Under building RMH

Im building a small 20 by 20 cabin next year in michigan. We plan to have it raised up 2 to 3 ft up to give a crawl space and was wondering if a RMH could be installed under the cabin floor with wood feed just outside the front door. Im trying to avoid having my fire pulling air from the room so the cabin doesn't draw in drafts. Has anyone tried this or would it end up failing?
I was also thinking I could do a taller riser with a square "barrel" for a cook top inside the building. My cousin is a master welder so I know he can build the box to be air tight. Would squared off edges create hot spots for possible failure?
Thanks to anyone giving input.

It's a very interesting idea -- sort of a modern, hybrid hypocaust. I have a cabin with a crawl space and deep, dry sand that could benefit from a similar concept.

Douglas Alpenstock wrote:It's a very interesting idea -- sort of a modern, hybrid hypocaust. I have a cabin with a crawl space and deep, dry sand that could benefit from a similar concept.

Hypocaust, thats the first time I've heard of that and it's opened a ton of research. Thank you.

Hi John,  The idea of having the word feed outside certainly has some advantages, but probably equally the same amount of disadvantages. One of them being that a typical RMH requires constant feeding and that going outside to feed it would get old pretty quick.
 Something to consider.
The traditional hypocausts that I have seen  have very large wood feed chambers where they can load a lot of wood in at a time and therefore only need to stoke the fire may be a few times a day.  They also didn’t appear to be as clean burning as an RMH.

Gerry Parent wrote: Hi John,  The idea of having the word feed outside certainly has some advantages, but probably equally the same amount of disadvantages. One of them being that a typical RMH requires constant feeding and that going outside to feed it would get old pretty quick.
 Something to consider.
The traditional hypocausts that I have seen  have very large wood feed chambers where they can load a lot of wood in at a time and therefore only need to stoke the fire may be a few times a day.  They also didn’t appear to be as clean burning as an RMH.

How often would you expect to feed it? I currently work for a tree care company outside for 9 to 10 hours a day. My thinking is ld like to feed it in the morning before work while i get ready and make breakfast. Then again around 5pm when i get home (so long as temp stays above 55* F I'm happy with that), maybe a little more before bed. I know most wood boilers are 3 times a day but they waste so much heat energy just sitting there idling and transporting heat to the house. Am I expecting too much from a RMH?

Feeding time intervals vary according to your system size, diameter, species  and dryness of wood and also which  style of rocket your building.
It can be as short as 15 to 20 minutes  with a J tube or up to 2 to 3 hours  with a batch box.

This is the equation I used to calculate btu requirements, if anyone can correct it for me please do.
80 degrees difference x 400 sq ft x .133 = 4256 btus per hour so just about 1 lb of dry wood per hour at 50 to 60% efficiency. Is that all correct or no?
If so it would require 10 to 11lbs of dry wood per 10 hour shift. Id be using about 25lbs a day. Which might be kinda challenging to make sure i can get enough heat into the cob to sustain it.

Does any of this sound correct or am I way off base?

Gerry Parent wrote: Feeding time intervals vary according to your system size, diameter, species  and dryness of wood and also which  style of rocket your building.
It can be as short as 15 to 20 minutes  with a J tube or up to 2 to 3 hours  with a batch box.

So even with a batch box it would never have enough heat to sustain a 9 to 10 hour shift?

John Smich wrote:
So even with a batch box it would never have enough heat to sustain a 9 to 10 hour shift?

Well, that’s where a rocket mass heater (RMH) comes into play.
Heat up enough mass to hold the heat  and keep your place from getting cold until you return.

Gerry Parent wrote:

John Smich wrote:
So even with a batch box it would never have enough heat to sustain a 9 to 10 hour shift?

Well, that’s where a rocket mass heater (RMH) comes into play.
Heat up enough mass to hold the heat  and keep your place from getting cold until you return.

I think I misunderstood what you were saying. If Im understanding correctly it's the time having to keep feeding it till it comes up to full temp. Basically id need an hour or two babysitting the fire before id have enough to go back inside.  

Yeah, RMH’s  generally need more upfront babysitting time than a regular wood stove but the pay off comes from heating the mass and having it keep the house warm for a fairly long time.
This is because they burn hot and fast, clean and efficient which is not often the case with a regular wood stove.

Thinking a bit more on this, I am so used to the idea of a wood stove constantly burning, drawing in drafts, where a RMH you don't have that constant syphon. Once the fuel is spent closing the baffle stops that from happening so it's less of a battle.

One of the main benefits of a RMH is not having to feed it repeatedly. Instead you fire it once or maybe twice to charge up the large masonry mass, and then that mass radiates heat over a long period. It depends on how well the space is insulated as well, a drafty cabin where it's -40 outside needs heating more often than a well sealed space when it's 10 outside. The latter might go a full day or two between firing.