Floor/space heater

Here's my situation: 625sf garage apartment built in 1921, with 12-16" thick limestone walls that go down at least 4' into the ground that we know of. Garage doors long ago failed, and were boarded up, leaving the garage as a dirt floored workshop area.

Landlord loves wood heat, and he and my family have tons (literally - between those sources, about 200 acres) of mesquite available for the taking. He's open to experimentation, an volunteered that the first tenants he had here had a wood stove down there, with the chimney going out a former window under the stairs.

My thought was to take advantage of the RMH's low exhaust temp to go out that same window without the need to go past the stairs; from what I've seen, temp after a 40' underfloor run should be lower than a dryer vent.

This is Texas, so I'd probably only want heat about 3-4 months out of the year, so unobtrusive is a key goal; I'd like to have only the fuel hole, barrel, and exhaust above the floor, and all close together, with the mass heating being a single u-shape across the center of the floor.

My questions, then:
How hard is it to do an underfloor RMH like that?
In a 625sf space, with thick rock walls, how much heat should I expect to rise into the apartment above? (The floor is reasonably permeable, but would I need the workshop to be a sauna to get warm upstairs?)
Any known issues with mesquite in a RMH? Mostly, I try to drop the trees in spring, so they'd have at least 6mo in the sun to dry and cure before burning, but it's still a really hot burning wood, and I know it's common to get grills and woodstoves bright red orange hot without even trying. (Killed a grill that way myself.)
Joists for the apartment floor are exposed wood about 8'6" above the garage floor. I'm sort of assuming there should be some sort of radiant heat shield between them and the barrel. Would a couple of standard 26ga roofing sheets suspended a foot below the joists be enough, or should I go overkill with two layers about 6" apart so the lowest one doesn't reradiate directly onto the joists?

Hi Joe; I'm not an expert but let me try to answer your questions. First , do you have a copy of ianto evans book rocket mass heaters ? If not you should get one. As to building underground , yes, that is done but... you must insulate from the earth or all but the radiant heat from your barrel will be trying to heat the ground surrounding your rmh. Your thick rock walls will gladly radiate your heat thru them and again try to heat mother earth on the far side. As far as heat rising thru the floor, i would cut a hole and install a grate or two , much faster way to get heat upstairs. One piece of sheet metal should be plenty to keep the heat deflected from the joists. Yes, exhaust temps should be in the 130-150 degree range, plenty safe to exit the building where you need it to go. No issues with any wood that i have heard of (i'm planning on planting black locust myself ) if mesquite burns hot then all the better to make your dragon roar ! In closing, if it were me I would build a mass bench above ground and not try to build in the ground ... unless you plan on insulating with 4" of foam anyplace the mass touches the ground. Good luck and post pictures of your creation .

thomas rubino wrote:As to building underground , yes, that is done but... you must insulate from the earth or all but the radiant heat from your barrel will be trying to heat the ground surrounding your rmh.

I had the hardcopy book at one time, (Used copy caught my attention at Half Price Books and got me interested in the RMH idea, but haven't been in a housing situation right for it until now.) but haven't seen it since the divorce a couple years ago, so it might be gone.

That raises the possibility, then, of putting the firebox and heat riser in the center of the floor, (or offset to be under the bedroom on the opposite side from the window I want to vent through) so the only undesirable direction from there would be straight down; after all, the basic concept is to heat the dirt floor. My assumption then would be that after a couple of turns to add length in the middle of the room, by the time the exhaust got near the wall, most of its heat would already be gone.

How much heat would actually be lost straight down through what (at that depth) is basically hard packed caliche and/or red clay? (Floor is about an inch of sand over a varying amount of caliche, then hard red clay. Haven't dug down far enough myself to see what's below that, but the landlord had a crew try to dig under it for a new drain line, and they were still hitting stacked limestone walls well past 4 feet. Makes me wonder if there was once a root cellar or cistern with a wood floor at ground level.)

Joe; In that case then, straight down is ok ! Put a couple of inches of cob with straw down first as an insulator and build away. Post pictures as you build to help the next guy along.

Hi Joe,

I like the floor heating concept. It reminds me of a Korean Ondol: https://en.wikipedia.org/wiki/Ondol
There's a concept proven over thousands of years.

"How much heat would actually be lost straight down through what (at that depth) is basically hard packed caliche and/or red clay? (Floor is about an inch of sand over a varying amount of caliche, then hard red clay. ..."
I read in an underground house book about northern USA ground temp.s averaging about 55 degrees F. Maybe there undergrond temps would average 60? There's is r-factor information on heat conductance in soils, so this heat loss downward is an answerable question.

"--- they were still hitting stacked limestone walls well past 4 feet. Makes me wonder if there was once a root cellar or cistern with a wood floor at ground level.)" Wow, I'm surprised at how substantial a footing was built.

