How much wood do you use with your rocket mass heater?

I was curious for those of you with a rocket mass heater, how much wood do you burn per year in cords? I’ve read before people saying they burn 1-2 cords per year, but I’m curious on those numbers. For instance how big is your home? Also is that cord number an I split wood cord, you is that your wood split to size for the rocket mass heater and then stacked and called a cord, because those two numbers will be a vast difference in wood split vs unsplit.

I use about one cord a year to heat my approximately 900 sf house (main floor and lofts) in upstate New York, USDA zone 5. I split the wood (mostly oak, ash, black cherry and maple) myself but not to kindling size, much of it around 4" x 4" equivalent.

Glenn Herbert wrote:I use about one cord a year to heat my approximately 900 sf house (main floor and lofts) in upstate New York, USDA zone 5. I split the wood (mostly oak, ash, black cherry and maple) myself but not to kindling size, much of it around 4" x 4" equivalent.

Hi Glenn, so that’s one cord of (roughly) 4”x4” split? Not one unsplit cord subsequently split into 4”x4” correct?

Hi Justin;
That number has many variables.  
Home size and design, how well insulated it is.  Personal preferences on how warm you keep your house.

I can tell you that in my uninsulated greenhouse/studio.  We were using 12+ full cords to keep it above freezing.
After building an 8" J tube with piped mass, we used less than five cords.
That is burning all day from 7 am  or so until 9-10 at night.

They work as stated but each family is different, some prefer it cool some prefer it hot.

Correct. I have sometimes started with a cord's worth (measured by the size of my woodshed) of commercial-sized firewood, sometimes with all properly-sized wood for my RMH, usually a combination. I haven't seen a noticeable difference.

65F through the day and 60F by morning is comfortable for us. If the sun is shining we don't need to make a fire all day in average winter weather, just in the evening. I have a lot of windows, mostly on the south side, and the house is supposed to be superinsulated (10" fiberglass in walls) but the carpenters I hired did a very sloppy job and it is fairly leaky at present.

Hi Thomas, now that less than 5 cords that you are now using, is that split to size for the j tube? And the 12+ cords you used before was that unsplit or even split big?

Well its stacked as rounds and halves but when it was a J tube it got split down more before it came inside.
Now that I have switched to batches wood is round or half.

Before it was as large a chunk as I could toss in the wood gobbler!

I use about 5ish cords of slabwood per season in my 8 inch J tube

Our heater is a 6" batch box rocket with a weight of 2.2 US tons. See https://batchrocket.eu/en/applications#redbell
The house's habitable surface area is 180 m² (1940 sq ft), the living room annex kitchen is 52 m² (560 sq ft). We like it warm, during winter the average temperature in the living space is 22 ºC (71.6 ºF), the rest of the house is never cooler than 15 ºC (59 ºF).

Very low fuel usage can't be achieved without special measures, in our case it's due to the house and in a lesser way the location. We live in a detached passive house, insulation 36 cm (14.2") of mineral wool in the walls, 34 cm (13.4") in the roof and 30 cm (11.8") polystyrene foam under and around the floor slab. Special insulating window frames, triple glass with plastic spacers, large windows facing south and balanced ventilation with heat recovery. Build to leak (much) less air than a standard house, the obligatory letterbox in the front door isn't even there.

We live in the Netherlands close to the coast, comparable to northern California I'd guess. About 2 to 4 weeks of frost during the day most winters, nothing lower than -5 to -10 ºC (23 ºF to 14 ºF), albeit it's generally windy.

At the risk of making someone jealous: last winter we used about 0.41 cords (1.5 m³) of resinous fuel, mostly lumber scraps and pallets, to heat our house. With outside temperatures above freezing we run the heater with one batch a day, no more than 5 kg (11 lbs), this is what the firebox can hold. With frost and snow outside this goes up to twice a day, in a sliding scale up to 8 or 9 kg (17.6 lbs to 19.8 lbs) in total per day.

My conclusion: insulation works, more insulation works even better.

Thats a great comparison for me to consider Peter. I am in a fairly similar climate in Northern California, and a similar sized house (1600ft sq, 2stories, well insulated). We are up around 1800ft elevation but just 7mi from the Pacific near the first major ridge, with similar winter lows. We do not have our passive solar attached greenhouse yet, but it is in the long term plan, as is an RMH.

We use about 2cords per year (split to my preferred sizes) in our “highly efficient” wood stove. It is the best wood stove I’ve used of a half dozen or so I’ve lived with as my primary heat source in and around the coastal mountains of the Pacific NW. I go for mostly 2” splits to get a hot and efficient (cleaner) burn. I like to see the stove reaching 600f and the chimney 3ft up at 200-300f when recirculation is working well. We have some wood up to 4” thick, but those are set aside for overnight burns. Peter’s numbers are a good motivator to overhaul this with an RMH and passive solar greenhouse, as that work will offset my collecting, bucking, splitting and stacking in just a couple years.

