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thermal mass

Building bench with heat transfer from adjacent stove

pollinator

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posted 2 months ago

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I would love to have a rocket mass heater but can’t afford to replace the 6” flue in our existing stove with the complaint options. My stove does get pipping hot and the wall next to it gets to 175 degrees F.

I was considering building a masonry, clay, cob, or similar bench. I was thinking of using a sheet of aluminum embedded vertically and sticking out next to the wall that gets hits with the heat. (The stove is in a corner. To the left would be the bench. To the right is a door so that side isn’t an option.

The idea being the aluminum would transfer the heat to the bench sorta like a thermal mass. I’m thinking aluminum because it’s cheaper than copper. The idea of using water or another liquid isn’t appealing because of the added complexity.

Does anyone have thoughts or suggestions?

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

Glenn Herbert

Rocket Scientist

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Location: Upstate NY, zone 5

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posted 2 months ago

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First off, you don't need to change your stovepipe/chimney if it is currently safe for your woodstove. If you have decent natural draft, an 8" J-tube RMH can work with a 6" chimney, since the system does not need or want the amount of airflow that 8" is capable of. Alternatively, a 6" batchbox system can deliver more heat faster than a 6" J-tube, so is more likely to easily heat what your woodstove heats. In Colorado, I expect you want constant heat for much of the winter, so a batchbox would take less tending than a J-tube.

S. Marshall

pollinator

Posts: 179

posted 2 months ago

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thanks, I didn't know a 6" pipe would be enough.

Last year I was in touch with my county and my local FD to see if I could get the ball rolling on any type of approval. My concern being insurance if my home burns down or something. They all pretty much thought I was crazy and had no idea what I was talking about.

My floor joists do run the wrong direction for a heavy thermal mass which is also a negative. The wife also likes to watch the fire burn in the stove. So... it's a bit of an uphill battle.

I was going to build a RMH in the basement until I was told I would need class A all the way up to my roof which would cost $15k+ just for that.....

Still, I want to bring in mass around the stove if I can and a bench would be perfect. I'll be adding XPS insulation to the outside wall for starters. Without that my mass could be giving some heat the that exterior wall.

I imagine burying an aluminum heat transfer rod/plate into the bench stickout out near the stove may not do much anyhow. But it got me exciting to think about. Imagine a repurposed baseboard radiator with a side sticking out near the stove (the majority being inside the bench surrounded by fine sand and then the clay/cob.

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

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posted 2 months ago

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Hi S;
Have you seen Glens 6" Batchbox build in Western Colorado?
Also, at 8000' elevation, built last winter and only used for a short season, he has been blown away this season at how amazing his 6" Batchbox really is!
https://permies.com/t/238503/Batch-Rocket-Build
https://permies.com/t/248275/Batch-Rocket-Double-Skin-Bell (material list)

If you build an 8" J-Tube, use 8" pipe until you reach the 6" roof jack

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

S. Marshall

pollinator

Posts: 179

posted 2 months ago

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Glen did an amazing job at that. Looks good too. For my space I’d need to have something with a more gentle look for my wife. Have you seen this woman’s masonry stoves?

https://shko.ca/about/

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

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posted 2 months ago

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Yes, she does beautiful work.
I have no idea what her work costs, but it is a fair amount more than you can build an RMH yourself.
This is my 6" Shorty core with a single-skin bell. Your wife might like how it looks.

20250112_152911.jpg

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

S. Marshall

pollinator

Posts: 179

posted 2 months ago

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That’s a beauty also. I’m sure my wife would love it, but our house is a weird layout. We could possibly find a spot for something like that but it wouldn’t work where our current stove is.

How heavy do you estimate yours is? Did you have a concern about the weight on your joists?

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

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posted 1 month ago

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Being a Single skin bell, I estimate around #2500 or so.
The footprint is apx 4' x 5'.

My home is a 100 year old cabin.
Built with larger logs. There is no easy access underneath.
Shorty spans 2 log joists, and so far, we have no problems.
Because it's a small house, we can avoid stomping around or doing jumping jacks in the living room. (Or anywhere else.)

20191114_142722.jpg

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

S. Marshall

pollinator

Posts: 179

posted 1 month ago

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I love your cabin! I'd much prefer that to the poor quality 1990's framed house we have. Builders didn't even bother to wrap the house in an air barrier. There's 1/2" gaps between the OSB in places.

I can dig around so no worries if this is too much to explain. But can you remind me again of the advantages of this type of bell stove vs the rocket mass heaters that popped up here originally?

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

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posted 1 month ago

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There really is no comparison.
J-tubes with a piped mass have been superseded by stratification chambers (brick bells)
Batchboxes are much more common now than J-Tubes are.

Here are a couple of threads to start you on.
https://permies.com/t/272271/distinctions-RMH-type
https://permies.com/t/270559/Stratification-chambers-Bells-explained

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

S. Marshall

pollinator

Posts: 179

posted 1 month ago

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Thanks. The idea being that the large bell enclosure does the same work as thermal mass as a piped mass? I did like the idea of warming my butt on a heated bench, but I guess that's out of fashion? Is this an efficiency issue? I do like the batchbox for reasons you've already mentioned.

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

3319

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posted 1 month ago

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The bell can be a bench or any shape that fits your home.
There are specific (ISA) internal surface area numbers for each size bell & matching core.

I will tell you that a brick bell radiates such awesome heat into your entire home that it is much more comfortable to sit in a recliner or couch than on a cob/brick bench.

Heating with bricks is a unique experience not found by heating a steel box with wood.

Once you go, Brick, you will never go back!

https://permies.com/t/267527/Shorty-Core-bell

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

S. Marshall

pollinator

Posts: 179

posted 1 month ago

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Do you think large rock would have a similar feel to brick? Just exploring ideas - there is a place to build one but it would require a new chimney and chase which i've been told would cost an arm and a leg.
I loved the thread with your build. Really incredible stuff. Why'd you suggest the bigger build at first? Guessing it heats a larger area because of it's mass?

West of Denver, Colorado @ 8,000'
Zone 4(ish)... Summers are still brutal!

thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

3319

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posted 1 month ago

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It is easy to brag about Glenn's build; after all, he was one of my apprentices.
His build is near you and in a very similar climate.
He also kept a very accurate material list, as well as good documentation of his building progress.
And he did an outstanding job!

Yes, the bell can be built with stone, metal, or even concrete blocks at the lower levels.
They can be pretty or utilitarian-looking.
Shorty Core and the DSR design stoves can have a large window in the door, and if you use my door design on the first-generation Batchbox, you can have a smaller window.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Joylynn Hardesty

master pollinator

Posts: 5036

Location: Due to winter mortality, I stubbornly state, zone 7a Tennessee

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posted 1 month ago

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Would it be a bad idea to build a bell out of brick, then cover the brick with pretty tiles? It would of course add a lot of expense. I happen to be a fan of brick and stonework.

'What we do now echoes in eternity.' Marcus Aurelius
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thomas rubino

rocket scientist

Posts: 6478

Location: latitude 47 N.W. montana zone 6A

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posted 1 month ago

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Yes, absolutely, you can cover up that beautiful brick... with equally beautiful tile or stone.
That is what Scott in Iowa did with his stove.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

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