8" RMH having too much draw?

Our RMH is still not quite complete. We can safely run it, but the bench seat back doesn't go all the way yet (still another 3' to go), and I want more mass around the barrel so there's more battery instead of so much immediate radiant heat.

Some details:
We live in a timber-framed, straw bale in-fill home in central Vermont. It is designed for passive solar--large windows set low on the south wall, some large windows on the east side, three small windows on the west side, and small inoperable windows on the north side. There are 3' roof overhangs on all sides. The walls average R34 as does the ceiling, while the floor is R26. We're still building, so we're still finding a few gaps that are quickly filled. We're in a snow belt, so we receive lots of snow, and our 1200' of elevation means the snow begins early and stays late. Being on the east-facing slope of a range of hills and mountains is not ideal, but south-facing properties cost 3x as much, and we lacked the budget. We do receive copious light from sunrise to late afternoon during winter, more so in summer.

We survived last winter with a rinky-dink wood stove. We had planned it would be the emergency backup, but due to construction delays, it became the only heat source. It sucked. Literally. Thank God last year was mild or we may have had to seek shelter elsewhere. But we made it through and this winter we're looking forward to enjoying it.

(For a whole series of reasons that I don't feel like detailing here) we have a wood floor over a crawlspace, so we built a replica of the Bonnie 8". We followed Ernie & Erica's book meticulously, remeasuring regularly, and actually dismantling and rebuilding the J-tube at one point. It is on a raised platform built of bricks (just like the Bonnie 8"). It has 3 easily accessible clean-outs. The mass is composed of a ton or two of rocks from right outside out door. These are mortared into place using the “masonry” cob recipe in the Wisner's book (mostly sand, a little bit of clay). The RMH has 3 runs—the first running from the barrel, then looping around behind that, and a third running back above the second run. The mass retains heat for hours, but we hope that by enclosing the rest of the third run and finishing the bench we can have the heat last longer.

Our issue/concern is with the draft we're getting. It seems a bit much. We have difficulty lighting the RMH because the air being sucked down the feed tube blows out the flame on our paper before it has a chance to light our kindling. It usually takes 3 tries to get the fire going. We're using itty-bitty twigs which are dry, so it should light immediately. The only time I've had it light on the first try was when I used a bunch of birch bark that I wrapped around the bundle of twigs. All this extra paper and birch bark is contributing to the excess ash we're seeing (less than we had last winter with the wood stove, but still more than expected). I clean the burn tunnel every few fires and get a few cups of ash each time. I'm seeing little to no ash build-up at the first clean-out, which is immediately after the manifold, and no ash further down.  

I imagine that a major contributing factor to the excellent draw is the extensive chimney we're running inside our house. After running 3 times through our 10' bench it goes vertical—through the second floor and up to the attic, exiting near the peak of the roof on the leeward side. Single layer pipe whilst inside changes to one section of insulated outside. Properly capped and everything.

Because of the excellent draw we never run the RMH fully open. We always limit airflow after it's lit. Despite having its airflow reduced we still get that roaring rocket and intensive heat.

Is this amount of draw typical? Is this just a feature we need to get used to? Are there any pointers on lighting on the first try? General thoughts?

I'm sorry I don't know anything about RMH but I just want to compliment you on your thorough question and maybe draw some eyeballs your way.  

Better too much than too little, because it's easier to restrict flow than try to improve it. It sounds like you are already on the right track. Sometimes it just may be a matter of figuring out exactly how that particular heater wants to be operated. I would keep experimenting, and if you still feel it is too drafty, you might install a damper or a more restrictive rain cap to tone it down. I recall Paul saying the RMH in the Fisher-Price House was too drafty, but don't recall the actions taken. Mine is quite drafty, and I like it. I start mine with a torch usually like Paul.

Hi Mark;  Big Welcome to Permies!
Congratulations on building a very cool looking 8" RMH!

Do you have a temperature gauge installed in the exhaust stack?  Monitoring the true gas temp versus external pipe temp can tell you how well your rocket is running.
An analog candy thermometer (3" dial & 8" probe) inserted thru a small hole drilled in your stack will give you instant readings, very useful if / when you make changes.
It is also called a T&G Dragon Breath Monitor here at the rocket forum.

