Rocket stove driving underfloor heating?

OK, so the great and wonderful Burra is determined that the new place will have a rocket stove/mass heater, and that's fine with me because rocket stove is about efficient burning of fuel and as an engineer that appeals.

However, I also like the idea of underfloor heating.  Now, I did wonder about running the rocket flues under/through the floor, but that might be tricky to get right, and also it's kind of a one-trick pony.  

Now, I also note that rather than what was the case, heating a big lump of earth/rock up with your rocket, people are also heating what is essentially a box of air (or so it came over to me and I freely admit I've not studied it in detail).

So here's the thing.  From what I'm hearing, the rocket stove is the engine that you use to heat *something* as a way to store heat and release that heat steadily into your house.

This leads me to wonder if I can use it to heat a water tank and use that warm water to supply more-or-less conventional underfloor heating.  The advantage I see in this is that I could also heat that tank from a solar panel - I've seen an installation done like that; the underfloor doesn't need high temperatures like conventional central heating (I believe it can run from 30-40°C) so a modest sized solar panel will work in winter on a sunny day, for example.  On days when the sun don't shine, I could burn the rocket stove some.

Another point is that a quick-and-dirty google leads me to the specific heat capacity of water being about 4x better than air, so it should be space-efficient too which is relevant as the house isn't all that big.

The same system could also heat (part of) domestic HW too, I guess.  Kinda like when you have a boiler and solar water heating, like we had on the house in Wales, both heating the same domestic supply - and the boiler had a split circuit so you could run CH or HW or both.


So... anyone built anything like that?  Pitfalls?  Pluses? Stupid idea?  

Another point occurred to me - having built in an underfloor heating circuit and provided I can find a credible way to cool the water, it could make underfloor cooling in summer.

Austin Shackles wrote:Another point occurred to me - having built in an underfloor heating circuit and provided I can find a credible way to cool the water, it could make underfloor cooling in summer.

Depending on the size of your floor, you might not need too.

In my other house, I put 400 tons of rock under the concrete slab. I put insulation down so as to isolate it from the outside edges where it would get cold. But last year was the first year it sat vacant all winter. Without heat, the temperature inside the rather conventional home never dropped below 40 degrees. This was with no heat, and no one in the house producing any heat, and yet it was a steady 44-45 degrees even down to 22 below zero. I thought that was pretty darn good.

In the summer, the cool concrete, a steady 57 degrees 365 days a year, kept the house cool. We never had air conditioning in that home.

When I did have the radiant floor heat running in the winter, if the power went out I lost 1 degree in temp per day. By the time you even noticed a temp difference, the power was back on.

The only issue is, radiant floor heat does not take high heat; on average, because the slab is such a huge, all encompassing "radiator", the water running through the floor is only 76-100 degrees. But it takes a steady supply of hot water to maintain the slab at that temp. That is kind of where the not so steady fuel consumption of a RMH kind of falls short. But I have heard of people doing it.

I wish to encourage calling it a rocket mass heater instead of a rocket stove.   Granted, the core of the thing started with rocket stove stuff, but mixing the terms up has been fueling some crazy.   People say they are running a rocket stove in their living room and other people freak out with "THINK OF THE CHILDREN!"  - all because they think you are running a wood burning camp stove in your house.

However, I also like the idea of underfloor heating.  Now, I did wonder about running the rocket flues under/through the floor, but that might be tricky to get right, and also it's kind of a one-trick pony.

like this?

This leads me to wonder if I can use it to heat a water tank and use that warm water to supply more-or-less conventional underfloor heating.  

Any chance you have watched the boom squish dvd?

paul wheaton wrote:
Any chance you have watched the boom squish dvd?

What's the "Boom Squish" DVD? Is it one of the Rocket Mass Heater DVDs?

