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We often get asked to produce numbers to verify the efficiency of rocket mass heaters.
People hear someone like Paul Wheaton say they use 1/10 the wood of a conventional woodstove, and they say, But modern woodstoves are rated at 75% efficient, up to 90% efficient, how can you possibly be claiming these are 750% efficient?  That's not possible. 

Well, let's discuss what "efficient" means in this context, and why rocket mass heaters seem to deliver more efficient performance in use compared to typical woodstoves.

This article is sponsored by my deep and abiding interest in the success of two 2015 Kickstarter projects:
- Paul Wheaton's new, improved 4-DVD set:
- our long-awaited Rocket Mass Heater Builder's Guide:

A) Rocket mass heaters have been tested privately with the same combustion analysis equipment used for woodstoves. Results have been posted on and, among other places.  When comparing burn efficiency alone (all that the equipment tests for), most rocket models are at least as efficient as good, certified, modern woodstoves.  Some are cleaner and more efficient than pellet stoves or natural gas furnaces (see graph below - the pink line is CO, the bad stuff, and the red line is efficiency between 87% and 95%). 
Some prototypes suck, of course, and we walk away from those.
Peter's Batch Box, heating a 2000-sf shop space with CO emissions lower than a candle.

The EPA does not test stoves for efficiency.  Their required tests focus on emissions, and the woodstoves' hot burn rates, to determine safe clearances.  Most users confuse the EPA or UL rating sticker with actual independent lab tests-  my understanding is that the EPA for some obscure reason gives a generic efficiency rating by type, but doesn't test this independently for each model. 

Manufacturers may publish their own test results in product data.  They are allowed to take off a factor of 15% for the legal minimum woodstove exhaust temperature, 350 F.  (Yes, that's minimum, not maximum - woodstoves are expected to smoke somewhat, so the chimney must remain above 350 F all the way to the top to prevent creosote from condensing on it.  Exhaust temperatures of 400-600 F are well within the expected range for a woodstove.)

So a light-weight woodstove that was advertised at 100% efficient would actually be about 85% efficient.  A 75% efficient stove would really be 63% efficient.... This is still pretty good, considering power plants and automobile engines are lucky to get 40% efficiency converting heat into mechanical movement.
But heat-to-heat efficiency is much easier than mechanical or energy transformation.  The results should be better  for straight combustion heat delivery than for multiple-transfer process, like mining and transporting and burning coal to make steam to make electricity in successive voltage levels to transmit across hundreds of miles of wire to run through a resistor... to make heat.

Given that heat is the final end state of almost all physical energy processes, the real questions are why are we not getting more heat for our efforts.  And that mostly comes down to heat recovery, not combustion efficiency. 

Does the design of the stove or home allow you to store that heat, or is it gone almost as soon as it is produced?   Can that heat be delivered as comfortable warmth, or do you have a choice between too hot and too cold?  It's hard to compare numbers like square feet, BTUs, or thermostat temperatures with actual delivered comfort.  The more space we put between ourselves and the source of life-sustaining warmth, the more of it is wasted before it ever reaches us.

The real-life problem is that woodstove owners want steady warmth, not hot-burn maximum efficiency.  So they are running their stove 24/7 at one of its least-efficient rates, to get the same warmth we enjoy with any masonry heater or rocket mass heater after a 4-hour burn.  The worst part of running a woodstove is when the fire goes out at 3 am and the stove keeps drafting for another 4 hours with no heat in it, functioning as a very effective cooling chimney, and you wake up to frosty-cold floors.  Woodstoves owners swap tall tales about their valiant efforts to defeat the cold-morning nemesis.  Many will scandalize the stove to make it smolder all night, resulting in much worse fuel efficiency in practice than was ever imagined in the lab tests.  (If you routinely run your stove with more than 1/4 inch of ash in the firebox, or have installed anything on your chimney without checking your stove manufacturer's recommendations, you may be among the inadvertently guilty.)

This smoky all-night-smoldering artistry results in a bad name for wood heat.  Despite the "clean" and "efficient" ratings of the modern stoves, the reality is that in many areas, overnight smog from wood heat is a reality.  Local jurisdictions will often ban wood heat altogether when concerned about air stagnation, health, and safety - which means old stoves and fireplaces are grandfathered in, and newer models face a tough uphill battle for legal approval.  Of course, banning wood heat is only a temporary solution, and tends to stifle progress.  Our devaluing of wood as a fuel source results in health and safety issues from unburned ladder fuels and wildfires, and the alternatives we use create global and local problems from extraction and transport of non-sustainable fuels. 
And as we lose the skills to be truly clean and efficient with wood, each political hiccup like the prospect of Russia-Ukraine sanctions cutting off the gas to the EU sends people scrabbling for alternative heat, meaning slapdash installations and soggy woodpiles where once was a viable tradition of efficient wood heat.

