Introduction

A Rocket Mass Heater is a variant on the conventional wood stove. With a conventional wood stove, the fire is typically around 1000 degrees F.  Heat is extracted immediately.   Smoke goes up the chimney due to a thermosiphon.  This design has the problem of creosote leading to chimney fires. If the chimney is insulated, the chimney gets hot enough that the creosote burns much more often - thus reducing the risk.  But the downside is that more heat leaves the house through the chimney.

With the rocket mass heater, the design is shifted to insulate the fire box.  With temperature now at 1800 degrees F, the creosote and the smoke are burned.  The exhaust is mostly steam and CO2.  We can now safely extract more heat.   We add a stubby chimney so we can push the exhaust wherever we want.  We add a mass to absorb heat.

• heat your home with 80% to 90% less wood
• exhaust is nearly pure steam and CO2 (a little smoke at the beginning)
• the heat from one fire can last for days
• can be built one in a day or two
• can be built for less than USD\$20
• less CO2 than natural gas or electric heat
• if you buy the wood, it costs less to operate than natural gas
• Some have reported heating their home with nothing more than the dead branches that fall off the trees.

Rocket Mass Heater Vs. Conventional Wood Stove

• a conventional wood stove uses a lot of the heat to push the smoke out of the house
• a rocket mass heater extracts as much heat as possible before releasing the exhaust

Paul Wheaton, of Wheaton Labs and Rich Soil says,

"I've now given presentations and taught people how to make these. The question I am most often asked is: "If my current wood stove is 75% efficient, it seems the most room for improvement is about 25%. But you say you can heat a home with a tenth of the wood. Isn't that claiming that a rocket mass heater is 750% efficient? Wouldn't 100% efficient be the maximum?" There are two ways to answer this.

Measure the temperature and volume of the smoke leaving a conventional wood stove (very hot and a large volume) and compare that to the exhaust of a rocket mass heater (a little more than room temperature and a trickle). Far more heat stays inside with a rocket mass heater.

Let's do the math.

A rating of "75% efficient" does not account for some of the heat that goes up the chimney to remove the smoke. The testing labs will use a number of either 14% or 16% for smoke going up the chimney. So the 75% number is actually 59%. Saying 75% is allowed and sells more wood stoves.

The rating of 75% was the most efficient result experienced in a laboratory with experts trying to get the most efficient numbers. So while a wood stove might be able to achieve 75% efficiency in a lab, it rarely does in a home. An experienced wood stove operator will probably experience something more like 35%. Somebody using wet/green wood and shutting the dampers down a lot for a "slow burn" will probably experience something more like 5% efficiency (or less!) with a 75% efficient wood stove. Thus leaving a lot of room for improvement. Rocket mass heaters have no way to reduce the air flow for a slow, inefficient burn. An inexperienced wood burner will probably have a 90% efficient burn every time.

Another question is about creosote. In a conventional wood stove, under inefficient conditions, creosote can build up in the chimney and start a chimney fire. The "chimney" in the rocket stove is the same thing as the heat riser. The rocket stove is designed to have a controlled chimney fire every burn.

Source: rocket mass heater vs. conventional wood stove

The question of rocket mass heater efficiency is demonstrated in this video by Paul Wheaton: "How is a Rocket Mass Heater so efficient."

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