamax on Nov 30, 2016 9:36:07 GMT -8
Let it soak!
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