And of course, poor practices and all-night-burn-artistry also result in the stove operating at a very minimal efficiency compared to what it could actually be doing, since a lot of the energy in the wood is being used to send unburned fuel up the chimney as smoke, and to evaporate the water from the unseasoned wood that smolders the "best."  Don't get me started on people's delusions about dry firewood. 

To sum up: A manufacturer-rated 78% efficient stove may only offer 67% combustion efficiency at its best.  When you consider common fuel storage and burn habits, you may have a burn rate less than 25% efficient.  And given that most "heat extractor" woodstoves do their best to extract heat by blowing warm air around, which lasts less than 3 hours (possibly less than 15 minutes) in a home with sufficient ventilation to be safe to breathe in while burning wood, we can expect abysmal efficiency in actually keeping us warm.
People whose wood burning skills are so superb they routinely start 0-match fires have repeatedly cut their wood usage to 1/4 of previous totals by replacing a modern, certified insert or woodstove with a rocket mass heater.  Ordinary people with any of the aforementioned poor burn habits can easily burn ten or even twenty times the amount a RMH would need, while priding themselves on the peculiar skills that keep their family warm all night and the neighborhood coughing all morning.

Smoldering wood is not a safe place to store your heat all night.

B) The rocket mass heater burns for only a few hours per day, so any excess air is only an issue for 4 hours in 24.  The exhaust is often measured above 90% combustion efficiency, at masonry heater minimum exhaust temps of about 200 -300 F.  (Some tinkerers have run rocket mass heaters with exhaust temps as low as 60 F, colder than the room temperature in the building, but those stoves are very finnicky to operate and need a condenser-style exhaust instead of a chimney.)
Compare sending smoke (unburned fuel) up a woodstove chimney 24/7, above the legal minimum of 350 F the whole time.  Compare 4 hours of chimney draft with NO heat production if the woodstove goes out at 3 AM.  Rockets and woodstoves both have air controls, some more foolproofed than others; most experienced operators constrict the air to the specific percentage that makes their stove run best.  But as in most stove models, too small an air intake runs the risk that it could be clogged by fuel or ash, and suddenly you go from optimal performance to smoldering creosote factory. 

In the balance between allowing a little excess air to ensure complete combustion of your fire wood, and trying to hold on to all that warm air for the full 3 hours it could otherwise spend in the house... putting holes in your floor that are fireproof vs. making heat-exchangers to pre-heat the incoming air to compensate for the 500 CFM going up your chimney... this is not a win-win situation.  The less time you have to make this tradeoff, the better. 

Run the fire as briefly as possible, and shut it down when not in use.

Store the heat in something other than air. 

C) Compare the heat capacity of air vs. brick. Losing some warm air is a negligible problem when thermal mass maintains warmth for days.  We still have 2 feet of snow outside, and we are only running our heater every other day.
To have good indoor air quality, avoid structural rot, black mold, depleted oxygen, and other toxic situations, most modern homes must be ventilated to ensure a minimum air exchange of 1/3 their volume per hour.  Living in a zip-loc bag just is not a good idea.  As Paul summarized it after repeatedly asking this question of half-a-dozen different rocket stove experts, "Why would you breathe stale air and farts, and feed the clean air to your stove?"  Only if you have gone off down the rabbit-hole of chasing the wind.  Air is a poor conductor, that's why most insulation consists of foams or fibers that trap lots of air.  It's hard to force heat into air, and even when it's at a high temperature, it doesn't actually store or deliver much heat.  Trying to store heat in a bag of air is a wasteful exercise compared to using dense thermal mass like brick or stone or hot water: something that has the heat capacity and conductivity to actually build up a thermal storage worth protecting. 

Imagine you are about 6 years old and just came in from the cold.  What's going to warm you up faster, asking your mother to blow on your hands, or some time in her lap with a hot mug of cocoa? 

Have you ever been given a warm brick, or a hot water bottle, to take to bed?

We read the Laura Engalls Wilder books, how they'd get a hot potato from their uncle to put under rugs and stay warm on the sleigh ride home. 
We had hot water bottles.  We knew how long heavy things like soup or potatoes would stay warm, compared to toast or rolls. 

We still didn't make the connection with Nan's brick.

My great-grandmother hooked rugs, elaborate nubbly-surfaced wall-hangings with scenes and characters and identifiable birds.  Nobody would let any of them touch the floor, except for one weird one that was a rug-wrapped brick. 

We used it as a doorstop. 

Boy, do I feel dumb now. 

School-day mornings, I would crouch over the hot-air register with my arms tucked in, like a parental penguin, waiting for the brief rush of warm air to balloon out my nighty and heat my toes. I'd usually wait through two or three furnace cycles, toes still nerve-jangling cold inside and hot outside, before I finally obeyed Mom's demands to Go Get Dressed!

I don't think any of us connected that brick with the idea of heat.  We took it out once, just to see that it was actually a brick.  That dense woolen brick-cozy has never been put to warm use since Nan gave it to us.

