Installing Rocket Heaters Safely

Rocket heaters work efficiently, in large part, because they maintain very high temperatures 1,200°-2,000°F in two sequential combustion chambers. This helps insure that all the volatiles and most of the particles are consumed.  The first and hottest combustion chamber is also usually located close to the ground. This means it is critical that the burn chamber be extremely well insulated to protect the substrate from intense heat.

Our tests showed that the area under a 6″ burn tunnel with 2″ of perlite insulation can still be over 800°F. This temperature range is much hotter than the feet of cast iron stoves which are near the floor. Consequently, it is important to mitigate the heat from a rocket heater in a different way than traditional stoves, which only require a hearth pad.

Installing a Rocket Mass Heater

If you have your burn tunnel surrounded by a lot of clay cob like a traditional rocket mass heater, this mass helps absorb the heat. However, be sure that there is enough mass to accommodate your firing cycles and intensity. This issue is probably at the root of some buildings being burned down from rocket heaters.

Installing on Concrete

Concrete can work like the mass in rocket heaters by absorbing and conducting heat away from the site, throughout the slab. Depending on the size of the concrete area under the burn tunnel and what type of concrete it is, degradation may still occur. For example, if the concrete area under the burn tunnel is not very big, at a certain point, the heat begins to accumulate and raise in temperature.

One solution is to mount your burn tunnel container on a platform that allows an air gap between the burn tunnel and the flooring. Heat, which might otherwise accumulate to high levels,  is taken away via air convection.

Another option is to place a layer of 1″ refractory board on the cement, this will lower temperatures from above 800°F to below 300°F.

Installing on Wood

Wood requires more protection than concrete. Over time wood’s combustion temperature can be lowered due to prolonged exposure to infra-red radiant heat. So while wood may start at a spontaneous combustion temperature of over 400°F, this number can be halved due to prolonged exposure.

This danger applies equally to wood that is below an otherwise non-combustible top layer, such as tile and backer board. The non-combustible layers can accumulate dangerous levels of heat which are conducted to the plywood underneath. Over time, the spontaneous combustion temperature of the wood is gradually lowered and the heat accumulation from the tile layer can cause the plywood to ignite.

An air gap above a non-combustible surface can work if there is sufficient additional insulation below the burn tunnel container. The size of your burn tunnel should dictate how thick the insulation will need to be.  You should test your burn tunnel prior to installing to insure your wood is being exposed to temperatures no more than 125°F.

Standard Hearth Pads

Standard hearth pads are typically constructed of a non-combustible top board over a layer of mineral insulation. They are designed to combat radiant heat from a cast iron stove many inches away. A rocket heater is much hotter, usually much closer to the floor, and has larger conductive surfaces in contact with your floor.

They can be used as a part of your floor protection system, but don’t rely on them exclusively. For example, a hearth pad on top of some bricks or other spacers to allow air flow under the burn tunnel might be sufficient, depending on the size of your burn tunnel.

Testing

We recommend installation of a permanent temperature sensor of some sort to be installed under the burn tunnel next to the surface of the floor to monitor the heat level there. Compared to not knowing that your floor is getting too hot, the cost is minimal.

NOT SPECIFICATIONS

Be safe! This post is not meant to be specific installation instructions, but as a starting point for your own testing and verification if you are thinking about building your own rocket heater.

 

 

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Comments

  1. I like the idea of a permanent temp sensor – always good to know what’s happening in (or under) the belly of the beast. 1). Could thermocouple(s) also be installed in the burn chamber, heat riser and/or other locations to monitor these areas as well (ie. to evaluate current burn conditions, not necessarily only fire hazard warning – data is a good thing, generally)? 2). Is there any merit to also installing a substantial sheet of aluminum or copper (1/8″ – 1/4″ or more) under the burn tunnel, as well as the insulating materials mentioned in your blog, in order to conduct the heat to where it could be radiated out to the surroundings (vs. being essentially “wasted,” assuming one does not have a thermal heat sink to capture and radiate)? Admittedly, there would be additional cost, and if radiant fins were incorporated, technical challenges, so unsure if the cost/benefit is there, but that may be in the eye (& wallet) of the beholder. A *cursory* search indicates Cu appx 6-7X the cost of Al (ie. 3/8″ 5052 Al plate 12″ X 24″ is appx $75 [& the plate would obviously have to be larger]). If over-engineered (ie. too efficient), could it cause problems by lowering temp inside burn chamber too much? Therefore, keep it simple, but does the concept have merit?
    The #’s below and comments were posted on the Permies.com forum by Konstantin Kirsch in Germany; I do not have the technical background to evaluate or verify them, but assuming correct, the comparisons are interesting. http://www.permies.com/t/32099/rocket-stoves/doubling-power-RMH-cooling-feed
    ***The thermal conductibility coefficient ? in W/(m · K) of some metals is:
    Silver 429 Copper, pure 400 Copper, usual 240…380 Gold, pure 314 Aluminum (99,5 %) 236 Brass 120 Iron 80 Steel 40…60 To compare steel and silver the thermal conductibility is about 10 times better, but the price for silver? Too much! Copper is nearly as good as silver but its much cheaper. The next interesting material is aluminium. Its not so good as copper, but extrem(ely) better than steel. And the good thing is: Its cheaper than copper. So my thought is: Copper or aluminium is the material you should use if you want to build a feed-tube cooling system.***

    Thanks for your website and blog. Fairly new to the Rocket concept, so learning curve is steep at the moment. I like your product offerings – looks like there has been plenty of R&D + thought put into it!

    • Thermal conductivities are right, but the other big factor for wood fired heaters is corrosion. Aluminum and copper are both bad around wood exhaust.

      Also the goal is to insulate the burn tunnel not cool it. Konstantin was cooling the feed tube, not the burn tunnel.

      • Thanks, did not consider corrosion factor, so employing the strategies you suggested to insulate floor beneath burn tunnel, plus a sensor to monitor floor temp. is the logical and simplest way to ensure safety. Could you please address my first question, re: additional thermocouples in burn chamber, heat riser, &/or other locations to monitor burn status? Perhaps there would be installation issues (ie. too complicated), and possibly the corrosion issue for a permanent thermocouple installation (ie. burn tube or heat riser, although there are thermocouples made for industrial applications), but is there any merit in having a few sensors/thermocouples strategically placed throughout the system if one is willing to tackle the installation challenges? Thanks.

        • SMathieu says:

          Thermocouples in the the burn chamber would be a bit expensive. They must be able to handle the very high temperatures there. My feeling is it would not really be worth it. We have found a thermocouple at the top of the heat riser will give you pretty good reading of the burn real time with lower temps. A 1000C one should work there, but also just air temps 6″ above will even be a bit lower and easier to read, giving some real time feed back if you want.

          The places that I think give the most valuable feedback is the temperature of the thermal mass. A few thermocouples placed there would tell a lot about when your mass in “charged” enough. Maybe one above the heat riser in the air and a few in various locations in the thermal mass would be very worthwhile I think.

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