Length of exhaust pipe.

My second question of the day.  

What is the maxium length of the exhaust on the horizontal before going up the house chimney? The plan is for the exhaust to run horizontally for 24 feet, then do a horizontal 'U' bend under my bed to then horizontal again for another fifteen feet before hitting the vertical. There will be a clean out at the usual places, plus one at the base of the vertical to use as a chimney warmer when needed. The vertical will be two storey and a tall attic with another few feet above roof line. Is this too much? All pipework will be inside the house (central chimney stack). Our house will be upsidedown, bedrooms and bathroom downstairs, living and office space upstairs (mainly because the view from upstairs is excellent and wasted if only seen when we are in bed).

Thanks.

This depends on the size of your system. An 8" diameter system can handle up to 50' of horizontal, minus 5' for each 90 degree elbow. A tall internal chimney is likely to improve the draft enough to allow more horizontal, but I would be reluctant to stretch it without a fallback plan in case it is too much.

If practical, I would include a bypass so the exhaust can go directly to the chimney for startup, avoiding the hassle of starting a warmup fire in the chimney base.

If you are planning a couple of RMHs, I would suggest getting Ernie & Erica Wisner's new book Rocket Mass Heater Builder's Guide. It has more detailed and concrete information than the Evans/Jackson book (which is still valuable for the essential concepts).

Thanks for that. I'm doing the build in Europe and the largest exhaust pipe I can find is 6 inches. How much will this size reduce the length I can have?

On the other hand I'm lucky that house I'm renovating has old, 60+ year old, soft bricks in the walls I am knocking down and theres a 50 cubic metre hill of clay on what will be my lawn, when I find uses for the clay.

A 6" system would have maybe half to 2/3 the power of an 8" system, and the recommended maximum length is around 35', with the same 5' per elbow reduction.

At this point, it becomes important to determine your heating load, as you don't want to have to run a smaller system all day to get the same amount of heat a larger system could produce in a few hours. What is your climate, and what size and construction is your house? How well insulated is it?

If you are really limited to a 6" system, a batch box might be a better option, as they put out heat in a one-hour blast comparable to a couple hours or more of a J-tube. A 6" batch box is said to be as effective for heating as an 8" J-tube.

There are also other lower-friction ways of constructing the mass beside the simple duct through cob, which might be useful for you.

The climate is inland continental European. Hot long summers, cold but short winters (minus 18C to minus 26C). At the moment, being the beginning of October it's 20C to 28C in the day time but evening times needs jumpers or a fire lit for a short while as it drops to low teens centigrade. It's been known to be high teens to low twenties into November. Spring starts late February early March. I won't be here this winter, so I can carry on looking for bigger pipe. Though the standard wood burner exhaust pipe is the 6 inch size.

Julie;  You do not have to use pipe 6" or 8"  A tunnel of any size can be built using soft red brick. Just smooth the inside with clay for easy flow of the hot air.

You do not have to use pipe 6" or 8"  A tunnel of any size can be built using soft red brick. Just smooth the inside with clay for easy flow of the hot air.

Now this makes it much easier. I have a need to recycle as much of the renovation/ build materials as I can, as I have no real way to dispose of it easily. So two walls full of soft red bricks and even more of the clay being used up is a bonus.
Neat clay or should it have a portion of sand?

My guess for this to work is that the clay would have to be smeared as the tunnel was being built (arm up the hole) or you would get a flat topped tunnel?

Is it as safe reference gasses leaking as a metal pipe would be?

Would you 'mortar' with clay between the bricks or lay them dry?

Thanks everyone so far for answering.

I would use a soupy-thin clay slip mortar, only thick enough to fill the gaps between bricks. Sand here, unless very fine and perfectly graded (no pebbles), would be an impediment to close fitting.

For lining the tunnel, I would use a sand-clay mix; experiment with proportions to get the best strength and least shrinkage. Reaching in and plastering a section at a time while building would probably be the easiest. It doesn't need to be round, just smooth. If the bottom surfaces of the top bricks are even and smooth, you may not have to apply a layer of cob there, just making sure the joints between them are smoothed over.

I hadn't realised there was another reply. Plus I've decamped from Bulgaria back to the UK.

In reply to the most recent reply, for anything requiring a smooth finish when using sand I have to sieve the sand as it's river sand with tiny pebbles.

The actual house chimney internal cross section is 20cm / 8 inches, but it has a bottle neck down to 6 inches where the internal 'pipe' will attach. What cross section should I make the horizontals if I use brick instead of pipe?

How long can I make the horizontal lengths if the above dimensions are taken into account?

The chimney itself will be at least 25 foot taller the height of the rocket stove.

I have the most recent book on my Christmas wish list.

Given the height of your internal chimney which will aid the draft significantly, I think the 6" constriction can be overcome as long as you make smooth transitions. I would make the whole system 8". The duct lengths you described, including the bends, would be feasible for 8" equivalent duct, but I think you would get better results with a bell cavity in your bench. Ordinarily I would say "half-barrel bell", but as you have so much brick you would obviously use that.

I would make a cavity say 16" wide x 12" high, with solid double brick walls on each side, and a row of bricks down the center with as many gaps as you can manage while securely holding up the roof bricks. Cob thickly all over the top and maybe sides if you want, and you will have a very effective heater with next to no friction. You would connect the bell cavity to the chimney from the bottom of the bell, keeping the top of the opening less than half the height of the cavity. This will hold the hottest air inside, while the cooler air falls to the floor and exits to the chimney.