Author Topic: Rocket Mass Heater Myths: "they cannot be that efficient"  (Read 1210 times)

Offline paul wheaton

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Rocket Mass Heater Myths: "they cannot be that efficient"
« on: November 06, 2017, 02:47:39 PM »
I sit down with donkey, peter, erica and mud to address an engineer's concerns about the claim that a rocket mass heater heats with one tenth the wood.

https://www.youtube.com/watch?v=t0cs8PWDfwg

Offline mountainmoma

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Re: Rocket Mass Heater Myths: "they cannot be that efficient"
« Reply #1 on: November 06, 2017, 03:13:51 PM »
I think lots of mass is good when you are in an area that is consistently cold, but I have had my doubts for areas that need a small amount of heat, sometimes, not everyday all winter, and not always all day, either. What are your thoughts on that ?

Offline paul wheaton

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Re: Rocket Mass Heater Myths: "they cannot be that efficient"
« Reply #2 on: November 06, 2017, 03:31:51 PM »
In that video I link to above, is a design for heating a shop really fast.  Triple barrels.  Lots of fast heat into the room and very little into the mass.  The idea is that most shops are used for a few hours here and there, unlike a home that has somebody in it constantly.


On the flip side, having a rocket mass heater in my montana home in the summer helps to keep things cooler when it is hot.  The mass absorbs heat from the hottest part of the day.



Offline surfivor

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Re: Rocket Mass Heater Myths: "they cannot be that efficient"
« Reply #3 on: November 06, 2017, 03:45:46 PM »

 I always wondered if the powers that be would like to ignore Rocket Mass Heaters because they want to push things like solar energy which ties into grid systems.

 I often think how compost toilets make a lot of sense but so many places require septic systems and flush toilets anyway.

Offline IKN

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Re: Rocket Mass Heater Myths: "they cannot be that efficient"
« Reply #4 on: January 03, 2018, 08:42:29 PM »
My wife and I have been playing around with different RMH designs. The one problem we've had is with cob. The heat transfer through cob is very poor.
 We've built a couple different J-tube RMH's up in our dog kennel for heat with cob as the mass. It will get pretty warm, but won't release enough heat to keep the building warm.
FYI, the building is a 24x36 with 8 foot wall pole barn that's framed in and insulated.
One of the designs used a 55 gal barrel with a 6" flue pipe ran 20 feet through a cob mass. Even burning full bore, it wouldn't heat the building except in the localized area of the RMH. Even tried fans to move the heat with no success. Another  issue was how fast the barrel bottom burned out from the heat and flame impingement
After a lot of thought, I'm considering a new design that will use a heavier, thicker burn chamber barrel and a 8" J-pipe burn chamber. The question is what to use for a mass ???
I'm considering using water with anti-freeze for the mass. Not the easiest to design or build, but water will hold a lot of heat/energy and is hard to beat for heat transfer.
Another idea was to use a pipe within a pipe. Use the Rocket stove exhaust to heat air in a reverse flow forced draft set-up on another pipe (either internal or external) to the RMH exhaust pipe.
One problem I have yet to figure out is a method for supplying outside air to the burn chamber. One thing I did notice on the other builds was the continued draft through the RMH after the fire was out. The mass would continue to heat the air in the exhaust pipe causing air to continually draft from inside the building to the outside. This caused cold air to be drawn into the building and remove heat from the mass.
To prevent this in the future, I want to install some sort of damper to minimize air flow through the burn chamber to outside when not burning. To this end, I've been researching some of the open source PLC's like Arduino and TI Launchpad MSP's to automate and monitor the system. I could use one of them to control the dampers as well as the draft fan speed to optimize the system. It may even be possible to monitor the burn chamber temperature, oxygen level, and air flow to maximize the burn efficiency and temperature.

Any thought/ideas for material to use for the mass would be appreciated.

Offline paul wheaton

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Re: Rocket Mass Heater Myths: "they cannot be that efficient"
« Reply #5 on: January 04, 2018, 06:54:00 AM »
I suspect that you don't have a rocket mass heater.

A lot of the things you have said are not the experience i have had or seen or heard of.  There are a lot of people out there that build something that they call a rocket mass heater, but it is not a rocket mass heater.   And then they teach it to others (typically through youtube) before they have event spent a winter with it. 

A few points

-  if your barrel has rusted through, it will be the very first barrel to rust through.  The lack of oxygen during the burn prevents rusting.  The oldest rocket mass heater is over 20 years old, has seen heavy use and is still using the original barrel.

Here is a video of that very old rocket mass heater:

https://www.youtube.com/watch?v=75sqmYAvHuc

- a six inch system should be fine for that size of building.   Does it get super cold where you are?

- cob conducts and holds heat extremely well. 

- I strongly recommend against using an external air intake

- If you want a LOT of heat really fast, you might consider an 8 inch batch box design.

- I advise against a damper

Here is a video of me covering a lot of these points:

https://www.youtube.com/watch?v=W2uZgRQ3uiU


Batch box:

https://www.youtube.com/watch?v=VL7O8qLXz24&t=8s


New batch box door style

https://www.youtube.com/watch?v=aX5DTNNZ9sg


Offline IKN

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Re: Rocket Mass Heater Myths: "they cannot be that efficient"
« Reply #6 on: January 04, 2018, 09:52:38 PM »
Thanks Paul, guess I should have explained further.
We're in central Illinois so is does get pretty cold during the winter.
Our original build used a 6" flue pipe in a J-tube configuration that I covered with a mixture of vermiculite, and fire brick cement. Kind of an insulating light cement.
It didn't have an outside air supply to the combustion chamber just drew from the building. We covered the burn chamber up pipe with a 55 gal. barrel with a 3" clearance from the top of the up pipe. We then ran 6" pipe from the barrel bottom out in a S shape in a 4x8 foot area that was edged with block and filled with cob. The pipe was ran out through the wall where we elbowed up a riser pipe up above the roof line.
When fired, it would draw very good giving that nice rocket sound. Some of the cob bench would get very hot, but other parts would barely get warm. The top of the barrel would get glowing red. Most of the barrel was covered in cob as well with just the top couple inches left bare metal.
Even after burning for up to 4 hours with well dried oak, the bench would be nice and warm, but the building stayed like an ice box. Even tried fans to circulate the air with little change.
I'll take a look at the videos you listed.
Thanks for the help.