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.
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.
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.