6" RMH Build

[mannytheseacow]

[matt walker]

[mannytheseacow]

Yeah, maybe too much math... and there are certainly other factors such as insulation, layout of home, and size of radiator/barrel. Glad to hear some input on it, though! Thanks!

[matt walker]

Dang it Manny, I think about this every day but just haven't gotten around to doing it yet. I'll put some rough numbers here now to get started.

Cubic feet of house. Damn it, this is the big one that stops me every time. Guessing here, +/- 1000 sq. ft. with 7' ceilings in two large rooms, a convoluted hallway, and two bathrooms.

Bench is roughly 16' x 2' x 18"

Roughly 32' of 8" flue pipe in there.

Currently 40°F temp. difference.

[mannytheseacow]

Thanks for posting this, Matt! I don't mind crunching the numbers if folks can even post just what you did. I'm just curious to see how this idea pans out.

Do you really have 32' of flue pipe in your bench? That makes your mass 25% duct!

Anyway, if so, then you have:
7000 cubic foot of house, 48 cubic foot of bench, and 67 sq. ft. of surface area in your tubes.

So your golden ratio is like 100 : 0.68 : 0.96 for a 40* rise. And it seems to be working pretty well for you! So, pretty close to that 100:1:1 thing. Maybe I'm crazy.

I actually made a mistake in my math the first time around figuring the surface area of my tubes. I used diameter instead of radius. My ratio is actually 100 : 0.87 : 0.31. Increasing my tubes to 8" would get me that surface area.

[matt walker]

Uh, I just actually measured the bench. It's more like 14' total bench, and maybe more like 26' of flue. Geez man, this is hard.

Now that you mention it, 25% actually sounds about right, when I look over there and visualize the mass as a whole. I tried to minimize my mass since I had concerns over the weight on the floor. It's one of the things that struck me about yours, it just looked huge compared to mine, and with much less flue. I think yours is better, but it's obviously harder to get up to the temps mine rides at. I thought it was very interesting to hear that Ray's was almost exactly the same surface temps as mine. Somewhere, there are some constants and rules in this. I'm glad you are trying to put some figures together.

So, we are having one of our cold snaps right now, which we seem to do about twice a winter. It's 27° outside right now, which is really cold for here. My house has terrible insulation, as I mentioned, so I can feel how cold it is outside from my seat here in the middle of the house. That said, I'm wearing a t shirt. It's 72° over there by the front door, which is on the far side of the room from the stove. My bedroom, however, is only 60°. I'm burning hard right now, whenever I'm inside, and going through about 60 lbs of wood a day with conditions like this. Man, I really need to insulate. On the other hand, all I have to do is read on the bench for a bit and I'm overheated.

[mannytheseacow]

[matt walker]

Yeah, as we spoke about, these things really are space heaters for modest spaces. I think both of us are trying to get more out of them than they really can deliver, and for sure your 6" is hard to run enough fuel through. I think the answer is not bigger heaters but smaller spaces, or dedicated heaters for different zones in a home.

Is your bench still just over 100°F even when burning for that long with it covered? Man, I had mine up to scary hot yesterday. My whole long section of the bench was >250°F under the cushion after burning all afternoon. Surprisingly it still felt great to sit on with the cushion on. I took off the cushion and the house jumped up a few degrees. I think one of the big factors here is what we talked about, surface area of heat exchange, or, flue circumference. Mine is 25.1" around and yours is 18.8", multiplied by length of flue. Big difference! Time for that upstairs heater. I'm for sure building one in my bedroom once the mud thaws out in a few days. I'm sorry to hear you are cold, but think of the children, and their tiny little lungs!

[mannytheseacow]

Yes, I definately have a lot of ideas for how I would do things different if I was building a new house- much smaller, one level, maybe a RMH on each end of the house. But for now I gotta make do. It would be awesome to have a place like they describe in Pattern Language where you sleep in the east and move west throughout the day, ending in the western end of the home, then have a RMH at each end. I might pursue turning my old studio space into a bedroom and then put a RMH in there to accomplish this idea. Pipe dreams for now.

I'm not sure what my temps under the cushions are when pushing the stove hard. I don't want to say since I'm not using a digital thermometer like you have. I'll say this- it's not too hot to touch. So I'm guessing 140* or less. I have wondered about just taking the cushions off becuase I think they might be overinsulating my oversized mass. I might try that next week and see what happens. I want to do it over several days so I would be comparing apples to apples. I can make my house warm on the weekends when I'm home all day. Just having time to feed the fire is one of the big limitations for me.... which leads to the question, "why heat the space if you're not there to use it?"

[matt walker]

That last quote is a big part of the learning curve of these things I think. When mine goes out, my house quickly is colder than I'd like it to be when I'm in the space, but it only will drop to about 60°F and then stay there for a long time. I was describing the cycle to folks in DC to being analogous to the way we run our thermostats, warm when we are in the space and cooler when we aren't.

I'll be curious to see what you think about cushions/no cushions. I still don't know what's best. Last night I had all the cushions off, and for sure I got a lot more heat into the space, the room was warmer than it had been. However, the bench lost 50° fairly quickly. I'm not sure if there's an advantage to driving up the internal temp before releasing it into the room, or just let it radiate into the room right off the bat. Longer warming cycle when the fire is out from driving the bench temp up? I really don't know, but I keep playing with taking the cushions off at different times. Removing them in the morning seems to bring up the room temps pretty quickly, but keeping them on brings up the bench temp by a lot.