Rocket mass heater outside air supply

[grainbinman]

Hello guys, I noticed you said something about a manual damper on intake air supply, that to me seems fine for shutting off out side air but I think you would want a auto damper/one way flapper. this should take care to the reverse chimney draft? Other positives I have been thinking, the colder the air the denser it is, I believe it would feed fire tube slightly better, no preheated air required? I would think bringing the supply air up from the bottom would also help... any thought?

Matt
I replied to your gmail you sent me, but it was returned to sender? Anyways yea that is me Dan.

[matt walker]

Hey Dan, good to see you here. I think a flapper would work fine. My thought regarding the damper is that in most RMH setups people typically use bricks or something over the feed to regulate how much air is going into the fire. Usually it's to create a little more speed near the inlet to suck smoke down more aggressively. I've found that different woods will have different intake air volume needs as well. I like to be able to regulate how much air is being fed, which is why I'd like a manual damper.

[mannytheseacow]

I have exactly the opposite thought. I have an old metal potbelly stove that eats about 4-5 cords of hardwood each year. I'm interested in switching to a rocket mass to reduce my biomass consumption (and hopefully ease the smoke damage to my home and lungs from the ramshackle potbelly). I'm concerned that because my home is such a tightly insulated envelope that I won't get enough draw for the rocket mass heater to work properly? All that air has to come from somewhere, right? I was thinking that I will need some sort of fresh air inlet. Any thoughts?

[matt walker]

If your house is that tight, you just might need to give the stove an air supply to keep it flowing.

[pa_friendly_guy]

You are already burning a wood fire in your home with no ill effect. Burning up 4 or 5 cords of wood should take more air supply then burning 2 1/2 cords of wood. Even though you may be having a slow burn in the current pot belly stove, burning 1/2 the wood should take less air than what is going up the chimney now. The RMH does burn a lot quicker and so your concern is that the quick burning fire will deplete the air supply before it has a chance to recover. That may indeed be possible, but I truly do not think you should have any problems. I would definitely try the new RMH and use it awhile before I started to cut holes for an outside air source. Those new air holes are a major way for cold air to infiltrate your home. You pride yourself on your home being tight and well built, I would not do anything to change that until you are sure you have a problem. I guess my feeling is, If it aint broke, Don't fix it.

[matt walker]

Right. I'd build it and see how it performs. Open a door and see if the performance improves. If so, give it some air.

[Nutcase]

Interesting question, Matt.

The rocket has a limited amount of force to work with in drawing air through the system. This force comes from the bouyancy of the exhaust gasses in the heat riser and in the chimney. For simplicity, let's ignore the chimney. The the theoretical maximum force that the heat riser can generate is the weight of the volume of room temperature air that it could contain, right? That is the force that you would have if the gas in the barrel outside the riser were cooled instantly to room temperature and the gas in the riser were infinitely hot and therefore of zero density. Right? Been decades since I took a physics course.

Given that the rocket has a limited max force to work with, the key to getting a better draw is low drag and minimal counterforce, once you get anywhere close to the asymptotic limit. The outside air has to get there without being sucked from a much lower level through a soda straw. Duh. I'm sure you already know that. Not everybody does, though, so maybe it's worth saying. That said, it's not easy to improve on direct supply from the room at the inlet.

It's nice to be able to see what is going on. Could still see down the fuel inlet if it was covered with a pyrex dish, maybe. Fuel burn rate mostly equates to the rate at which the wood is boiled off, and this is something I don't have enough experience to predict for wood in a vertical chamber heated by convection with the air inlet where exactly? But with the right sort of system you could play around with the geometry...

I suppose that there is a risk that fuel in a dead end vertical chamber could heat up enough to burst into flame when the cap comes off. of course, with a horizontal loader, that's less of an issue. If you know what you are doing and can see what you are doing, this is also less of an issue. However, you have to assume that a kid will mess with it some day.

I've thought about having the air supply rise through a shroud around the burn chamber leading into a large diameter tube concentric with the air/fuel inlet. Sort of like a barrel and heat riser. Could actually be a barrel and heat riser, except for the issues with fuel loading. Not sure anything like that is worth the infrastructure in my situation. Not sure it solves any actual problem, but the geometry is attractive.

[matt walker]

Thoughtful post, Nutcase. Good reading, and good stuff to think on. I've also enjoyed re-reading this thread from before I heated with the RMH. I still believe outside air could be useful to some folks, as Manny points out. I agree though, it's hard to beat unrestricted air in via the feed. It just plain works.

Nutcase, that unburnt fuel bursting into flames is why so many tall feed schemes fail. They make unintentional gassifiers as the heat eventually creeps vertically up the fuel feed.

I believe that the suggestion made to Piet in another thread is a viable way to go for outside air. An air inlet separate, but adjacent to the feed opening. A cover large enough to cover both, that could be placed over them to couple them temporarily when desired. Typical operation would just have the feed open, with the air intake supplying air into the room, close to the feed, to reduce drag on the system from a tight house. I dunno though, it would take some fiddling with for sure. ZeroFossilFuel has a video on youtube of his stove utilizing outside air, and does a nice job of it. It's worth a look for anyone thinking of going this route.

[pa_friendly_guy]

I aint so good at Geometry Nutcase, I like Matt's idea of opening a door and see if the draw improves a lot. If it does them add the outside air intake. The only draw back to that idea would be if adding the outside air after the stove is already built would be much more difficult. If it would be much tougher you have to think about that issue. I suppose to be safe you could incorporate the outside air vents but not hook them up to the outside. It would leave you options. Of course if you are going to all the work to design the vents and incorporate them into the stove design I guess it does make sense to just install it and use it. Burning outside air does have some advantages for a heating unit. My only concern for using the outside air supply is the potential for smoke back.

[Nutcase]

Pa-guy, I know I ddn't explain by idea very well, partly because it was still vague. Let me take another shot. Imagine cutting a barrel in half, perpendicular to the long axis. One half can serve as a stand for the barrel around the heat riser. You have to modify it so that the air flow can go in through the burn chamber and out to the thermal mass/chimney. Take the other half and put it over the fuel inlet! Of course you have to make the necessary provisions to get fuel and air in and the burning gasses out, very similar to what needs doing for the barrel stand. If the barrel has a removable lid, that would be one way to go for putting in fuel. From the POV of the fuel, the situation is little different from open to the air, if the opening into the half-barrel is big enough and short enough. Or you could pump it or choke it or whatever.

Also, I think the aesthetics could work.Plus now you have any mess generated around the fuel inlet contained.