New build questions

[abarlow]

I am trying to understand the physics of riser height. It would seem that the taller you make it the more the exhaust will cool off. Is that what we want?

But then doing less insulation seems it would also let it cool off.... My other thought was that you could simulate the additional riser height with more insulation.. I don't get the physics going on here. Does anyone understand what's going on and why a taller riser is better? I assume it has to do with creating more draw. But not sure how it does that.

[matt walker]

It's a damn good question Adam, I'll take a crack at it but I might be way off. So, firstly, insulate the heck out of the riser, always. The reasons for this are twofold. Firstly, and what's normally stated about this feature, is that a large differential in temperature inside the riser vs. outside the riser helps contribute to motivating the gasses. If the riser is insulated the gasses in contact with the barrel can cool quickly, give their heat to the room, and fall inside the barrel. This goes a long way towards making space in there for the new, hot gasses coming up the riser and adds to the draft and overall performance. Without enough insulation, if temps equalize inside vs. outside the riser, the system will stall. Secondly, insulation in the riser helps keep the gasses as hot as possible, aiding in completely burning any remaining fuel and again helping to keep that draft strong and the gasses rising.

As for height and insulation and draft, well, here's my take.... Gasses that are hot and getting hotter are expanding and getting less dense and rising. The higher they go, the more they expand as atmospheric pressure decreases with height. Due to that, the restriction of the riser in terms of size means that as they expand more they accelerate as the expansion works along the same axis as travel. So, higher riser means faster moving gasses at the top means more draft. It helps me to imagine a very short riser, if the gasses can leave the restriction of the short riser and expand unrestricted it makes sense that the draft would slow right there at the point where they expand. Keep 'em contained, and they are essentially racing to get up and out so expansion can happen. That's my take on it, not sure it's entirely correct, but maybe it's a starting point.

[abarlow]

[mannytheseacow]

Great job, Adam! That's looking really good there!

If you read back through my 6" build you'll see I was really frustrated with mine too. All I can say is hang in there and burn baby burn. Dry that thing out. Pack it full of wood and cover it with a pot and burn it, burn it, burn it. I was experiencing the exact same things with creep and smoke, and smoke in the house is a big concern for me too. Once it dried out, it has been running beautifully, and I'll never go back.

Having said all that, also try getting your exhaust outside to turn 90* up and run the exhaust higher than the top of your barrel. You definitely shouldn't need a fan. Once you get the horizontal run cobbled in it will run way better too. You need that heat through the long exhaust to keep the gases moving. Stick with it fella, you're almost there!

[abarlow]

Manny,

Thanks so much for the encouragement. You don't know how much that means to me, I have been pretty bummed out and was feeling pretty bummed since this is the centerpiece of the kitchen and I can't finish the kitchen till it's done!

What do you mean cover it with a pot?

Also the exhaust would get rain in it if it points up.. so do you mean run it up then elbow it again?

Thanks everyone,
Adam

[mannytheseacow]

Hey Adam,

Just curious if you've checked out the book on RMH... it's not quite as cool as some of current stuff that's going on but there's some great background and general information on there.

I cover mine with an inverted 6 qt. soup pot when I'm away or starting to see any smokeback issues. It was an integral part of drying my system out. I just invert it over the feed, and keep it propped up on one side an inch or so to keep air going in. Any smoke gets caught up in the pot and then sucked back down. any kind of metal container will work.

As for the exhaust, yeah you will need some kind of cover to keep water out. You could try running it up and then put a 180* bend at the top or just take some sheet metal and run a horseshoe over the top to keep rain out. Not sure how clear that is... I don't really have a way of drawing a picture right now. I know duct work gets expensive.

Looking more at your earlier post, I'm thinking the fact that your duct work isn't cobbed in yet is a major part of your problem there, though the drying part is a problem too. Functionally though, you're just fine. You just got to finish it up and dry it out!

[matt walker]

Yeah man, great job there! So good to see you and your progress. What Manny said, you are almost there, and what you are going through is completely normal, so don't get discouraged! You won't have smoke in the house once you get it dialed in, I promise.

So, a couple things. First, you are going to have to go up with the chimney quite a bit higher than you are right now. Ideally, you'd have insulated chimney going higher than the highest point on your roof. Insulated chimney is crazy expensive, so if I were you and didn't want to spend the cash on the expensive chimney I'd start sourcing sections of single wall ducting and going up as far as you can. A chimney cap will keep the rain out, don't turn the exit back down. Down the road you will want to get the chimney up above the roof line so you aren't subject to wind driven pressure differential, but in the mean time you can start to improve performance incrementally by adding sections as you get them. For better performance, the outside chimney should be insulated. It will never be hot there, so you can box it in with wood and insulate with fiberglass or perlite or something, but that can probably wait until you get a little farther along.

Secondly, the exposed horizontal ducting is cooling the gasses way more than they will be cooled once it's all cobbed. That rapid cooling to below condensation point is a big part of your troubles. Heavy, wet, dense cold exhaust is really hard for the heater to push out. Keep cobbing, keep burning, use your fan to overcome this stage. Once you have it all cobbed in and the cob is warm, it will be WAY better. My home heater is a bear to start when it's cold, but ideally that only happens once a year. You will most likely have the same characteristic with yours, so be aware of it, and again, you can use your fan to overcome this stage.

A feed cover is a huge help, as Manny says. I like a pot turned upside down, I posted a pic here somewhere, hold on.....

Like so...this way the smoke is captured and sucked back in to reburn...



I'm confident you will get to the point that it will run perfectly without the fan just by having warm, dry mass and a vertical chimney, so I say, keep going man!

[abarlow]

Okay great! I actually have an old chimney right there that i can hook into right there. I thought you WANTED to have it come out horizontal (saw that in a bunch of folks heaters). Matt, when the pot is over it, how does it get air?

[abarlow]

Nevermind , had missed Manny's last post. I do have the Into Evans book. I think I'll pick up the new edition too. I'll do the pot and prop it up. Thanks for the help guys. Still trying to find a clay source on my property. Then I'll cob the exhaust.

[abarlow]

Guys, I was reading in Manny's thread and just thought of something and it has me worried. For the exhaust exit from the large barrel i just squished a 6 inch pipe a bit. I thought squishing it would keep the same CSA, but on further thought I think it reduces the CSA.

The CSA of that exhaust hole, sounds like it's pretty critical. Should I tear it all up and open it up? Could that be part of the problem?