Venturi vacuum for RMHs

I was listening to the recent podcast about the rocket mass heater in the teepee. In general there seems to be a issue with draw/ cold plugs when the mass is  cold. I was thinking about ways of increasing the draw using a fan at some point in the final part of the chimney. Using a fan is a bad idea as anything motor related would melt. I researched venturi vacumes and this seems like a viable solution.



The idea is to have a nozzle in the exhaust that is used when the mass is cold. There are no moving parts and the only part exposed in the flue would simply be a pipe with a nozzle. I think for a full working version this could be fan driven rather that using compressed air. Due to the pressure and the nozzle the pipe doesn't have to be large so won't affect flow during normal use.

A more advanced version could have the fan speed automatically set via a pressure sensor before the burn chamber. This way the fan can automatically turn off when the system is up to temp.

I don't know if this has been suggested before. I googled permies and couldn't see anything mentioned related to this. If you think the idea has legs then I can maybe follow up with some plans/drawings. I love the alternative technology stuff so this is fascinating to me. I really like the recent stuff about mycelium insulation.

Keep up the good work.

Mark

Hi Mark;
Welcome to Permies ! And Welcome to the wonderful world of rocket science.
Well your idea of compressed air blowing down the pipe would certainly start the daft.
A fan however would not be able to supply enough air without a 6"or 8" opening.
Not having made the trip over to the Wheaton ranch. I'm not sure but I suspect that electricity is not supplied.  (makes running a fan harder). Compressed air could be a bit hard to get there as well.

No, the real answer is called a Bypass .
Matt Walker and Peter Berg both suggest this is the most expedient way to convince hot air to travel thru a confined space.
Heat the chimney and the draw will start.
I am not familiar with the Teepee arrangement . It might take a manifold rebuild to place a bypass gate in the system.
This is a small amount of work to fix an ongoing problem.

The Teepee being  an uninsulated tent has no residual heat in the mass.  
Cold is cold and until your chimney can create a warm air draft the smoke is going to seek the path of least resistance...
A bypass gate is relatively inexpensive to purchase .  If installed during a build it is no big deal to accomplish.
Installing one after a build is complete,  is more work but its cob! Easy easy to modify.
We had a recent thread all about retro fitting a bypass into an existing system!  It worked perfectly and his previously slow cold starting dragon now starts roaring in just moments.
Here is a link to that thread.  https://permies.com/t/152857

I wonder if you could use a bike pump to power the venturi?  Or a larger piston arrangement that does less pressure but more flow?  Halfway towards a blacksmithing bellows.  That way if you need draw you just give the piston a few pumps to get the venturi helping.

I agree Thomas. The bypass has made a world of a difference to (not just cold plugs) but startups in general.
In many of Matt Walkers stove chats, he has mentioned that a bypass can overcome a long pipe run or a much larger bell than what Peters recommended ISA (Internal Surface Area) numbers suggest.
To me its a win-win. No electricity, no fans, nothing complicated. The hardest part is to just remember to close it once the draft is going good enough.

The venturi idea could be something to experiment with because after all, we are rocket scientists, but when a simple bypass works so well, its a hard thing to beat.

FWIW

Shiny stainless is great for passing through structure, but an anodized dark surface (say the final four feet of a chimney) will catch the sun and heat starting natural convection.
.....Unfortunately that's useless in the dark, or a grey day.
The reason the bypass works so well is because you reduce the cold mass, to just a short (read easily heated) stack.

Anyway the easy way to test your idea, is to install a short section (5" or even less?) of reduced cross section pipe in the vertical chimney, say a piece of 6" in an 8" stack, or 5" in a 6" stack ....a short section will create a venturi ....a long section will create drag. (If this is confusing read up on annular venturi)
Rather than using a starter blast of air to start airflow, I'd make a double jacket with a flared top and pour hot water in it, the hot water will start the column of air moving.....and when the fire gets established, the water will evaporate adding (usually needed) humidity to dry winter air, (obviously this is the perfect place to use stainless.)

Here is a video of someone whom has built a venturi system for the exhaust of his (conventional) burner;

https://www.youtube.com/watch?v=W6c54IFrs3M&feature=emb_rel_end

Thanks for the replies. I had not heard about the bypass idea before. But as you say low tech has a lot of benefits. When the bypass is closed, does it need to be done in steps or is the draw of the riser enough to circumvent the cold plug?

Hi Mark;
Yes ,simple is foolproof.   The gates can be set partially at any amount of flow.
To start you would want it wide open.  If you have a dragon breath monitor you will see the vertical stack temperature start to rise .  
This is heating the core while allowing the smoke easy access to the vertical chimney.
Once temperature has risen in the core and stack, the gate can be completely closed or partially closed if your system is not ready to draw yet.
Bypass gates are available from dragontechrmh.com

I just went back to the Ernie and  Erica book and its in there under the section for cold starts.
It's good to know about, not sure how I missed it. Thank you for the information.  One day i'd love to build one. If I do ill make sure it has a bypass.

I had basically this idea. Plumb in a 1/4" air hose from an air compressor with a valve. Have it located after the mass, pointed directly up the chimney. If the stove isn't drawing, just open the valve a little. Shouldn't take much, wouldn't need to be wide open.  Once the system is warmed sufficiently, it can be turned off.

Seems much more utilitarian and usable than trying to burn a wad of paper or candle up the chimney or heat riser.

Hi Solomon;
Your idea would work great.  If your RMH was in your auto shop (like mine) that would be easy to do.
Anyplace with compressed air.   In my shop I have been known to use my cutting torch to heat the chimney to convince it to draw.  
Let me tell you, turn that pipe cherry red and your draw starts rite away!

A lot of folks with an RMH have not got an air compressor or a cutting torch. Heck quite a few have no electricity!
For them it is alternate methods. The best of which is a 4" bypass gate.