DSR update 12/25/2018

The double shoe box rocket is elevated off the floor about 18 inches, and instead of an exhaust on top- in front, it is channeled horizontally under a glass stove top from left to right.

Having the water tank inside the bell was an idea suggested at http://donkey32.proboards.com   that led me to my design, which in appearance is like a glass stove top, with an extended simple porcelain covered metal counter top.

This extended bell houses a naked 20 gallon water tank with two fittings, one for cold water at the bottom and one for hot at the top.

The top fitting connects directly into the hot water feed for my house system, and while in operation the hot water valve to the shower head in my tub is left open, and the cold water flow to the tank is shut off. This allows excess pressure to escape safely, and notifies me when the water is hot as steam starts to escape. Note that this is primarily proof of concept, and not yet ready for prime time water heating, it could easily have safety features added that would further ensure a more automatic type system.

note the new visions fry pan  “door” to the batch box, The lid I was using had broken into two pieces possibly from rough handling, possibly the lids are not as thermally  robust as the pans

Last night I was burning the third batch of wood in the firebox,low grade poplar and some mystery wood, likely not completely dry, when I started to hear the steam. That produced a luxurious long (10 min at 4-5 gal/min), very hot shower (mixing lots of cold water). I was concerned about stratification, and the possibility that all the water would not heat evenly, but the length and relatively constant heat of the shower indicated this was not an issue.

The design inside takes the combustion gases exiting under the stove top into a very broad vertical opening that directs the hot gases forward into a circular motion around the whole vertical surface of the tank, with some small horizontal space over and under the tank. These gasses that are further cooled start to sink to the bottom as they circulate. The “stack” entrance is below the bottom of the tank in back of the system, so the exhaust comes in contact with about 270 degrees of the surface of the tank, with some minor contact top and bottom. Note that this stack is actually a powered exhaust by a very cheap, low wattage (about 10)fan. This provides a more or less guaranteed exhaust even at startup, and the exhaust is so cool (around 100F) that more robust (and expensive) equipment is not needed.

The test run last night reached temperatures on top of the port between 900 and 1000 F during the third batch of wood, with a very robust secondary burn at the port. Without testing equipment I have no way of knowing just how clean this is burning, but it appears that the system gets more efficient into the second and third batch of wood by the size of the secondary burn. Perhaps using insulated Fire brick at the port would get the port to temperature more quickly (it is currently standard , full fire brick), and adding ceramic fiber blanket over the stove top might also enhance the temperature build up by reducing convection and radiation losses there.


4 thoughts on “DSR update 12/25/2018”

  1. I like it Bob. I’d like to get a better idea of what the secondary port is like? I understand the concept of secondary combustion is but don’t have a clear idea of how it’s built into this particular stove? I’d want to have a clear idea of how to build the secondary. High temperatures mean less creosote condensation on the water tank. Do you see much gunk on the water tank?
    I’m very interested in building a stove similar to this for my school bus. I need a stove that heats lots of hot water and weighs a few hundred pounds not thousands of pounds like cob rocket mass heaters. And running the luke warm exhaust out the side would be great. A small fan is no problem. Induced exhaust makes total sense to prevent back draft pollution inside.
    One idea that pops to mind is extra bigger water tank in parallel with the small tank. I’d like to have the option to store more heat once the small tank heats up. A valve between them could be opened giving the choice to heat more water or not.
    Thanks, Jeremy

    1. Having a primary and secondary tank is an interesting idea. I think my next build will have a taller tank. 30- 40 gallons with about the same foot print. but I don’t have enough space to fit another tank as a primary. If I was to use two tanks, I would set it up with the primary tank hooked in a thermosiphon loop with the secondary, . Make sure the thermo siphon distance is short with no constrictions to the flow. In this case consider the smaller tank as the heat source so the direction of flow is hot primary to the secondary hot fitting, through the secondary tank and cooler water at bottom of secondary running back to the cold primary. I have removed drain fittings at the bottom of the tank to access the cold layer of water –using a cold input fitting at the top will likely work very poorly for a thermo siphon. Another trick I’ve heard is to have the secondary tank cold outlet higher than the primary, but that’s not written in stone. I had lots of success with a one way flow valve in the cold part of the loop blocking a back flow to the secondary tank. This will work with the system under pressure, but is safer during heating with pressure off the system. also, I have found normal tpr valves will blow where water may reach higher temps (even though there is no pressure at all). You will likely want a straight pressure valve instead.

      The secondary port would be the same as for the DSR2, except overhead at the back of the chamber. I forget the length, but about 6 inches long, 2&1/2 inches wide, 2&1/2 inches deep, (I used full firebricks with good square, clean edges). The secondary air supply is about csa 3&1/2 sq inches on the back wall and ending midway up in the port.

      And there is a lot of smoke etc condensing on the tank and everywhere the exhaust touches till it is past the water tank -after that there is just a slight film. Remember, even though the relative efficiency is high, there are start up and ember phases where combustion is not as complete, and areas around the tank will never get hot enough to burn off the soot. Without having internal readings, the top of the stove closest to the water tank is already down to 3-400 even when the port temps (measured on top of the glass) are above 1000(F)

  2. Thanks for posting. I had thought I would build a DSR until I saw comments that it is recommended. Now I’m stuck wondering what to do. I’m glad to see it seems to be working for you.

    1. One of my posts on the pro board asked a similar thing, The problem during testing seemed to be runaway heat and dirty burns. and I don’t think there is any real data on CO efficiency, etc, but this all seems fairly easy to control and the key is a bypass so you can preheat a chimney, or an assisted exhaust like mine allowing for maximum extraction of heat.

      All that being said, after playing with a ceramic fiber blanket as insulation on top of the glass it appears best burn characteristics happen with more insulation around the port than simply the glass top. For this reason the DSR2 might be a better method. My bb burn chamber is 18 inches long, and I think it will be possible to fit the DSR2 with a back port and short insulated combustion space before going into the horizontal riser on top under the glass where it starts to radiate heat. I still have plenty of winter left to think about pros and cons before I tear it apart and start the next build cycle.

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