designing a rocket mass heater/stove to heat 250 square feet or less.

Hello all. I am currently designing a rocket mass heater/stove to heat a small cabin or cottage. I have a few questions that are holding me up. the first one is i planned on using a 55 gallon drum design. Do you think that is too big for the square footage? another is how hot the mass gets to the touch. i would like to have a seating area that would double as a bed be the mass. is there any chance of the mass getting too hot to have cushions or a thin mattress on it? anyone with a RMH bed here? Does the heat transfer well to your sleeping pad/mattress? I am debating on if i should cut the barrel down some and make it smaller. maybe 3/4 or half the original height? how would that affect the unit in terms of wood use and efficiency? Oh and one more thing. firebrick on inside of heat riser and cob/perlite on the outside or firebrick on the outside of the heat riser and cob/perlite on the inside. I see people filling the gap between 2 different sized stove pipes for their riser, but ive seen more than a few where the stove pipe on the inside burnt up. does the cob/perlite mix degrade in any way? Thanks again for the help.

Paul Delaet :Let me take a small piece of your Post and I am sure you will hear from several more of your Fellow Members . If the Constant cross sectional area C.C.S.A.

of a 8'' Rocket Mass Heater RMH is 50 sq.in. ( too big ) and the C.C.S.A. of the 6''RMH is 27 sq. in. (still to big ) Then the C.C.S.A. of a 4'' RMH is 13 sq. in.

Note that when we went from 8'' to 4'' we reduced size by 1/2 but decreased the C.C.S.A. to 1/4.

That means that we barely have enough Volume in our smaller piping to promote a Good draft Which is critical to give us the "Rocket effect'' we need to create a High

temperature Efficient good clean burn.

The usual practice is to build one of the larger 6'' or 8'' Rockets and live with them for at least 1 heating season. Generally the smaller 4'' RMH is considered a more difficult

or an Advanced build Because you are already working at the smallest size that can work, Plus the amount of horizontal run is also very limited. Because the resistance to flow

of 1 elbow Counts against your permitted length the same as 5' of horizontal piping - you Must limit the elbows on this size Rocket !


This is where I ask if you have been to Rocketstoves.com to download your copy of the brand new 3rd Edition of Evans' and Jackson's Rocket Mass heaters ?

This should have answered most of your questions an guaranteed you can come back here and use the same words to discuss the Correct sizing, Length Ratios, and Parts

Orientation both to the whole and each other ! And don't worry about the math it is really all 5th grade/Elementary stuff ! Remember every single Rocketeer here started

from where you are now , For the Good of the Crafts ! Big AL

For a very small space you might be better off finding a 35 gallon or so barrel, rather than shortening a 55 gallon barrel. A 6" system will be easy to build reliably, and if it is bigger than you need for heating, you can just burn less often. With sufficient mass to absorb the heat, you will have no waste and constant comfort. What sort of structure are you thinking of? That makes a difference in what sort of thermal mass you can use, and what kind of clearances you need.

Glenn Herbert wrote:For a very small space you might be better off finding a 35 gallon or so barrel, rather than shortening a 55 gallon barrel. A 6" system will be easy to build reliably, and if it is bigger than you need for heating, you can just burn less often. With sufficient mass to absorb the heat, you will have no waste and constant comfort. What sort of structure are you thinking of? That makes a difference in what sort of thermal mass you can use, and what kind of clearances you need.

The structure is going to be timber frame with insulated cordwood and cob. No more than 250 square feet with a ceiling height of 8 ft. I was also thinking of running a 6" system but use 8 inch duct through the mass to slow the air flow down. But considering 55 gallon drums are way more common around me, and i just picked up 3 for free, its where i decided to start. i definitely think that a 6 inch 35 gallon system will do me just fine. but if a 55 gallon one wont burn me out, im ok with just going with it.


Allen- No i have no gotten a copy of that particular book. As much as i would love to have a book that has most of the answers to the questions i have, the holidays are stripping me of my spare funds right now.

A 55 gallon drum instead of a 35 gallon one won't overheat your place, it will just be wider and occupy more room (if only a little). If you can fit a 55 gallon drum and be comfortable with the layout, by all means use it.

The Rocket Mass Heaters book, on the other hand, is pretty much essential. It has about all the information you will need for building your RMH, and you won't miss something critical that might slip by if you are getting your info off the web. It will also give you the standard terms so that you can talk reliably with other people and both know what you mean. If you are really strapped, ask for it at the top of your Christmas list

A cordwood/cob wall is sturdy and fire-resistant enough that you can locate the hot parts of your RMH without worrying much about clearances.