Peter, you also maybe the best person to answer a question that has occurred to me about the relative cooling effects of cold air, snow, and cold rain. I have been a backpacker and ranger in the coastal PNW mountains for awhile and have found myself out alone in some rough weather for up to a week.  If I were to set up a test of hypothermia avoidance  survival skills, the toughest scenario would be rainy and 35f, not anything in snow or sub zero temps. The cold rain just wicks warmth right out of you like a beer wort chiller, which uses cold liquid not air or ice. Is this similar for a house? How cold would dry air have to be to compare to the thermal loss of just above freezing rain? We can get 5+” of that rain per day, and combined with the lack of solar radiation on our large windows (which heat the house to 67-70f  on 40f sunny days), this makes for our most fuel consumptive situation. Any thoughts on that comparison of coastal and interior kinds of cold and its effect on heating fuel consumption?

I think a house is not really comparable to a person. With a rain-shedding skin and some overhangs, there should be little conduction or evaporation of heat from the surface relative to the overall insulation value of the envelope.

About six years ago I very carefully measured.  0.60 cords of wood with an average temp of 69 degrees F.  Montana 3-bedroom house (double wide) about 1300 square feet.  So-so insulation.  Zero insulation on the windows.  And that was a particularly cold montana winter.



I am slowly improving the house.  And I hope to improve the rocket mass heater.  And then do the test again.  I am hopeful that I can get this under 0.30 cords of wood.

I am in the first winter using my RMH. I had a Woodstock soapstone woodstove for the previous 16 years i lived in my house. This house is 900 sq.ft. I averaged about 2&1/2 cords /year over that time. I am carefully measuring my wood usage this year and, so far, it looks like i will be using appx. 1&1/4-1&1/2 cords for a rather mild (so far) winter for my area of south-central Colorado (San Luis Valley). My build is a 7" system using clay and stone. My house is warm and i love the look of the stone in my living room. I also cook most of my meals on top of the barrel-something i could not do on the soapstone stove-BONUS! That being said, my main motivation for partaking in this endeavor was the reputed lack of creosote build-up in the chimney pipe and i have yet to know as i will check at the end of burning season. The reason this was my main motivation was that i had to get up on my roof every year to clean the chimney and a few years ago i was up there and the wind blew my ladder over and i had to jump off the roof. At 50 years old that wasn't too big of an issue. I'm now 63 with two wrecked knees (took my football a bit too serious back in the day) and i can see myself blowing a hip if i have to do that again. So, if the creosote issue is not a real problem then the only issue left is what i see happening to my metal roof. Whatever is coming out of the chimney seems to be degrading the painted metal and rusting the screws adjacent to the pipe. I'm not sure but i think this would be mostly water vaper due to the high temperatures of the burn. I'm wondering if this vapor is acidic in any way. Any thoughts or suggestions would be greatly appreciated! By the way-i want to give a huge shout out to Thomas Rubino at Dragon tech for his generosity and technical advice-just a stellar dude.

Hi David;
Thanks for the Kudos!
How about a picture or two of your new stove?

Did you notice that the new 7" batch box at Paul's has the door I built for you on it?
You gave me a good deal on returning it and I gave Paul a good deal on purchasing it.
It has found a happy home!
https://permies.com/t/193821/permaculture-projects/inch-batch-box-rocket-mass

I live in Michigan, the south west part, zone 5.  My home is about 600 sq/ft, an old mobile home in which I've replaced all the doors and windows and added a whole lot more insulation to what was here.  I'm not sure if really qualifies as a super insulated home though.

Over the past several years heating with my 6" pebble style RMH I find I'm using between 1 to 1.5 cords of split wood.  I split it fairly fine for the RMH too.  I should also note that my RMH has less heat exchange mass than it probably could due to lack of space in my home.

I did a blog post some time back about the difference between a cord of wood roughly split and finely split.  Here's a couple photos from it showing my wood hod filled with rough split wood and then those logs after they've been split down finer.


This is before I split it finer.


These are the same logs split down finer and fit back into the hod.

Thomas-you earned the kudos-you're welcome! I checked your link to Paul's 7" batchbox and i'm glad to see my old (new!) door off the streets and out of trouble. I am in the (slow) process of cleaning the stone front of the stove but when i am through i will snap a few pics and upload. Thanks again for everything Thomas!

I couldn't find a specific thread for this topic so i will just post here. Does anyone have experience with metal roofing suffering from the RMH exhaust? 16 years with a woodstove and i didn't notice this. Not even halfway through my first year with the RMH and i'm noticing rusting of screwheads and the metal roof being impacted around the chimney exit. Any input would be highly appreciated.

Hi David;
That's odd, I have not experienced that at all.
My cabin has a standard box stove and the metal below it is stained with creosote and some surface rust.
My shop has had an RMH for 7 years and the metal roof is still fine.

Is your roof older?
Could it have slowly lost its protective covering and you did not notice?
Now with all the steam from your RMH, it is quickly showing up?

Eventually, all metal roofs will show surface rust.
I have noticed that around a chimney, it always seems to be the first spot to show.

I’m in Missouri zone 6b, I use roughly 1.5 cords with a rmh. Well insulated house, designed for a rocket mass heater with great circulation. Probably around 1400 sq ft. I like to keep it 74-80 plus. Anything below 70 degrees is strictly forbidden and more fire must be burned immediately.