Forced exhaust RMH, with or without barrel.

I know I'm not supposed to innovate on my first RMH, but I want to put it in my walkout basement as I finish it, and I want it to exhaust out the side of the house about a foot above grade level.  I might also be forced to neck the exhaust down to a smaller cross sectional area, and maybe even into the ground for 10 feet and away from the house, so that oxygen depleted air doesn't have the chance to sneek back into the basement via the walkout exit.  So with all of those factors, I would like some input/advice from the woodfire experts.  Ernie mentioned on one of his videos that he uses 22 pounds of dried wood each day, so since dried wood is pretty consistently 8600 btus per pound regardless of species, that would mean that he is using about 190K btus in a cycle.  First off, this means Ernie & Erica live in a pretty small house.  But what I don't know is how long it takes to consume this 22 pound charge.  If I knew that, I could calculate the power rating of a RMH well enough to know what size of an exhaust fan I would need to replicate the natural draft of a proper RMH.  The next question then would be, is the exhaust cool enough across the entire cycle that I can expect that it will not damage a normal exhaust fan, such as found inside a bathroom, or should I forget about that idea and just pay the money for a purpose built powered woodstove draft fan?

Oh, and I'm looking to build the mass bench out of standard red brick & mortar, and perhaps without the barrel at all.  I haven't decided if the insulated "riser" portion will still be vertical inside a brick column, or laid down inside the brick mass bench, since I shouldn't need the natural draft that a normal RMH produces. Partly because I don't want any hot metal surfaces on the bench anywhere, and partly because I won't be asking the Department of Making me Sad for their approval.

Has anyone tried something along these lines?

How cool the exhaust stays depends on how much heat extraction you have. If you have enough length of duct or size of bell compared to the core size, it could stay cool enough. This would be likely to reduce the draft in a normal system, but as you are talking about a system with no natural draft anyway, I wouldn't see a problem with a regular fan.

I would see a problem with a system that is guaranteed to smoke back and possibly even send flames out the feed if the fan stops working or the power goes out.

You can put the core and riser inside a masonry box instead of a barrel, and retain some self-generated draft without the instant radiation and hot metal surface.

Matt Walker has developed a riserless core that gives clean combustion, though it does depend on having good draft from a chimney.

I don't think you need to worry about moving the exhaust away from the house, as the exhaust will be well above outside air temperature and rise so it will not be a hazard for coming back into the space. I would locate it away from windows to be safe. I would also locate it at ceiling level to maximize draft. Is the walkout on the usual windward side, or the lee side, of your house? Any time the wind is blowing on that face, you risk reverse flow.

Glenn Herbert wrote:

I don't think you need to worry about moving the exhaust away from the house, as the exhaust will be well above outside air temperature and rise so it will not be a hazard for coming back into the space. I would locate it away from windows to be safe. I would also locate it at ceiling level to maximize draft. Is the walkout on the usual windward side, or the lee side, of your house? Any time the wind is blowing on that face, you risk reverse flow.

My walkout basement exit faces a huge copse of trees, on my back 10 acres.  So when there's wind, it's always on the lee side, because the trees effectively block anything from that direction.  That said, I do think that I have a solution to the backblow problem, with using a ground level dryer vent, that was mentioned by Erica on that video. The solution is to pipe another vent to the far side of the house, so that when the wind is on the side of the dryer vent, the wind doesn't produce backpressure through the RMH, but has a path of less resistance to the other side of the house.  Worst case, the vent pipe to the other side of the house would have to be full cross sectional area, but one could probably get away with less for that vent pipe.  Downside is that adding this additional vent is likely to 'whistle' whenever the wind blows, so it would have to be secured rather well to prevent resonation.

But, again, I'm not allowed to innovate yet, and I have no way of practically testing my theory in the backyard.

Glenn Herbert wrote:
I would see a problem with a system that is guaranteed to smoke back and possibly even send flames out the feed if the fan stops working or the power goes out.

Yes, I have considered that issue as well.  I'm thinking about putting in a small solar & battery system, just to run my fridge, anyway; and might make the forced exhaust vent a DC unit.  That would definitely increase cost, though.

I'm unlikely to ever go to sleep unless the fire is out, however; so back-drafting due to power failure isn't really a great risk so long as I have a well fitting cover that I can drop onto the wood-feed to suffocate the burn if necessary.

One thing to consider is competing drafts in the house. For example if there is no basement door you can close to prevent warm air from rising up the stairs from that basement space into the rest of the house, then there will be a definite draw of warm air up that path and your RMH will be an open air path from the exhaust back to the wood feed. If the house has additional stories then the draw could be even stronger, especially if there's any windows open even a bit. Even the bathroom ceiling fan vents will draw air. Just something to consider, you could light a match and put it out, then hold it next to the existing vent you want to use for the RMH exhaust while it's open and see if the smoke flows back into the house or not.

If your heat riser is made from bricks which aren't insulating (but the riser is also wrapped in insulation), then those bricks will retain some heat and help create a proper draft when starting a new fire. If the system is cold then competing drafts will cause more trouble.

Yes, forced induction works very well.

Check out this thread over at donkey32 on pro boards; He runs a better blower on page 2

http://donkey32.proboards.com/thread/3473/mini-rocket-forced-induction