Support for Masonry heater over unconditioned crawlspace?

So, I am really liking the idea of having so much thermal mass that a house stays warm all day from a single burn. However, the cabin in which I would like to build the rocket mass heater or masonry heater is just a wood joist-and-beam floor.

one can certainly sister joists/beams in order to get more strength. however, one thing I'm wondering is: could someone build the masonry heater all the way through the floor and down into the earth, and insulate around the whole things while it is below the conditioned space?

or could there be another method? has anyone dealt with this?

one other thing I'm considering: a 55gal drum of water for thermal storage (not sealed, obviously). a full drum will be around 500lbs, but it will store the same energy as 2500lbs of brick (science rules).

thanks

Brandon Hands wrote:I'm wondering is: could someone build the masonry heater all the way through the floor and down into the earth, and insulate around the whole things while it is below the conditioned space?

Yes, you can. Pour some 8" thick slab with proper reinforcement first. Then you can use CMU blocks or bricks that would be topped with perlite cement for insulation at the floor level and then you can continue with your build.

Yes, absolutely, as Cristobal described.
Cut through the floor and pour a large pad as your base.
You can also use pier blocks with steel beams supporting the floor joists.

Brandon Hands wrote:

one other thing I'm considering: a 55gal drum of water for thermal storage (not sealed, obviously). a full drum will be around 500lbs, but it will store the same energy as 2500lbs of brick (science rules).

thanks

I would be very much interested in your "science rules" where your calculations of 500 lbs of water, will hold as much heat energy ( BTU's ) as 2500 lbs of brick.

If 1 pound of water is raised 1 degree to equal 1 BTU  and you have a range of 80 to 200 degrees for the water?  ( 120 degrees of range in temp) so-120 x 500 for storage calculation of 60,000

And 2500 pounds  of the same 80 degrees to 200  would give a mass weight  (120 x 2500) for storage calculation of 300,000 Plus adding the fact that you could take the brick to 300-400 degrees safely and the water remains at 200 (safely)

I don't mind being corrected so please do tell,  I know there efficiency factors that could get involved, but...............just trying to keep this on a apples to apples sort of math.

Hey Brandon;
Scott is correct, that volume of water is not comparable to 2500# of masonry.
If you want even more storage, build a double skin masonry bell.
You will have 5000-6000# of mass, but your floor needs to be stout.

In Russia, many cabins had masonry stoves installed on post foundations.  At least some of these foundations failed, eventually, which often lead to failure of the stoves, as well.  However, these masonry stoves also required regular maintenance, and sometimes a total rebuild, anyway.  They were usually constructed of low fired country made brick (on site, or near to it), so no fire brick.  Even with due care, durability was finite, because of the soft, low strength brick.

I'll try to find a good YT video showing this foundation arrangement.  There is a nice series of a teardown/post-mortem on an old Russian stove, if I can find it again, with good details (and auto-generated closed captions which were at least passably understandable in English).  I'd suspect that cedar or tamarack/larch would have been good choices for such purposes, but no doubt many things were pressed into service.

I'm not suggesting that you use posts - the other recommendations are better from a structural and durability standpoint - I am simply pointing out that there was an historical approach to this same problem.  With more durable stove materials, a more durable foundation is imperative.

When I install a thermal mass heater in our house, I'll need to build a masonry plinth from the basement floor to the ground floor level in the middle of the house to support it, even though I'll be using a pre-fab double-wall metallic chimney (in a chase) for the stack.

On edit: here's the first video (in a series of 5), covering the investigation of an old Russian stove

This one shows the post foundation for the stove (and that, in this case, the sill logs of the walls rotted first).  But, if you are interested in old masonry stove construction, this is a good series to see how they used to be built (or at least, one version).

Scott Weinberg wrote:

I would be very much interested in your "science rules" where your calculations of 500 lbs of water, will hold as much heat energy ( BTU's ) as 2500 lbs of brick.

If 1 pound of water is raised 1 degree to equal 1 BTU  and you have a range of 80 to 200 degrees for the water?  ( 120 degrees of range in temp) so-120 x 500 for storage calculation of 60,000

And 2500 pounds  of the same 80 degrees to 200  would give a mass weight  (120 x 2500) for storage calculation of 300,000 Plus adding the fact that you could take the brick to 300-400 degrees safely and the water remains at 200 (safely)

I don't mind being corrected so please do tell,  I know there efficiency factors that could get involved, but...............just trying to keep this on a apples to apples sort of math.

The formula for heat energy is Q = mcΔT. m=mass, c=specific heat, and ΔT being change in temperature. Water's specific heat is c = 4.184 J/g°C. Brick (and cob) is around c = 0.840 J/g°C. So the same mass and same rise in temp results in about 5x more energy stored in the water.


