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rocket mass heaters

Full sized firebricks for heat riser

Chris Vee

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posted 1 year ago

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So here’s my heat riser; I think I’ll cut the top layer of bricks to create an angle that allows the ash to naturally fall… then think I’ll use loose duct around the riser with perlite/clay packed in…

All thoughts & critiques very welcome.

cheers!

F415DB1F-04C6-4B89-BEA8-7F007F818480.jpeg

Butt warmer heat riser made of firebricks

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Fox James

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posted 1 year ago

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That is a huge chunk of mass to heat up, it looks like you used something much thiner and lighter to line the fire box?

Chris Vee

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posted 1 year ago

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Yup Yup!

the plywood is just the form... plywood & wood glue..

then the core is perlite & refractory cement... a huge chunk of mass? The core or the full-size fire bricks?...

Chris Vee

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posted 1 year ago

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Do you have critiques or recommendations?

thomas rubino

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posted 1 year ago

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Hi Chris;
A heavy firebrick riser will work, it will just take longer to get warmed up to rocket temperatures.
This is not a big problem, all the early-built RMHs were made this way.
I like your idea of angle-cutting the top bricks to help the fly ash to slide off.
I recommend wrapping the riser with insulation, Morgan superwool would be my choice but Rockwool will work as well.
If you want to use perlite then craft a sheet metal wrap, retain it with tie wire, fill the inside up with perlite, and skip adding any clay at all.

Fox was referring to the heavy brick riser as a (chunk of mass)

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!

Fox James

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posted 1 year ago

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Hi Chris, as Thomas points out, you can run a J tube with a full brick chimney, I have done similar myself however there would be a considerable performance increase if you used insulating material or even split bricks with insulation on the outside.
I would say it depends on how you want your stove to work or how much heat you want in a given time?
Perhaps you have an insulated small house or perhaps if you want to heat a large space quickly?

thomas rubino

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

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posted 1 year ago

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Hi Chris;
While looking at your photo I noticed how close the wainscoting would be to your barrel.
Were you planning on a metal heat shield to protect that?

Your core casting looks great, it should perform well for you.

I agree with Fox about a highly insulative riser outperforming your current riser.
Full-size insulative firebricks are hard to beat, although I would still wrap them with a second insulator.
At my house, I would use Superwool and build a 5-minute riser... Of course, having 25' rolls in my shop makes that easy to say.

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!

Chris Vee

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Location: South Carolina

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posted 1 year ago

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I appreciate all the feedback,

We are heating the coldest room of a fairly large 1 story home, it's an old prefab home and there are a lot of small "cracks"/ "air-leaks" so this should help heat our chilly family room and we will be working towards cycling some of the heat into the other main living area.

I angled the top bricks and used perlite to insulate. In the bottom-most and top-most of the insulation I mixed some clay slip in to give it form and to try and keep an angle to support fly ash falling toward an easy-to-clean access door. (only about 20% of the perlite is mixed with slip)

Then I used two 10" duct pieces cut to length and used metal screws to secure them where I wanted them snug.

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A butt warmer core and riser made from firebrick with chamferred top

Glenn Herbert

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posted 1 year ago

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A heavy riser like that is going to work best when run for a long time each firing, so once the bricks heat up they can coast without taking more energy from the burn.

Chris Vee

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Location: South Carolina

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Glenn Herbert wrote:A heavy riser like that is going to work best when run for a long time each firing, so once the bricks heat up they can coast without taking more energy from the burn.

Awesome, the plan will essentially be to run it from about 11 a.m. to midnight each day throughout winter... does that sound right or should I consider starting it earlier each day so there isn't as much time to cool between night and morning?..

thomas rubino

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posted 1 year ago

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In South Carolina, 11 am is probably fine.
Here in Northern Montana, when it's cold (Around zero) We burn from 7 am through at least 10 p.m. Sometimes Liz will load it up again at midnight.
We keep an uninsulated greenhouse warm all winter long with a 6" batchbox.

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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