Aside from a hot water bottle or bath, there were very few dense, warm surfaces in our house where you could actually put any part of your body in contact and warm it up quickly.  We tried to save energy by turning down the heat at night - still a good idea if you can adapt to it - and if we'd had a warm patch of floor to stand on in the mornings while we put our socks on, I think my mother would have been able to spend less energy on heating bills AND less energy herding kids off to school.

These days, I live on a snowy mountain.  If I happen to get my toes chilled outside, I put them on one of the warm stones of my heater for a minute on my way back in.  If I feel like luxuriating, I sit and tuck them under the cushions while I have a tea and snack.

D) The real question is, what kind of efficiency can a heater deliver in practice? 
Is it comfortable? 
Does it get your kids warm enough to change their clothes without a fight?
Does it get the heat to the people who need it, rather than heating mostly ceilings, attics, and birds overhead?
Does it use less than a day's wages, or a day's labor, per month?
Does it use less wood than your yard debris naturally produces each year?

From the results we've observed in the field, thermal mass heating works awesome.

Rocket mass heaters run on a convenient schedule: crank it up after work or after dinner, shut it down before bed.  Skip the dangers of sleeping with an unattended fire in the next room, or the inconvenience of all-night fire tending; eliminate vulnerability to winter storm power failures that shut down many thermostat- and fan-driven devices. 

Sitting on the rocket bench is kinda like a mini-sauna for whoever wants it, available at all times.  No need to argue over the thermostat.  Anyone with cold toes or sore backs can snuggle into comfortable contact warmth, like a full-body heating pad or heated seats. 

Ever notice the difference in comfort between a car with heated seats and one that just blows hot air in your face while you drive?  Which one keeps you more comfortable and alert? 

The thermal mass is also compatible with passive-solar design (which I consider the ultimate in efficiency, as it's literally zero fuel usage).  Thermal mass soaks up ambient warmth from the sun, cooking, kids and pets, or whatever is around, including excess heat during hot summer days.  It averages out the differences between too-hot days and too-cold nights. In many climates, the mass of the heater completely eliminates heating and cooling bills for several months of the year, providing better comfort than fans or heaters with literally zero fuel, zero utility costs.
Over the remaining part of the year when heat is needed, it's common to see owners use about 1/4 the wood (8 cords down to 2 cords). Getting to 1/10 the fuel is not unusual in coastal and mild climates, where the heater can 'coast' more months: users go from 4 to 5 cords down to 1/3 to 1/2 a cord.  The mass heater can also make up for some deficiencies of insulation or weatherization, since it doesn't depend on keeping the air warm in order to keep the people toasty.  They're a great zone heater for a sun porch or solarium, where you want passive solar most of the time, with the option to crank things up to decadently comfortable for a big house party. 

Of course, these are old tricks, and not every woodstove operator is entirely ignorant of them.  Where the home was already storing the woodstove's heat with thermal mass, or an efficient passive-solar design, they'd be using less energy already, so the rocket mass heater results may be more like 1/2 the former fuel usage. 
I've never seen anyone use anywhere near the same fuel in their rocket mass heater, compared with a previous wood-burning stove or insert. Or spend a fraction of the money on cordwood they used to spend on gas. 

Rocket mass heaters work best when given pride of place in the occupied parts of the home. Don't expect to park a skin-temperature rocket mass heater in the unheated basement and heat the whole house like a furnace - you've just doubled or tripled the heating load by trying to heat that basement, while removing all the benefits of contact and convenient tending.
    If you already have a masonry heater, good for you; I don't think I would switch.  If you can do passive solar, or triple your home's insulation, go for it; these may save you enough money to pay for themselves out of your heating bills, and even improve your property value by a significant part of the cost you invested.  But if you want a heater-to-heater comparison between a well-designed rocket mass heater with any other woodstove or natural gas heater in the same price range, I hope you can now see why it's an order of magnitude difference in performance efficiency.

In 2015 we are encouraging our readers to support two Kickstarter projects:
- Paul Wheaton's new, improved 4-DVD set:
This supports Paul's efforts to promote this technology, including an annual Innovator's Gathering where we continue to work on ways to make these heaters better: easier, more affordable, even cleaner and more reliable, and find compatible ways to use them for a wider range of functions like cooking, hot water, and a wider range of heat outputs for specific needs.

- our own upcoming launch for the long-awaited Rocket Mass Heater Builder's Guide:
This book will put the current state of the art into the hands of a lot more people, make it simpler for first-time builders to avoid known errors, and for professionals to customize a heater for each new site. 
A stretch goal is to use the proceeds from this book to get the ball rolling toward EPA approval, eventually getting the best models out there with that reassuring sticker that your local authorities know and love.  (We'd like to test the most popular DIY versions, too, so that these heaters can remain affordable to a much larger number of people.)