The downside of water as thermal storage is, as you point out, that you can't store it above 100c without incredible risk; so you obviously need some brick/cob for the parts that could get very hot, but many masonry heaters have portions where they are well below 100c even when run with their full load of wood. Another advantage of earthen materials is that they give off their heat slowly whereas a steel drum full of water set in the middle of the room would lose its heat much faster, making it less ideal for storage, but that can be adjusted by insulating it or wrapping it in earthen material.

So you're right that 1lb of water 1 degree F is 1 BTU, but raising 1lb of brick or cob by 1 degree F is only about 0.2 BTU


Does that make sense?

Water has an abnormally high specific heat, I believe because of the dipole nature of the molecule. It does not just bounce around, but it also spins with an off center charge like a little flywheel

Material tables list bricks at around 840 J/(kg*C). Water is 4181, so almost exactly 5 times more. Clay/cob will be similar to fired brick.

If you want even more bang per kg, consider a phase change substance like wax for the thermal mass.

Phil Stevens wrote:If you want even more bang per kg, consider a phase change substance like wax for the thermal mass.

Haha, I've considered it, but the only one that is somewhat economical is, like you say, wax. If I accidently overheat the water, it will just boil in the barrel (not sealed). The wax on the other hand, becomes a very large molatov cocktail 🧨.

Also the per kg energy from transforming the wax is around that of raising water temp by ~100f. But wax has half the specific heat, so by the time you get the wax to its max safe temp (~180f) and the water to its max safe temp (~190f), you're only getting 40%-45% more energy storage from the paraffin. I'd rather add another half barrel of water and avoid the risk of fire.

Water is basically the best thermal storage up to its boiling point in terms of cost, safety, weight, simplicity, etc. The only downside is the upper limit in temp, so you still need it sonme distance through the brick/cob to moderate it. But a lot of Nordic style masonry heaters heat parts to moderate temps. Some are designed to sleep on top

Phase change materials really make more sense away from the primary heat source, and not just for this reason. If you consider the masonry and/or water your main thermal mass, then the wax could be decoupled from that and be installed somewhere else in the living space. Ideally, you'd choose a type that has a melting point around the comfort level that you're aiming for and not have a cauldron of 200-degree hydrocarbons sitting around waiting to go boom.

What about Glauber's salts (sodium sulfate decahydrate) as a phase change material?  Maybe the economics just aren't there for this use case...

Kevin Olson wrote:What about Glauber's salts (sodium sulfate decahydrate) as a phase change material?  Maybe the economics just aren't there for this use case...

it's also around 1/4th the specific heat of water, so once it's melted, from there until max temp water will be storing 4x more. so by the time you get close to water boiling point, it will likely be well ahead again. I don't even want to look at the cost of a 55gal drum full of the stuff, haha.

Phil Stevens wrote:Phase change materials really make more sense away from the primary heat source, and not just for this reason. If you consider the masonry and/or water your main thermal mass, then the wax could be decoupled from that and be installed somewhere else in the living space. Ideally, you'd choose a type that has a melting point around the comfort level that you're aiming for and not have a cauldron of 200-degree hydrocarbons sitting around waiting to go boom.

yeah, you could get one of those hot water bottles that people sleep with and fill it with wax instead.

Cristobal Cristo wrote:Material tables list bricks at around 840 J/(kg*C). Water is 4181, so almost exactly 5 times more. Clay/cob will be similar to fired brick.

As I stated, I don't mind being corrected, and I will have to look into this further, So these are good points to investigate.Nearly all water vessels can be made safe, so more investigation for sure, thanks all.  I guess, I will have to take 8 lb of water and 8 lb of granite/brick and heat both to 190 To show myself "the test"

Scott Weinberg wrote:

Cristobal Cristo wrote:Material tables list bricks at around 840 J/(kg*C). Water is 4181, so almost exactly 5 times more. Clay/cob will be similar to fired brick.

As I stated, I don't mind being corrected, and I will have to look into this further, So these are good points to investigate.Nearly all water vessels can be made safe, so more investigation for sure, thanks all.  I guess, I will have to take 8 lb of water and 8 lb of granite/brick and heat both to 190 To show myself "the test"

The two will give off their heat at different rates, so you have to test carefully in order to control that. One good way way to test it would be to put both inside a well insulated container with an electric heater and thermometer. Using a watt meter on the heater, you can multiply the power by the time it takes for each to reach 190F. Watts times time is energy.

You'd have to make sure the brick/stone is fully heated to the core, so prob need the thermometer probe drilled into the center. Or maybe using sand mixed with 10% mineral oil, that way you can stick the probe down into the sand and you don't have to worry about thermal conductivity causing error. The error that could come from solid stone/brick is that it will take a while to get the center hot, but the whole time you will be losing heat through the insulation, making it appear to take more energy. The mineral oil will increase the sand's average specific heat a bit, but only by about 10%, which you can subtract out at the end.

Something like that. Maybe I will perform this test myself; it sounds fun.