6" Batch Box ... 30 or 55 gallon drum?

OK, one last design question and then I'll be ready to start building my RMH soon.

I'll be using a 6" batch box foam core constructed by Luke Parkhurst based on Matt Walker and Peter Van den Berg's work, well illustrated and referenced in this forum.   My addition is 546 sq ft.   My understanding is that I could use either a 30 gallon or 55 gallon drum.  A 30 gallon would look and "fit" better in my small space,  a 55 gallon drum is so much larger visually, and let's face it, not that aesthetically pleasing, IMHO (oh, I an hear the wise ass remarks from here...)  I'm mighty tempted to go with a 30 gallon drum, but I'm concerned that the heat would be too intense, that it would flash very hot when fired and it would burn out the drum quicker than a 55 gallon.  

Any feedback, suggestions, opinions out there?   I see lots of 55 gallon drums in use,.  Anyone using a 30 gallon drum?  What's the size of your combustion box?  How do you like it?

Thanks folks, been loving this forum.  I look forward to posting photos when I start my build.

Mark DuPont

Hi Mark;
I have the same barrel I started with 7 years ago.  Common for the top to be 1000-1100 F . It looks as good as new.
Both sizes are the same thickness and have a very long life expectancy.
What kind of riser are you planning on using?
Are you aware that you can use a brick bell in place of a barrel?
Also consider using a barrel and then stacking brick around it to catch some of the radiant heat.

Hi Mark,    I would say they both would last the same amount of time. One big difference you would notice though is how much more instant radiant heat you would get from the 55 gallon barrel than the 30 gallon. If your interested in putting more into the thermal mass than go with the 30, more into the room, the 55.
Also, if you need more space on top for cooking, heating water etc. could also be a deciding factor.

As Thomas suggested, a brick bell could also be used to replace the metal barrel but it would take a much longer time to start to feel any heat from it (very little radiant heat) and raise the temperature downstream dramatically. A much larger downstream bell would also be needed to capture this heat before it goes up the chimney.

When I first switched over from a water tank which has an 18" diameter to a 55 gallon barrel, I too thought it was simply huge. Within a week or so though, I got used to it.

Thanks for your input Thomas and Gerry.  Very useful insight, and counter to my intuition, but now I see how it makes sense - I was imagining lots of heat concentrated in a smaller area, thus higher surface temps on the barrel; but what I get from your posts is that, with less reflective area more of the heat moves through the barrel and to the mass, which makes sense.  Given my small area I think I could get enough radiant heat from a 30 gallon, and I am going for more heat in the mass given that I already have a woodstove for the other half of my house (which also provides hot water).  I will be cobbing up and around the barrel to a degree.  

Yes, I'm aware of the masonry bell option.  Ryan McCutchen, a mason heater builder, was here last week for a site consultation and laid out several options.  I'm opting for the barrels for reduced weight on the wooden floor, and also because I prefer the look of a continuous, sculpted cob mass.  

Thanks again, this info is extremely helpful!!

Mark

Hi Mark;   Cobbing the barrel 2/3 up will work great. Mine has been that way for 7 years now. Thick at the bottom and thin at the top.
I found the best way to help keep that mud from cracking and falling off is to use a Fiberglass mesh. No it will not get hot enough to melt and off gas.
I don't have any photo's from the job, but basically put some mud on the barrel and then wrap several times with mesh. Cover with cob.
This only works where your cob is thick. Further up the barrel your mud will be to thin.
Your cobbed barrel will show cracks, every season.  Just get it wet at the crack and use your finger to rub in more sandy clay.

If planning to repurpose a metal barrel, this excerpt from the Rocket Ovens video could be helpful: Repurposing Metal Barrels