Neat concepts, hope to read how it turns out.

Joe : I am sending you a link to a different build because you could have problems getting an in ground system like you are thinking of to ''Draft'' when cold !

You would ordinarily have an inspection and clean out ''T'' located where your horizontal pipe goes vertical, but in your situation that part would be in the floor
and lighting a fire there to 'kickstart' you draft would be a difficult build, so instead this system is designed to divert the heat from after your barrel to your
chimney, then when your chimney is warm you flip the damper to let the hot exhaust gases flow through the horizontal piping !

https://permies.com/t/5831/wood-burning-stoves/underfloor-heating-system-rocket-mass

For the good of the Craft ! Big AL !

The broken link has been fixed, it worked 5 times in a row and I got cocky

Brian Cady wrote:I like the floor heating concept. It reminds me of a Korean Ondol: https://en.wikipedia.org/wiki/Ondol
There's a concept proven over thousands of years.

That's what made me think of it; the floorboards are exposed to the garage, so there's basically just 1.5" of 93 year old wood and some worn carpet between the garage and the living area. Shouldn't slow the heat down much at all, and will give me an incentive to work on the garage projects (tuning up my bicycle and fixing a couple of old chainsaws) while waiting for things to warm up enough to rise each day.

"How much heat would actually be lost straight down through what (at that depth) is basically hard packed caliche and/or red clay? (Floor is about an inch of sand over a varying amount of caliche, then hard red clay. ..."
I read in an underground house book about northern USA ground temp.s averaging about 55 degrees F. Maybe there undergrond temps would average 60? There's is r-factor information on heat conductance in soils, so this heat loss downward is an answerable question.

63F at 4" Yeah, our frost line is pretty much measured in grains of sand

"--- they were still hitting stacked limestone walls well past 4 feet. Makes me wonder if there was once a root cellar or cistern with a wood floor at ground level.)" Wow, I'm surprised at how substantial a footing was built.

You think you're surprised; imagine that poor plumber's laborer digging through hardpacked bone dry clay with a mattock, and every time he tries to go out he hits rock again. By contrast, the dirt outside is so soft and sandy I almost need to put pneumatic tires on the lawnmower to keep from scalping the grass. I got a pretty good discount for accepting the task of mowing the vacant lot behind the houses, (120x60' takes about 20 minutes twice a month when it's wet enough to grow) and also get full use of it, so I think a garden just about has to be on next year's agenda.

allen lumley wrote:Joe : I am sending you a link to a different build because you could have problems getting an in ground system like you are thinking of to ''Draft'' when cold !

You would ordinarily have an inspection and clean out ''T'' located where your horizontal pipe goes vertical, but in your situation that part would be in the floor
and lighting a fire there to 'kickstart' you draft would be a difficult build, so instead this system is designed to divert the heat from after your barrel to your
chimney, then when your chimney is warm you flip the damper to let the hot exhaust gases flow through the horizontal piping !

Something didn't work on the link, but since the pipe will have to come up from the floor to the window, would it make sense to just put the priming tee in right where it leaves the floor, and encourage the draft from there? With 30-40' ahead of it for the exhaust to cool, I would almost think about putting a fan in there on a 5 minute timer as sort of a "lighting supercharger" until the riser warms up.

Knew I still had these around somewhere; I design PV arrays for a living, (sorry, we only cover Texas right now unless you have a job (or maybe several smaller but very firm jobs) over 100kW to make it feasible to hire out of state electricians) so one of the first things I did when I started planning repairs and repainting was to build out what I was starting with in Sketchup. ~550sf enclosed on each of the main (garage and living area) floors, and around 300sf in the loft. (No, I don't have a Jefferson stair to the loft yet; that's something I've thought about to move the foot of that stairway back several inches and give me clearer access to the rest of that room.)

Under that center wall in the garage is a line of studs on a concrete footer that goes down a few inches, so the ducting will have to pass under that. Otherwise, it's all open dirt down there. Ideally, I'd probably put the firebox and riser just west of that footer, under the bedroom, and maybe run the exhaust west, then north under the kitchen, east under the bathroom, and then south to come even with the window. Actual run TBD based on what I can find in used stovepipe The window for the chimney isn't shown but is just behind the man door on the east side, (roughly between the two upstairs windows) so well sheltered from the prevailing winds. It's the same size as the others, but has been boarded over with cement board for 20+ years since his first tenants added the porch (east from the northeast corner) and rerouted the stairs that had just stuck straight out to the east, adding a lot of extra walking from the parking on the south. Now the bottom step is just behind the garage man door.

Joe : That link is fixed now ! I am getting help with linking URLs and got 5 in a row right and got cocky !

