Heat riser length

[dato1200]

Hi All,

I'm planning on making an 8 inch RMH, casting my core like Matt and others have.

I want to make the barrel, or part of the barrel removable. Initially I thought of getting a barrel with removable lid and lock ring.

Then I thought "why not use two barrels? Isn't bigger always better?". I could get two barrels at cut them both around one of their ribs.
There are two ribs on each barrel, so after the cutting, I'd have 2/3's of a barrel and another 2/3's of a barrel. I could cob in the bottom 2/3 barrel, then stack the other 2/3 barrel on top of that (using a lock ring to hold it together and maybe welding in some strength so the lock ring doesn't collapse the barrels).
Now I'm 1 1/3 barrels high and want to and I want to increase the length of my heat riser by the same amount (1/3 of a barrel).

My question is: when it comes to heat riser length, is bigger always better?

[matt walker]

Dato, I would say yes. I've seen discussion ages ago about speed of air/gasses through the burn varying with riser size and there being a sweet spot, but I've played with all sorts of riser heights and in my experience there's only advantages as the riser gets longer, as long as it is highly insulated.

The one big variable you are throwing in that a lot of folks don't discuss enough, in my opinion, is the size of your radiator. The amount of exposed barrel surface area is a major factor in flue temps through the bench, and by adjusting this you can adjust the balance of quick radiated heat vs. slow radiant/convective heat in the mass. You can adjust after the build by building up the cob surround up the barrel.

[dato1200]

This is purely a guess, but I'd say 1/2 a barrel cobbed in, the rest will be exposed. I could easily do less or more exposed barrel.

I'll have the exhaust flue from the barrel to the mass cobbed in, so there goes 10 inches of barrel cobbed in.
I want to add in water heating, running copper pipe through the mass, then have it coil around the bottom part of the barrel on the outside, above where the barrel exhaust is.

Heated water will be stored in an open top barrel. I don't want to re-do any work, so I figure build the copper pipe in now. Next year, when I get an electric water heater, I'll build a closed loop system with pressure relief valves, etc. to run through the baseboard heat in my house.

[matt walker]

Sounds like a good plan Dato. I like your style, plan what you can and get started. The details tend to reveal themselves along the journey in my opinion. Hope you'll share your build with us, it's sure been exciting for me to follow along with all the builds here. I'm really interested to see how the water heating loops end up working out for you and Manny and others here interested in that type of system. My neighbor is talking about doing that on his second build, and it would be great to point him towards you guys as examples.

[mannytheseacow]

Just chiming in here with the limited experience I have on my own system. I would think you would want to try to get as much heat from your barrel into the bench. I'm not seeing temperatures yet that will facilitate any serious water heating other than in an emergency situation. I think extending your barrel is the wrong way to go. The problem is the high differential between what is comfortable ambient air, and comfortable hot water. It takes a lot of energy to heat water, and pretty warm water to feel warm, by comparison. That's my 2 cents anyway. I would SERIOUSLY love to see it work, though. I'm at the point I think you would need two systems. One for heating your space and one for heating your water.

[pa_friendly_guy]

I understand your point manny. Feeling warm in a room at 72* verses 90* for warm water. The one thing that may work for him in this situation is the fact that he plans to run the copper coil around the barrel before it goes to the tank. The higher temperatures generated in the barrel may be enough to get the water temperature where he wants it. If the water intake starts in the bench and gains heat as it goes through, it will really gain a lot of heat by the time it goes past the barrel. Dato, you may want to run a good bit of copper around the barrel to insure enough heat gain. This does drain heat from the system and may effect its operation if you rob too much heat. Steam explosions are always a concern when you are heating water with a fire, you are addressing that issue with a open ended system for now. I would think about putting a check valve on the system to insure what direction the water is flowing. Other that that you just have a hot water syphon that should work on gravity flow with out pumping. You will know what results the unit will produce after you build it, unfortunately I don't know how to calculate how it will work before you build it. There are people much smarter that I for that task.

[matt walker]

Not to derail Dato's thread, but perhaps this will be something he can use as well.

Manny, a simple thing you might try to change the balance is pack a layer of insulated cob on the barrel top. It won't take long and is easy to remove if it doesn't do what you want. Might be worth a try to push more heat into the bench, the top radiates a TON of heat as I'm sure you're aware.

[mannytheseacow]

Yeah, I don't doubt that adding some cob to the top of the barrel would get more heat into the bench. Overall, I'm just having a hard time getting enough heat out of the 6" RMH to heat my house, let alone taking any away for water heating. It keeps my downstairs very warm but not much heat is getting upstairs. If anything I'm needing all that air coming off the barrel, so in that respect having a bigger riser would be better for me- if I was home to feed wood into it 24/7. It's just not cranking enough to do both. That's just my situation, though. It might work in a different setup- smaller house, bigger design.... I'll cross post info about my temps and such in my 6" build thread to keep things focused.

[dato1200]

Almost everyone is using a standard barrel as the radiator, and that dictates the length of the heat riser. Is this heat riser length really optimal for burn efficiency?
In the grand scheme of things, it probably doesn't matter too much, but maybe an extra 6 or 12 inches of heat riser length might be good. I have no idea.
I don't mind welding two barrels together to go anywhere up to 1.5 barrels high (any higher and the ceiling might catch on fire).

As for the water heating, maybe I should be coiling the copper pipe inside the barrel? I'm guessing I'd want the copper pipe something like an inch in diameter. Now am I messing with my barrel wall to heat riser wall dimensions? I could always cut down the side of a barrel, cut 12 inch wide piece out of another barrel to increase my radiator circumference by 12 inches.

[matt walker]

I wouldn't put the coil inside the barrel Dato, there's times it would be too hot, and it would almost always be on the edge, in my opinion. As for riser height, I think the extra little height would be a very good thing. As you noted, most folks are just using a 55 gallon drum and that dictates height. I end up building up the drum landing a bit to raise it up and get an extra inch or two. I wouldn't go bigger in circumference. The radiator can be too large and that can shed too much heat too early.