Byron Campbell

Hi Linda,

Ideally the exhaust / flue pipe diameter should match RMH system size, i.e. 6 inch inside diameter for 6 inch RMH, the stove's system size being defined by the size of the wood feed-tube opening. What size system did you all build?

BTW, only the portions of the flue that are outdoors and in un-heated attic spaces need be insulated.

Hi Linda, the ideal situation for a wood burning stove is that there be a good strong natural draft even when the stove is stone cold. A strong natural draft helps keep the stove under negative pressure at all times, pulling any residual combustion gasses up the chimney and out of the dwelling. With the proper chimney, even a freshly built, wet, stone cold RMH will exhibit a good natural draft. This is of even greater importance for stoves of "single skin" construction.

What's your chimney like? Is the exterior (outdoors portion) of the chimney insulated? Does the chimney exit at a height of 3 to 4 feet above the tallest point of the roof? For us mortals that don't reside in a magical valley with consistent unidirectional winds (re Ianto Evans) there's just no getting around having a proper chimney attached to our RMH.

Hi Nate,

For heat riser insulation, you could try fiberglass. I.e. to protect the fiberglass, perhaps make a two layer insulation by using fiberglass for the outer most 25mm thickness layer, and plain sand for the innermost layer, to let the sand take the full brunt of the heat from the riser's brick.

What's your heat riser to barrel top gap like? I.e. the distance from the top of the heat riser to the barrel's inside top horizontal portion? From your photos it looks like the top gap may be a little tight. I typically shoot for a minimum riser top gap of 75mm. Making it a little larger won't hurt unless you intend to use the top of the barrel for cooking etc. But an advantage of a slightly larger gap is that it will allow the stove to draft stronger on marginally short chimney systems.

thomas rubino wrote:

Still making custom cuts to come up along the side of the barrel. Using Super wool as an expansion joint to hopefully keep it sealed...

 

Interesting build Thomas. The steel barrel to masonry vertical  gas seal, that's going to be the trick maintaining a tight leak free seal in that critical area. Lots of thermal cycling forces at play there. Perhaps some kind of spring loaded strapping could be implemented to hold the barrel tight against the compression seal? Myself, I'd probably just go the second route, i.e. add/build the bell's fourth wall straight up against but not touching the barrel. Best of luck, and looking forward to the test burn results.

I would like to have, but no, my build notes come from E&E's early permies posts, that have long since been buried in all the thread traffic over the years. If you dig deep enough you'll find them. Good that you have the Rocket Mass Heater Builder's Guide -- that's a must have and simply the best J-tube RMH publication ever!o) IMHO. My own stove is patterned after their compact Cabin-8 model, via their plans.

Jim, sounds like you've got a good plan of action there. BTW, when these stoves are running full tilt it will be uncomfortably hot within 3 feet of the barrel. Needless to say, the intense radiant will heat up the stovepipe that's angled above the barrel, so there is really no need to insulate the first (lower) three feet or so of that 45° run, if any at all. And it goes without saying, it's a good idea to closely watch the temperature of the tent's ceiling directly above the blazing hot barrel.

As far as priming time, typically only a few minutes is required with the torch, as Max mentioned. Once the stove is run consistently for a time, the thermal mass bench will dry and warm up. Then daily start up will become much easier and exhaust priming should no longer be needed. To get a bit more heat from the combustion unit into the mass, you may want to consider insulating (Rockwool, CFB, etc.) the half barrel manifold, at least from the base up to the barrel clamp.

If you have any sections of 6" dia. HVAC duct left over, with insulation added, that would serve as a quick and light weight way to test the draft improvement of adding chimney height. Because it's very lightweight, none guyed support could be done indoors using something like sections of inexpensive TV mast pipe,  running from the floor up through the same ceiling opening as the stovepipe, and extending above the tent's roof. You will be amazed at the draft strength improvement that a few feet of additional chimney height makes.

Another suggested area to improve upon is J-tube's wood feed. Build up the area around the wood feed entrance with several courses of brick (fireclay/sand mortared in place) to form a vertical wood feed "tube". I'd make it exactly the same measurements square as the current opening, and tall enough so the depth of this vertical wood feed channel (measured from it's opening down to the burn tunnel floor) is about 16 inches (common firewood length).

