Issues with Rocket Mass

Hi All,
I am another first-timer here with my own set of "problems" with our newly created rocket mass heater. First off, I will be the first to say that my hubby and I did not study the design as thoroughly as we probably should have, and put together a unit based on some of your You Tube videos and designs sketches. Please be kind...lol. Now, here's the problems we are having. Please note the pics I attached. I have not finished cobbing it as I did not want to go further until we work out the bugs.

My hubby, built the unit using a terra cotta tubes for the feeder (see pic) and the air chamber? (whatever that opening is below the feeder?). The combustion chamber was made from a 8" pipe with another terra cotta tube inside it. He used 8" pipe in the bench...but did not run it back to the barrel.

1. So, the unit does burn but when starting it from cold, it must be started from the access hole on under the feed to get it burning sideways, otherwise it smokes back.
2. We cannot seem to burn anything larger than small twigs and sticks in the feeder tube without it getting smoke back. He seems to have to put the wood in the bottom access hole to get it to burn sideways, laying them down on their sides...can't use feeder tube.
3. The darn thing burns really fast and have to keep feeding it. Of course we are using small wood but it still burns super fast, even with bigger pcs of wood.
4. Question: The bench is very wet due to cold weather and we don't start it often as we are not living on premise right now. How long to they take to dry out if burning it daily, esp in cold weather?
5. Question: since we did not bring the pipes back around and only have one 8", will we still get any warmth from the bench?

I told my huddy that the feeder tube is too long and there is a design issue with it. What did we do wrong? or rather...did we do anything right?? lol.

Go ahead and post your pictures. It will be easier to offer advice if we can see what's going on.

Off the cuff:
- I don't like the sound of 'access port' near the fire area. The combustion chamber is usually a J-shaped 'thermosiphon'. If you add air holes or access holes down in the bottom, it's just like poking holes in a hose and then trying to use it for a siphon. Leaky siphons allow things to flow out both ends of the 'siphon' - in this case into the room, as well as up the chimney.

- There are also some common problems further down in the barrel-to-bench connection.
Everything needs a minimum cross-sectional area that's about the same as your firebox and your exhaust. The only exception is the barrel and 'manifold' - the junction between barrel and bench. The barrel interior and the manifold can be substantially larger flow volumes, we often end up with 2-3 times the cross-sectional area here when possible.
It's common when building your first manifold to make a bottleneck either right at the bottom of the barrel, or in front of the mouth of the pipe into the bench. This will cause sluggish flow and can cause smoke-back into the room.
Fly ash will build up here, we usually include a cleanout that can be reached with hands or a shop-vac for annual cleaning.

- We have not had very many encouraging experiences with terra cotta chimney liners in the combustion area. They just don't seem to handle the 1200+ F temperatures, the few times we've used them they always cracked. We tend to use firebrick if buying new material, or kiln brick or older building brick if working with reclaimed building materials. You might be able to work the kinks out with your current system, but consider upgrading when possible.

- Make sure there's good insulation between the heat riser and its liner. 1" of refractory wool (rock wool, ceramic-fiber blanket insulation), 2" of perlite (stabilized with a little runny clay slip if you want to reduce settling), or 4" of loose-fill vermiculite.
This entire area - the firebox, and the bottom of the barrel - must be completely sealed and well insulated. This area is an up-down convection cell: hot air rises inside (1200 to 2000 F) and 'cool' air falls down the outside (600 F or so). But if any heat can leak in between, the temperature can equalize. And any room air that leaks into the barrel will tend to want to move upward or displace the falling 'warm' air. Stagnating air inside the barrel can cause smoke to build up and choke the fire.

(When there is no fire, and the ceramic liners cool down over an hour or so, this stagnation is actually a good thing: it slows down the hot air being drawn out of your home if you leave the heater open after the fire goes out.)

- The feed can only be about 1/3 of the height of the 'heat riser' - that's the insulated 'chimney' inside the barrel, that is the longer leg of the thermosiphon. You can check yours and see if it's close.

- If you are not using the heater routinely, you won't see much warmth from the bench. It takes about 2 days of burning for at least a few hours per day to bring the thermal mass up to temperature. Until yours dries out, some of the heat is sacrificed into evaporation too. In our experience it's usually about a week or two of firing before things are completely dry, but it varies enormously with local humidity, building techniques, and so on.

