Starting the draw

Am looking into a rocket mass heater for our new addition after decades of woodstoves. I get the general principles once it's operating, but have only seen videos of ones with the firebox already drawing, or being lit outdoors (these latter would be the simple cinder-block constructions). So a simple question--if your firebox is indoors, how do you get the stove drawing without smoke puffing back up around your fuel?

Other than when first built while your cob is still wet. A dry / warm rmh will suck out your match before you can get the paper lit.  Anybody who happens to come by is amazed, if I happen to light it while they are here.  I throw kindling in the bottom crumple paper in my hand ... light it and push it down to the kindling and whoosh its drawing.

Thanks--I appreciate the information.

Hi Robert,

Don't know if you may wish to incorporate something similar into your design, but look at photo #2 in the following thread:  https://permies.com/t/42529/FWIW-Rocket-stove-heater-concept

It may be possible, if desired, to place a side port just before the chimney that allows you to start a small burn for the purposes of pre-heating the chimney stack.  The unit shown in those photos sits in a very cold building so there is no temperature differential between inside and outside until I fire the beast up.  When running, it's throwing off pretty tremendous heat from the barrel, but until the building is better insulated, it just keeps me warm while doing some chores in that workspace.   Maybe something of use here.....

Since my rocket stove was usually off and cold more than it was on and warm, I usually had a bit of smoke back depending on how carefully I lit the stove.  usually I would wad up a piece of newspaper and stick it far into the tunnel, very close to the heat riser.

Then i would put the kindling and another piece of newspaper in the firebox.

Lighting the first piece of newspaper would start the draw, but remember, everything has to be super dry, ready to explode into flame given the chance.  Any green or damp wood will be a nightmare on startup.

If a bit of smoke starts back into the room, lean in and blow hard into the firebox directing the flame and heat through the tunnel, and that works too.

A clean out T with a removable cap placed in the chimney pipe can solve smokeback as related above. Mount a small metal cup on the inside of the cap and replace the cap while burning a fire starter in the cup prior to lighting the rocket. This will heat the chimney and establish draft. No reliance on electricity.

It does work. I have one on my back yard rocket and I’ve done start ups in cold rainy weather with and without it. With it there is NO smoke back. Without it, in cold rainy weather, smoke back conditions sometimes persisted 10-15 minutes.

When building my indoor Batch Box RMH I built in an opening cast iron door near to the base of my brick 'bell'. This was to allow access to the masonry chimney flue for cleaning, but also to allow me to start a priming fire at the base of the chimney to induce draft in very cold weather or when I haven't used the RMH in a while.

I've only been using the RMH for a month or so and I haven't had to light a priming fire so far...

This is my fourth winter using this 6" RMH.

I use a firestarter to start the fire. We make them using small paper cups (a little larger than cupcake size, but those should work), melting old bulk candles, mixing the melted wax with sawdust we scrape from the workshop floor.

When the system is cold I might need to blow hard or use a small fan. When it is warm the draft is already there so I don't need to. Lately, it has been so cold outside that I couldn't light the firestarter inside the feed tube because the draft is so strong.

One thing to watch out for is that cleaning it out with a vacuum can cause the draft to reverse. Whenever I clean out the ash I have to blow harder to restore the proper draft direction.

But I ALWAYS have a small fan, with metal blades and metal body, that fits over the feed tube. Even when it is pulling well I will find times where for some unknown reason it starts burning too high up the feed tube or smoking back. Or if I need to step out of the room for a few minutes I'll use the fan just to be sure.

It took me a while to get used to the system, so the walls all have these small round grey spots where the soot hit the colder spots on the wall where the drywall screws are; from the thermal bridging. I could paint over it but it does mean I don't need a stud finder.

The ease of starting a particular RMH will depend on the house and chimney layout and characteristics... if you have a good tall chimney in the middle of the house, you may well have draft with no fire at all.

Mine has a short, undersized temporary chimney going out the sidewall and partway up, and still draws consistently even when stone cold unless there is a strong east wind (very rare except in spring).

I find that a quarter of a cardboard egg carton makes an excellent fire starter, with a good handful of tinder on top of it and kindling ready to put on as soon as the fire is going. The draft fans it to a roar within a minute or so.

Here is the very first lighting in a brand new rocket mass heater:

have been wondering just how much it takes to induce the draft into the rmh  Was watching paul W lighting his and got to thinking about his layout. There is a nice size clean out sticking out of the side of the chimney.  One of the things Ernie and Erica was saying that it only takes a bit of a temperature difference to cause a draw.. So putting this together .. said hum need to heat the chimney a bit to induce a draw.   I actually like to keep oil lamps around the house because they really put out a lot of light. No electricity needed.. but there is also a fair amount of heat going up the chimney of these lanterns.   Put a lantern under the clean out.. it will heat the chimney and cause a draw.  Likely need to let it run 20 minutes to heat the chimney since it is a lot of metal.. but sure is worth trying.. perhaps less ash to clean out along the way.