1. If I replace my wood stove with a RMH can I use the existing chimney? It 39' from floor to the exit point in the roof 2 1/2 stories above?

2. Can I "share" the chimney between a RMH and a regular wood cookstove?

I'm waiting for my DVDs (via kickstarter) but I think I need to start planning for the build and I don't know if these questions are addressed.

Cj Verde wrote:1. If I replace my wood stove with a RMH can I use the existing chimney? It 39' from floor to the exit point in the roof 2 1/2 stories above?

2. Can I "share" the chimney between a RMH and a regular wood cookstove?

I'm waiting for my DVDs (via kickstarter) but I think I need to start planning for the build and I don't know if these questions are addressed.

Hi CJ,
These are great questions and only briefly addressed in the DVDs.
- Existing chimneys can usually be used - the main issues are location, condition of the chimney, and whether it's a smooth, consistent cross-sectional flow area that will work well with the rest of the system. So a lined masonry chimney or a relatively new, manufactured (stovepipe) chimney could work great. An older, possibly leaky brick chimney without a liner might not work as well; an oversize chimney would need to be lined at the new system's size.

- "Sharing" chimneys is not a great idea. It's impossible to properly size a chimney so that it works correctly for the varied flows needed when one, the other, or both appliances at once are in use.
In the case of the rocket mass heater, there is also the issue that rocket exhaust is cooler and denser than woodstove exhaust. The cookstove might help 'prime' the chimney for the rocket heater under the right conditions, but under the wrong conditions rocket exhaust could back-flow down past the cookstove damper and into the home. Most cookstove dampers are not airtight - so the cookstove would always be a potential source for exhaust leaks into the room. The rocket heater could potentially do the same thing if the cookstove was lit while the chimney was cold (less likely, but could happen).

If the chimney is large enough, you could potentially divide it into 2 flues to handle rocket and cookstove separately. We've done this on at least one project where the owner wanted to keep a fireplace and heater as two separate options.
If not, I'd recommend putting in a new manufactured chimney for the rocket mass heater.

Hope that helps.

Yours,
Erica W

A related question...

I am in a similar situation in that I will be replacing an 'existing' conventional woodstove with a RMH. But my situation is also unusual in that I haven't yet actually installed the woodstove, as strange as that may sound. I am still building my home, and I have already purchased a woodstove and had the roof pass-through installed, but not the stove itself. After completing the house, installing the woodstove, and living with it for at least one winter, I will uninstall it and build a RMH in its place. I know that sounds wasteful, and it certainly isn't the most economical path, but it was necessary for two reasons. First, I'd already purchased the stove before truly "drinking the coolaid" on the virtues of RMHs. Second, there was no way I was going to ask my building code official to approve a homemade woodburning appliance as the sole heat source for my home. As it was he took a bit of convincing just to approve a commercially manufactured stove.

But this leaves me in a unique situation in that I can plan ahead for the retrofit of my RMH. For instance, I can still build an alternate, horizontal chimney into my walls to which the future RMH could mate. It would have to rise above ceiling height, to about 9-10' above floor level, then travel horizontally about 40' or more to exhaust at the side of my home, between a carport and a garage but away from any windows. I have read that horizontal chimneys are not uncommon with RMHs since the final exhaust temperature can be as low as 100 degrees F, or even lower.

Or, I could take the much easier path and mate the RMH to what will be the existing 6" vertical stovepipe chimney from my conventional woodstove. This would have the obvious advantages of simplicity, knowing that the existing chimney is already built to code, and not having to remove the existing chimney. On the other hand, the existing chimney will exhaust much higher, I'd guess 20-22' above floor level. To what extent would I need to design my RMH around the need to ensure a higher final exhaust temperature? I understand that, the taller the chimney, the hotter the exhaust must be to ensure that it drafts up quickly and completely. For what final exhaust temp would I have to aim, and how accurately can one calculate final exhaust temp given total RMH ducting length and mass, anyway? Will it make any significant difference to double the height by using the 20' existing chimney, as oppoesed to an alternate 10' horizontal chimney?

I purchased ernie and erica's "Annex 6" and "Cabin 8" plans tonight and was slightly surprised to read that both use vertical, through-the-roof chimneys. In fact they write in there somewhere that this is what they usually recommend. If Erica chooses to respond to this post herself, can you explain why you usually recommend this approach?

A little background on myself: I've watched all of Paul's videos on RMH, read his RMH article, and listened to all podcasts with Ernie and Erica. I just bought Ianto Evans book, but haven't had the chance to read it yet. I've also never had the opportunity to see a real RMH before. So please forgive if any of my questions are hopelessly ill-informed.

Thanks for any feedback.

Erica Wisner wrote:
Hope that helps.

Yes, thanks.