Rocket Mass Heater for my home

[matt walker]

Had my first fire of the season a couple nights ago. Feels so good to get the stove going again. I should have done some plastering around the feed while it was off season, but I didn't. Here's my first fire of the season. I cleaned up the wood shed and burned some sticks and splitting scraps and such.

[mannytheseacow]

There's something special about sitting around a fire. Something primal in us. Looking really good there, Matt. Here's to staying warm this winter, Cheers!

[4seasons]

I have noticed on your last updates the red/pink plaster you added around the feed and viewing door. Did your original cob not hold up in this area? Also what did you use to get that color as opposed to the mustard color of the rest of your bench?

[matt walker]

The feed itself wears from wood abuse. I consider adding a few handfuls of core mix every couple months a matter of regular maintenance. I enjoy it as it gives me a chance to try different mixes for the core patching, as well as different plaster mixes and colors. So, yep, it's got new stuff on it, regularly. The colors are concrete pigment additives, yellow for the mustard color and red for the latest application.

[ByronC]

Nice build Matt. I'm pleased to have found this board to check your RMH construction details / photos. Been looking into building an RMH for my place for a number of years now, and presently studying Ianto's "the book" on R.M.H.

I'm curious to know what the stove operating characteristic differences are between that of your cast core and cast riser and a stove constructed solely of dense fire brick?

And then there is the burning question of whether to go with a 6" or 8" RMH. Space is tight so there's room for only 32 ~ 33 ft. of total exhaust pipe length in the form a bent 16 ~ 17 ft. "U" in the thermal mass bench. I.e. burn unit, 3' of duct to 45s' to 11 to 12 ft of ducting hairpin in the main bench. The RMH will be centrally located in the house and against an interior wall of an otherwise open 20 x 25 ft. combined kit/DR/LR where the box stove currently resides. The existing chimney is 6" ID stove pipe extending up to a ceiling thimble where it transitions to 6" ID Selkirk Metalbestos double-wall stainless steel insulated pipe (from the ceiling thimble all the way up to and above the roof's ridge-line). The total chimney vertical length from floor to cap is about 14 ft. This appears to be screaming 6" RMH. I've thought about building an 8" RMH and trying it first on the 6" ID chimney stack pipe, but I'm unsure how the 6" chimney's bottle neck "exhaust damper" effect would affect performance of an 8" RMH. Any ideas y' all?

[mannytheseacow]

Welcome Byron! Key piece of information missing is what part of the world you're in or what your climate is like and how big of a space you are trying to heat. If it's small and you're in a mild climate you can get away with a 6 but unless both of these are true I'd go with an 8. I've seen several 8s in the forums now that are piped down to a 6" exhaust at the chimney and run just fine. You've got the chance now to go big so I'd say do it. This is some information I wish I had when I built mine.

As for cast vs. firebrick, I'll let someone else tackle that. I've no experience with brick.

[ByronC]

Ah heck, I knew I left out something! Sorry about that. My location is about 30 miles north of Knoxville TN, or about 16 miles south of the Cumberland Gap. During the heart of most winters we have low's in the mid teens, and highs into the 30s' (F.) Things were interesting with that Arctic Vortex this past winter let me tell ya, with -4° F. here one morning. Had the box stove running full tilt (Hudson River Stove Works medium Seneca, which has the secondary burn combustion air supply tubes in the firebox ceiling) and could have used a bit more BTU.

Insulation just keeps on paying for itself, so a few winters back I added an extra layer of R19 on top of the existing R19 in the attic space. The exterior walls remain R12 ~ R13. This house is only 950 sq. ft. and all one level, and there's a 420 sq. ft. scabbed on shop addition. I've been using a 12" fan to move warm air from the house through the interior connecting doorway into the shop. I could open up that 36" doorway between the heated space and the shop, if needed. Anyway, that arrangement been working out just fine for several years now keeping both house and shop at around 67° ~ 68° F. on those days where highs are in the mid 30s', with the existing box stove. For the RMH build I'm thinking about ducting some air under and up the back of the heated bench and routing that into the shop.

[ByronC]

Thanks for the welcome Manny. BTW, I checked out your RMH build a few days back. The brick work is especially nice. I'm leaning towards doing something similar, mainly to save on the amount of cob work.

[matt walker]

Welcome Byron, good to have you aboard. As Manny says, there are examples of folks using 6" existing flue with 8" systems, and they report good results. I think there are some variables to consider, but I feel pretty comfortable suggesting going with that arrangement. I prefer the insulated cast core over heavy fire brick construction due to the brick builds taking a long time to come up to working temp and the large coal/ash build up due to the mass of the bricks wicking heat away from the combustion zone. Not only that, but they tend to get super hot up high in the feed making smoke back a significant issue, not to mention burned hands from the edges of the feed. I think an insulated cast core is a big improvement over the brick builds.

[ByronC]

Hi Matt, thanks for the welcome and info. Good to know about the coal/ash build up issues, and the feed tube flame creep. And from what I've read, the typical start up for an RMH constructed of dense firebrick core and riser, is that for the first hour and a half to two hours involves feeding it enough kindling and small sticks just to get the thing up to operating temperature, before going to larger wood etc.

In mild weather and climates, where firing an RMH is needed only every other day or so, this slow start-up characteristic would quickly become old for the likes of me.

Which begs the question, what is the typical time for your stove to come up to operating temperature, from a cold start with kindling to burning the largest sticks of firewood?

Thanks for making those how-to youtube vids of your core and riser, BTW. As they say, a "picture is worth..."