- We do sell as-built drawings of existing heaters that have been successfully tested for a year or more, if you want a set of detailed drawings and builder's notes to compare your installation with. We only publish the designs we feel are most widely useful across many situations. There's also a pretty good diagram for free on this page: http://www.ErnieAndErica.info/rocketmassheaterpermitting and lower down on the same page it also goes over some basic requirements for how to lay these out.

The one thing that was left a bit loose in the Portland code is the chimney. If you have a conventional home (shingle, tile, composite, or metal roof, with proper roof vents), you will want to use a conventional chimney. Any chimney parts outside the home should be well insulated, and the chimney should stick up above the ridge line.
Many of the original builders were working in single-story houses, some with a membrane-lined 'living roof'. In these structures, there is rarely negative pressure inside the house no matter the wind conditions or temperature. But with conventional homes you will encounter many puzzling problems as the weather changes unless your chimney opening is the tallest opening in the house.

Hope that helps. We are travelling now, so we may not be able to check this thread right away, but others will probably give you some good additional tips after you post pictures.

Yours,
Erica W

Hey, Sorry I took so long to respond back. Have not had a chance to sit down in front of the puter much this past week. I did post the pics? I thought? But re-posting them here for you to take a look out. After reading over your thoughts, I am definitely sure the feed tube is too long. Plus the opening under the feed tube?? A pic speaks a thousand words. Thanks for your valuable time.

First of all, your feed tube needs to be shorter. I'd say eight to ten inches from the bottom of the burn tunel to the top. No more. And you seem to have an ash cleanout in front, made out of the same terracota tube. That needs to be blocked when the stove is lit and when in use. As well, i can't realy picture what is the thing with ashes in it. Is it a metal chanel? If it is a part of the stove, there's something wrong. This shouldn't exist.

My husband is asking: what will we gain by shortening it and how do we keep the fire burning longer without having to constantly feed it every 5 minutes? I assume covering the ash clean out would help.

The metal that the ashes are in is just an insert he created to hold the ashes and pull it out, to make it easier to clean.

Kim Hostettter wrote:My husband is asking: what will we gain by shortening it and how do we keep the fire burning longer without having to constantly feed it every 5 minutes? I assume covering the ash clean out would help.

The metal that the ashes are in is just an insert he created to hold the ashes and pull it out, to make it easier to clean.

Well, by shortening it, you'll have more presure differential between the short leg and the long leg of the J, this giving you more draft. You have to remember that cold air entering the stove weights far more than the hot gasses exiting the heat riser, even tho they try to raise being lighter, they have to set in motion the cold air entering the stove.

And about feeding every 5 minutes, that's the nature of the beast. Well, with good wood, well dry, some mass in the feed end of the burn chamber, you could may be go an hour without refueling. Don't expect more. If you want to hold longer branches, a perforated steel gutter of the same size as the feed could help.

There's two other questions i have. How are your hear riser and burn tunel insulated? Are you sure of your barrel gaps and flue connections.

How are your hear riser and burn tunel insulated? Are you sure of your barrel gaps and flue connections.

If what I think you are talking about is the "heat riser"...the chamber inside the barrel: we used a square terracotta tube with 8" pipe in the center with vermiculite filled in around the sides. I now know we did not put enough in it...as per the advice we should've used 4" thick. We have about 2". The burn tunnel? Don't have any.... What are we missing without the proper insulation? I assume less heat being held in barrel?

It does get quite hot quickly. Just did not like the idea of having to feed it little twigs all the time. Once we change the feeder and cover the ash clean out below it, I am hoping for better results.

Kim, you will have better results. Tho, you won't burn as clean with the burn tunel not insulated, but it can be an advantage too, as whatever twigs, shavings, any little piece of wood will light itself up just from the heat of the burn tunel base. I've done it in my workshop one, sometimes after the flames have gone out, barely no embers, just stuffing some wood in there restarts the burn.

The Freaky Cheap Heat movie may provide the answers. Here is a clip from the 2hr movie. I think it makes a huge difference to be able to see a build and hear about the thinking behind the decisions about the design.