It seems that people confuse the term rocket mass heater with any type of rocket stove heating system. While all rocket stove heaters have a rocket stove at their core, how they handle the heat output is where they all differ. I believe some clarification is needed starting with this popular term:
Rocket Mass Heaters
Rocket mass heaters use a rocket stove to efficiently burn wood. The hot exhaust is then tunneled through a maze like pipe that goes through a thermal mass. As it does so, the heat is transferred from the exhaust to the thermal mass, where it exits the outside relatively cool. So what is this thermal mass? Think of it as a battery for heat instead of electricity. It is made of materials such as rock, brick, and cob that absorb massive amounts of heat and then slowly release that heat energy to the surroundings. Many people have turned this thermal mass into something decorative and functional by styling benches and furniture out of it.
Rocket mass heaters have the advantage of only needing an active fire for a couple hours at a time throughout the day. By burning a hot fire for a couple of hours, the thermal mass is recharged and can pump heat slowly into the room for several more hours to come. Once the mass has cooled, it is time to start another fire and the process continues. To balance the slow steady heat release of the mass, often rocket mass heaters will incorporate a radiant system to provide quick room warm ups when the fire is going. This is often in the form of a metal barrel placed over the heat riser. This provides a quick heat transfer to the room and a cooktop to boot. This radiant system brings me to the next type of rocket stove heater.
Radiant Rocket Stove Heaters
Most radiant type of rocket stove heaters are small and are used to heat garages and workshops quickly. As mentioned in the previous section, sometimes they form a hybrid with the mass type heating systems to provide quick heat. Typically, these tend to be a bit inefficient because they are often built without insulation. The lack of insulation leads to more combustion byproducts. By taking time to add insulation, their efficiency can be improved and the exhaust made cleaner. Heat from the exhaust is radiated to the surroundings by exposed metal surfaces from the stove and pipe, much as is the case in traditional wood stoves. Between often times lacking insulation and a lot of heat escaping through the exhaust, the radiant rocket stove heaters are of dubious advantage over traditional wood stoves with a small hot fire.
Forced Air Rocket Stove Heaters
By building a small insulated shed around a rocket stove with the hot exhaust passing through a couple of barrels, one can build a outdoor forced air furnace. A blower transfers the cool indoor air around the barrels, heating the air, and transferring it back indoors. With a well made rocket stove, efficiency can be better than commercially made units. The main drawback to this system is that air is a poor heat transfer medium and those blowers consume a decent amount of power to do so. The main advantage is simplicity.
Rocket Stove Boilers
By placing a coil of pipe above the heat riser in a rocket stove and pumping water through it, a rocket stove boiler can be made. From there, the hot water can be pumped into a in-floor radiant system, radiators or baseboards, or a heat exchanger in an existing furnace. It is for this reason that I recommend rocket stove boilers for most people with existing homes. The main drawbacks in a boiler system are complexity and cost. Much of this is offset if you are using one to replace an existing boiler and using the remaining system. The advantages are the fact that they are easily integrated into existing homes, can heat greenhouses, barns, pools, etc., and can provide domestic hot water. It is the type I chose to use myself for these reasons.
So we see from all this that not all rocket stove heaters are the mass type. Which one is best for you depends on a lot of factors. I hope this makes you able to make a better decision when choosing one for your needs.