To heat he total sq ft you have, a 6'' system is more than adequate, however even with a good exterior vertical chimney to help your draft with a 6'' system
30 feet of horizontal run is all that we can comfortably tell you you are going to be able to count on ! However, that 30' loses 5 feet for every 90º Elbow in
your system, And -if we have to come up out of the ground with a 90º elbow, turn 90º to go out through a window and then turn up with a inspection / clean
out ''T", we have used up the better part of 15' and we have no pipe in the ground yet.

I am afraid that an 8'' system which would give you 50' of horizontal run - 5' for every elbow will barely get going before the heat energy coming off of the
barrel will be too much !

You can certainly see if you can find a source for stove pipes for a 7'' system, not everywhere stocks that size ! I do think before you continue to let us plan
your life for you, you should go to Rocketstoves.com to Download Your PDF Copy of the Brand New 3rd Edition of Rocket Mass Heaters.

I spent several years as a Fossil Fuel Fired Forced-air Furnace Technician, and I always regarded the use of a booster fan in the exhaust stove pipe as a
sign of someone who did not know his craft, and had a problem he could not figure out, in this case you may need a bypass damper and a inline exhaust fan

For the Good of the craft ! Big AL

allen lumley wrote:To heat he total sq ft you have, a 6'' system is more than adequate, however even with a good exterior vertical chimney to help your draft with a 6'' system 30 feet of horizontal run is all that we can comfortably tell you you are going to be able to count on ! However, that 30' loses 5 feet for every 90º Elbow in your system, And -if we have to come up out of the ground with a 90º elbow, turn 90º to go out through a window and then turn up with a inspection / clean out ''T", we have used up the better part of 15' and we have no pipe in the ground yet.

That's actually just about right to put the firebox and riser under the bedroom, then do a straight run across to the window, which might be just about right. I do have a gas heater in the living room that can supplement quite nicely, (though it does a better job of heating the loft if I forget to seal the access...might take a lead from the previous tenant and sleep in the loft for the winter) but I'm stuck with just an oil-filled electric in the bedroom, so heating that is the primary goal, with a warm workshop being a nice fringe benefit.

Of course, the amount of heat actually needed is a consideration too; last night's low was 27F, and we generally only get a few nights much below 20 in a given year, so it's not a case of trying to bring the place up from negative temps. I suspect 6" will do a pretty good job, at least keeping the electric from running very much. That's been killing my usual usage and tripling my daily electric cost since it cooled off enough to shut off the A/C for the winter.

Joe : Yes I know, our temps here yesterday near the border with Canada was high 50s and todays highs will be mid 40s with high winds as the stuff the midwest
is getting now arrives early tomorrow !

For the longest time, say the last 40 years, not much was happening to stir development of new ideas within the Rocket Stove and RocketMass Heaters RMHs
Communities.

The rocket stove people are trying to introduce the rocket stove to the40% of the 8 billion people on this planet who are still doing their cooking outside in a
pot over a 3-rock fire, and their results up 'til now has not been impressive !

The RMH community also has been setting on its Laurels for years, but lately there has been a gather of wizards and great things are being spoken of as tho
they were already common. Our goal right now is to help you build your first RMH, after that You will be one of the Permies Rocketeers that are telling newbies
exactly how you did it, and probably how you are going to build your next one !

For the good of the craft! Big AL

allen lumley wrote:The RMH community also has been setting on its Laurels for years, but lately there has been a gather of wizards and great things are being spoken of as tho
they were already common. Our goal right now is to help you build your first RMH, after that You will be one of the Permies Rocketeers that are telling newbies
exactly how you did it, and probably how you are going to build your next one !

Don't know that I'll have much call for another heater, but I am planning to build one of the portable Rocket Stoves for camping, maybe a permanent one at my "base camp" spot on the family land to brew up some hot tea while stargazing and cook when I decide to camp out there, (Guess I could go really fancy and make that one a 7x4' mass heater, so I could even have a warm camp bed.) and maybe a small one for the porch. Mesquite grilling goes best without a too-efficient burn, but a Rocket beside the grill would be a perfect way to sear the meat and/or cook up other stuff without having to run back and forth to the kitchen. Only issue there is that the porch deck is plywood; my little 14" Weber doesn't scorch it, but there would have to be a good bit of extra insulation under the firebox for me to feel safe about putting a Rocket there. I've been wanting to put some cement board on that end anyway, just to be safe.

Going to borrow a mattock from work tonight (another annoying loss from the divorce that I still haven't gotten around to replacing) and see how hard it will be to trench all this stuff in.

Just too darn cold to work in the garage right now, especially since I don't get home until after dark. Going to pick up either a kerosene or propane heater next week to warm it up a bit down there and be a supplemental heater once that's finished.

Still looking for a source for some used 6" stovepipe and a barrel that isn't plastic too.