New member, question about RMH chimney

Hi all.  I'm really interested in rocket mass heaters and wondering if I build one in my basement can I exhaust the chimney out of an altered standard basement window which is just above grade?  I don't have a chimney to accomadate and don't wish to go through the floors and out the roof.

Thanks,
Tim

Hi Tim;
Welcome to Permies!  And Welcome to Rocket science!

Yes you could exit the building thru the window.
But at that point you would need/want insulated pipe up past the ridge line of your roof.
That is certainly easier than going thru your roof, but much more expensive.

If you go less than the roof line you can expect wind issues.
If you try using uninsulated pipe you will have serious draw issues.
Could you just stick some regular stove pipe out the window a few feet to try it ??? Yes you can as a trial.
But it will not do in the long run.

Let us know what your thinking and by all means include photo's when you build.
We are more than happy to help you build a successful RMH.

thomas rubino wrote:Hi Tim;


Yes you could exit the building thru the window.
But at that point you would need/want insulated pipe up past the ridge line of your roof.
That is certainly easier than going thru your roof, but much more expensive.

If you go less than the roof line you can expect wind issues.
If you try using uninsulated pipe you will have serious draw issues.
Could you just stick some regular stove pipe out the window a few feet to try it ??? Yes you can as a trial.
But it will not do in the long run.

Thanks Thomas.  So is there a standard height of vertical chimney that will eliminate draft problems?  I'm proposing going from about a foot above the basement floor up about seven feet, horizontally for about 24" and then vertical again for about two feet.  I can go higher without obstruction.  Just wondering what height is needed on the vertical portion.  I am in a ranch style house.  If I was in a two story are you saying I still must go above the roof line?

Hey Tim;
You Know every build is different and every micro climate is different as well.
We encourage experimentation!  Keep your expectations low and give your idea's a try.
Here are my thoughts for you.
Let me start by saying no completely horizontal pipe.  It must slope. You will get condensation, it needs a place to go.
If you use uninsulated pipe then you will always have a condensation problem and hard starting when cold.
If you switch to insulated pipe outside the building and you have wind issues. Then keep adding pipe until it stops.
Common practice though, is to go all the way up past the peak. Very expensive with metalbestos pipe!

Thomas offers good advice.

I would add: If your site is nicely surrounded with big beautiful trees, you may get a downdraft that messes with proper draw of any stove. Not only will you get hard and smokey startups, you could get backflow into the house. And, your stove efficiency will suffer. As suggested: go higher.

Insulated pipe is spendy, but the difference in performance is huge.

Gotcha!  I'll design with appropriate chimney height.  

Another question... I've read about difficult to start RMHs.  I understand that the mass will retain heat for a long time.  However, if not burned for some time and the mass is cooled off to room temperature if a person opened the "flue" a substantial amount of time before attempting lighting the RMH would a natural flow of warm air begin to flow and help with start up?  If so I see this as preferable to fighting to get the draw going while lighting it.

By opening the flue I mean removing the firebrick I will be using to block the throat of the fuel feed for air control and to keep the mass warm without drawing air when no fire is present.

Hey Tim;
Well no not really. Your mass is still warm or its not.
Removing the firebrick's covering your feed tube will not help on a cold stove.
It sounds like you are planning a J tube build.  Depending on what you build.  Once any cob is completely dry you should not have any troubles after the first fire of the season.
The mass holds heat a long time.   As you are still in the planning stage it becomes even easier!
Build in a bypass!  
By diverting some of the hot air before the mass and sending it up the chimney.  The hot air creates the necessary pull to start the flow thru your mass.
Here is one type of bypass, I sell this style at my store. https://dragontechrmh.com/
There are many ways to accomplish this same thing.

If all else fails, there's nothing wrong with adding a fan that forces an updraft in the chimney when you're starting up. Yeah, maybe it seems like a cop-out, but so what. Sometimes the general topography makes this a practical choice.

Hey Douglas;
Have you ever tried using a fan on cold RMH?
It does work BUT...it takes a long while before you can walk away.
Believe me I've been there and done that... its not fun.
Much simpler to open a bypass.

Hey Thomas, fair enough, you're the RMH guru!

thomas rubino wrote:Hey Douglas;
Have you ever tried using a fan on cold RMH?
It does work BUT...it takes a long while before you can walk away.
Believe me I've been there and done that... its not fun.
Much simpler to open a bypass.

It also depends where your fan is. I had mine on top of the "chimney" (an elbow sticking out of an old chimney,about midway up the wall, facing down)

And it saved me many times when the wind was blowing from the south.