So until this year, when I converted it to a 6" batch box. I lived with an 8" J tube.
Once the mass was warmed up for the season you were hard pressed to get a fire started with the draw.

I developed a style of lighting my J tube rocket that worked every time.
First off I placed 2 split full size fuel in the feed tube. Effectively blocking 70% of the burn tunnel.
Behind that I toss in small kindling pieces directly on the floor, over that goes one crumpled piece of paper.
Then normal long skinny kindling.  Usually room to place in more full size split down piece's at the back.
One final piece of crumpled paper on top and light it off!

In my J tube I thickly cobbed the barrel  3/4 of the way up to avoid that excessive  radiant heat.
On my 6" Batch box I am using cob and clay bricks to surround the barrel for the same reason.

As you cob up the back section of your bench your draw may even become stronger.
This is good as you want that heat to be stored in the mass.

Running a rocket with the feed tube restricted is not how they are designed to run.
Wide open or going out is how to fly your Dragon.  Restricting air is how you run a box stove.

Excellent job so far!
Try my method  of fire starting, I'm sure it will work for you.
I strongly recommend a Dragon breath monitor for your stack!

A propane hand torch is something I've started using with the Batch box's.

















 

Hi Mark,  Congratulations on creating a super rockety mass heater!  Its going to be wonderful for you this winter in staying warm and not having to suffer again.

The RMH that Jordan mentioned was having the same issue as you are having and one of the things they did was shrink down the wood feed to help slow the draft amongst other things.

As you haven't finished the last run of pipe to the exhaust, you could either add more pipe somehow or make a bell that could extract more heat and slow things down a bit also.
Thomas Rubino made one here and he reported that it cooled his exhaust down too much so he had to decrease the size of it to get the temps back up.

One way I found to light a fire with a strong draw was to make a small wall of wood at the back end of the feed tube to block the burn tunnel somewhat so that the fire you are trying to light behind it doesn't get snuffed out before it catches. Open the 'wall' gradually as the fire gets going and can sustain itself.

Thomas, I thought I read a brief passage in Ernie & Erica's book that suggested folks in northern climates should restrict the feed tube opening by using fire bricks. They had a few pictures showing different configurations for optimal flow.

Now that we're shifting to using the RMH more regularly I will run some tests on running full bore versus limiting flow.

I can test how much heat is escaping up the chimney by hand-testing the chimney on the second floor--it's usually just warm to the touch. Sometimes it's a little too warm, but I hope that finishing the bench and wrapping the barrel will solve that issue.

I don't have any thermometers set up yet. Perhaps in a few weeks after we pass the latest funding hurdle I can begin getting hard numbers.

Thank you for all the advice!

I will try to take some better pictures of our rocket in action.

Hi Mark, I'm close to finishing the build on my 8" RMH, very similar to yours, also in Vermont. I can verify the advice from Thomas and Gerry: for start up, I recommend blocking off the burn tunnel with as much wood as you need to, then lighting your paper/kindling/etc behind that screen of wood where the draw is not as strong (but still strong enough to pull the flames into the burn tunnel, incidentally starting your wood screen on fire). It works for me every time, and I can consume less kindling this way. If you need a visual aid, let me know and I'll post a photo of my starting setup.

Like you, I wish I had less draw, so that more heat would stay in my house, and less of it fly out the chimney. I may extend my bench in the future (at the 180 degree turn), or else find a way to take more heat out at the beginning of the system, with a heat-operated fan on top of the barrel for instance. The fan idea is tricky because various models can be damaged by high heat, so proper placement is critical.

In summary, I think we live in very different climatic conditions than some RMH builders, and we have good natural airflow shooting up the chimney, plus there's a culture of good, strong chimneys with proper draw in Vermont, because they're absolutely necessary!

Your RMH is looking beautiful, thanks for sharing the pics. -Richard

I've been paying attention to my draft each day, and it was a lot lower last week when I allowed the burn tunnel to accumulate some ash. Worth trying out and seeing if that diminishes your draw in an advantageous way or not. Best of luck!
-Richard