All-in-one rocket cookstove with secondary heat riser: how much is too much?

Hi there!

This spring, I will build a RMH in a small single-story, 75m2 house I am renovating in Slovakia (sorry for metric units ) I've heard so much about rocket mass heaters, and here in Slovakia, firewood is (relatively) cheap and abundant, plus I don't have a gas line in this house.. so in terms of both space heating, water heating and cooking, I want to take maximum advantage of renewables.

I thought: RMHs have been used in all three of those areas, so why not creating a kickass, all-in-one RMH, that would let me cook, heat the house and heat my shower & tap water in winter?!

So I came up with this quick draft, of a "rocket mass heater cookstove heat exchanger":


I would really like to have your opinion on this design, because I have some doubts:

1. In my design, the firebox ends up having some sort of a heat riser over it.. that's not standard in the rocket stove designs i've seen so far, although I have seen a similar design during my research (googling)(Himalayan rocket stove).  I don't know how this secondary heat riser could impact the stove's efficiency, or cause trouble with its "rockettiness", cause smoke issues through the feeding chamber etc...?

2. I'm not sure the oven could get enough heat, without even considering the uneven temperature between the right side (against the steel barrel) an the left, by the exhaust, which could make uneven baked goods...

3. related to the 2nd question..: would the heat exchanger still have enough heat to exchange, after the fire has gone through so much heat losses, through the oven, and the two cooking surfaces? how would the placement of the heat exchanger be the smartest? around the exhaust as shown in the drawing? under the oven? inside the barrel, around the vermiculite heat riser (maybe too hot for copper pipes on the long run)?

4. On this design, the firebox must be very long (because of the cooking surface on top), compared to what I usually see in RMHs designs. is that a big issue?

I've read somewhere that when considering the length of the exhaust pipe buried in the cob, a rule of thumb for an 1/1.5/3,  8-inch pipe system, was to count 50ft of pipe, and subtract from it 5ft for every 90° elbows in the system. that sounds like a lot of cooling mass, so it seems like there's quite a lot of heat to extract from a RMH, which keeps me optimistic.. but I'd love to get your opinion!

I'm looking forward to your ideas and answers!
- Bruno

Both of the cooking surfaces in your sketch would be high-heat elements, and I expect you would really want a range of temperatures. If you simply make the main riser barrel top (about 60cm wide) all exposed for cooking surface, you would have a wide and continuously variable range with at least two or three simultaneous pot locations. Scrap the "extra" heat riser and make the combustion core to tried and true dimensions. If you can dig into the floor to set the whole core lower, you can get the cooking surface to normal stove height.

The oven as shown would probably work well enough. If there is insulation all around it there should not be significant uneven heating on the barrel side. Are you thinking of a white or black oven? I expect either would work at that location, but opening a black oven while the fire is going would cut the draft through the feed tube and likely cause smokeback every time you check the cooking progress. A white oven (isolated from combustion air) with steel floor and possibly sides would cook well without danger of smoke leakage.

For the water heat exchanger part, a copper coil directly in or touching the airflow path risks getting too hot very quickly and having steam explosions, or if it is in a low enough temperature zone to be safe, might not work well. The standard recommendation for this is to have a large container of water at atmospheric pressure exposed to the heat with a coil of copper in it to heat the water for use. As pressurized water has a boiling point higher than atmospheric water, the coil would never be in danger of explosion even if the container boils. You would want an automatic water makeup feed similar to a toilet tank so the container never runs dry nor overflows.

Some other details of your core are not according to current best practice. It has been found that there is no need for separate ash pits; making the burn tunnel/feed tube floor flat and smooth allows easy ash removal with a small scoop like a tuna fish can, and I have found that I only need to clean the floor once a week or so. Ash buildup in the burn tunnel does not impair combustion at all unless it chokes half of the flow area. I have a metal poker that I use to scrape hot coals from the feed tube base into the burn tunnel so new wood can sit down all the way. They burn down to pure ash.

The manifold floor (under the oven in your sketch) has no need of an ash pit. Simply making the floor accessible to a scoop works fine for users, and it generally does not need to be cleaned more than once a year or so.

The feed tube works best if it is vertical, not slanted. Wood reliably feeds itself in a vertical feed while it can more easily hang up in a slanted feed.

Two sides to this coin: One side says that rocket stoves are all about experimentation, so go for it! The other side is drawing from the experience of others, and from what I’ve seen it’s hard to have one stove that does it all. For example: if you’ve already extracted heat from two burners and an oven, will you have enough to realistically heat a water exchanger? If you DO get enough to heat the exchanger, will you then have enough warm air still rising up your chimney to provide the proper draft needed for the system to work?
I personally would be afraid this system could do a lot of things, just none of them very well. Usually you would focus all your immediate heat usage (a burner, oven, or coil exchanger) in the hottest part of the stove above the riser and put the rest of the heat generation into mass for room heating.

On one hand, trying to do multiple things with one appliance can result in doing none of them really well. On the other, if you can cook, bake, and heat water, all of that heat will end up warming the house. If you are concerned about having enough heat for all of the immediate tasks, you could make the heater a size larger to give more capacity. Ultimately, you don't need to fire the heater as long to warm the house if it is larger than you need, so you won't overheat the place.

Thanks for your replies al of you =)

Glenn Herbert wrote:On one hand, trying to do multiple things with one appliance can result in doing none of them really well. On the other, if you can cook, bake, and heat water, all of that heat will end up warming the house. If you are concerned about having enough heat for all of the immediate tasks, you could make the heater a size larger to give more capacity. Ultimately, you don't need to fire the heater as long to warm the house if it is larger than you need, so you won't overheat the place.

I've had a hard time understanding the other answers until I read yours @Glenn Herbert:
To me it seemed a bit nonsensical to say that I could "draw too much heat" from the rocket stove and thus ending with a stove that doesn't properly work for the given tasks: If my "appliances" require more heat than given by the stove, don't I simply need to burn more wood to "push" more calories into the system?

To me, the hassle of starting two fires, one in a cookstove and another for a heat exchanger/space heating stove, makes little sense. But I understand from your answer that the question is probably more about the firebox capacity than the burning duration, am I getting this right?

But if a well-built 8in RMH duct system can decently warm up 50ft of straight pipe buried in cob without affecting draft, as I read in another post, that's A LOT of mass, much more than what I seem to need, no? Sadly, I can't experiment with different designs currently.. only read, theorize and ask stupid questions...

I have tons of questions so I bought "The Rocket Mass Heater Builder’s Guide".
I'll come back to this thread once I learned all I could get from there