Hi Roger, from my notes (via E&E Wisner) the maximum horizontal ducting length on a really good quality built 6" RMH is about 40 feet with no turns. Subtract 10 feet for each 180 degree turn, 5 feet per 90 degree turn, 2-3 feet per 45 degree turn. Don't count the last 90 degree turn exiting your mass that goes vertical to connect to the chimney pipe, nor is the vertical chimney pipe length counted.

One way to help avoid a sluggish system is to test for natural draft before making your build permanent. I.e. lay out all of the horizontal ducting where it's going to be, all connected up with elbows, Tees and etc. and connect it to the chimney. Test for a natural draft at the other end that will be connected to the RMH barrel manifold. With your 14 ft. chimney you should have a fairly good natural draft, and with that you're good to go. The draft can always be improved by adding additional chimney height too.

I'll mirror what Max said about the chimney pipe insulation. Exterior stove pipe insulation is mandatory between the ceiling up to the chimney cap. Inside (living space) stove pipe insulation is optional. Insulating the stove pipe between the thermal mass exit to top of barrel is counterproductive, that's where the bare stove pipe is strapped to the barrel specifically to facilitate the draft.

RMH rule number one: avoid like the plague having firewood sticking up out of feed-tube opening, it's a recipe for smoke back, to say the least.

Starting a stone cold stove, especially in really cold weather, in a cold dwelling, is often problematic and frustrating. Break the cold starting procedure rules, attempt to rush it, and the RMH will bite you in the butt, rewarding the living space with smoke-back. Experienced RMH folks have their individual ways of dealing with cold start scenarios. Here's my tried and true method:

1) Remove the cold air plug in the chimney, using a propane torch to preheat the chimney's interior air. This is done at the exhaust Tee that's located right above the thermal mass bench exit's 90-degree upward turned elbow. The torch flame is directed upward into the open Tee until the stove pipe is warmed up and a good natural draft established.

2) Cap the exhaust Tee and light the J-tube kindling fire at the base of the heat riser. As the fire grows continue feeding it and work it back towards the base of the J-tube vertical wood feed. Use only small kindling wood. Continue feeding the fire until the barrel and J-tube warms up and a hot bed of coals form.

3) If number-2 fails, repeat starting again at number-1.

4) Once a well burning hot kindling fire is raging, then and only then, begin stepping up the size of the wood to larger pieces. 2-inch diameter and up firewood is added after about 30 minutes into the burning of fine splits and or small sticks, only when the stove is warmed up to operating temperature.

Hey Jim, I like that tent setup, nice. The condensate issue will improve with improved (hotter, better drafting) stove performance, by pushing the condensate (as steam) up and out the chimney. Guess that's pretty obvious. Nice photos, and the one of the truck head-on with tent behind clearly shows the draft problem -- all those close in surrounding trees. Want to know the optimum height of the chimney?o) Make it a little taller than the close in trees. I'm thinking erect one of those telescoping radio/TV mast, guyed off of course, insulated chimney pipe strapped to the mast, if you want to experience the best an RMH has to offer. Using insulated double-wall stainless steel Class-A chimney pipe sections, this top notch setup shouldn't set you back much more than the cost of the tent itself!o)

Okay, maybe that's extreme, on to the second plan: add insulated chimney sections one at a time, inching the height up in stages (about 4 feet at a time) and testing for draft with each new addition, until you get the desired RMH performance. You'll know when it's right, firing the stove will result in all that blue barrel paint flashing off in a cloud of noxious stinking smoke, if it's not 1200° F. rated paint of course. After burning off my barrel, plus a bit of sanding, it was repainted with high temp 1200° F. woodstove paint. The high temperature paint is holding up very well, with exception for looking gray'ish at the very top of the barrel, only the horizontal portion. That hottest area easily hits 800° F. to  900° F.

Anyway, with the right chimney setup you'll get that mass warmed up in its current configuration.

Hey Jim, nice looking build. H'm, how is your draft when the stove is cold without a fire going? A really good natural draft will pull the flame off a lit match held at the entrance of feed tube.

Max mentioned the chimney and that reminded me of my system. Living in a hilly area with shifting wind patterns, I've found that my RMH performs best with a tall chimney. Currently that consists of the following: single wall stove pipe exits the RMH rising to the ceiling (Selkirk ceiling box) where it transitions to double-wall insulated Class-A and passes through the attic and up through the roof to a height of 4 feet above the roof's peak. Total length from RMH to chimney hat is between 15 and 16 feet. My RMH initially had problematic draft issues on a shorter chimney, and extending the chimney made a world of difference.