I’ve seen a few houses designed with water-based radiant floor heating that was hooked up to a wood boiler, so I don’t see why you couldn’t do this with a rocket mass heater instead. Hook the radiant tubing up to a manifold and you could switch whichever heating source you desire at any given time. If you place the rocket mass heater below floor level, it seems like you should get a natural water circulation as well.

I think the biggest problem you might have with a design like this is lag time. It’s going to take a long time between you starting a fire and the house warming up. In fact, it’s the reason I know that everyone who designed these systems with wood boilers ended up using a natural gas / electric heating mechanism in the end. The trouble is, I’m not sure how you would ever know what that lag time might look like until you build the whole system and use it for a long time.

My 2c... put in the radiant tubing (you can always power the heated water with something else later), try it out, and maybe build another more well-tested rocket-mass heater above floor level for reliable heat. After all, why build one RMH when you can build two? Plus it’d give you experience using a well-known design before playing with modifications.

PS — I’m not sure your cooling theory will work that well. One of the reason radiant floor heating works so well is that heat rises, so it moves from the floor to the air very naturally. When the air is hot, it doesn’t transfer to the floor very well, which means the cool water wouldn’t have much heat to absorb.

Well for one the house isn't that big, and there isn't really room for 2 RMH/S in the current plan.  I've been learning about the climate here this summer, and one thing that does keep the heat inside the house at bay is mopping the tile floor with water.  It only shaves a degree or two off the temperature, to be sure - but the difference between above and below 30°C is quite marked.  I'm going to design an airflow system which will take air in low on the north wall of the house and exhaust it into the roof-space, which isn't insulated or sealed.  Insulation will be between the roof space and the 1st floor.

And don't worry Paul, I'm well aware of closed systems that can go boom, I had a while firing steam locos on a preserved railway.  I'm not trying to make a pressure boiler here, any system heated by the RMH/S will be open and vented

As for heater vs stove, Burra wants a stove.  So that part is not real negotiable - and I like the idea; if you're burning the RMH anyway for heating, why not cook on the top of it at the same time?

Austin Shackles wrote:OK, so the great and wonderful Burra is determined that the new place will have a rocket stove/mass heater, and that's fine with me because rocket stove is about efficient burning of fuel and as an engineer that appeals.

This leads me to wonder if I can use it to heat a water tank and use that warm water to supply more-or-less conventional underfloor heating.
...

So... anyone built anything like that?  Pitfalls?  Pluses? Stupid idea?  

There are a few ways to do this right, and an infinite number of ways to do this wrong.  Hence the phrase Paul used, "Boom Squish"; which refers to the catastrophic form of failure that occurs when you heat pressurized water wrong.  The house goes "boom" immediately followed by the residents going "squish"  

That said; there are both some of Paul's videos, and threads on this forum, that attack this exact problem.  The easiest solution to the "boom squish" finale is to not use a pressurized system, and instead use a hot water pump loop with an atmospheric vent or 'standpipe'.  If you have a sight-glass or some other means of gauging the water level of your standpipe, you can have a manual valve provide water from whatever household source that you may have.  So long as steam pressure cannot be trapped on it's way towards that standpipe (and hopefully boiling water spray coming out of the top of that standpipe is not a safety hazard itself) a boiling temperature event simply becomes a loss of efficiency and not a catastrophe.

In short; look around these forums some more, invest into some of the DVDs that pertain to your goals, and forget about a pressurized water tank. (unless you are going to do the Rocket mass water heater trick with a pressurized water line that coils through an atmospheric water tank, working as a 2 stage heat exchanger and buffer.  That one was pretty slick)   This is dangerous business, proceed with caution.

Just for reference, this is the kind of 'stove' that I'm hankering after - Matt Walker's Tiny House Cook Stove and Heater

The beauty of radiant floor heat is that you can use any heater that heats water to power it, but some types of systems are going to be better than others.

I think the absolute best sustainable way to power a radiant floor heating system is compost heat. I say that because it simply mimics radiant floor heating requirements the best. It is low heat, but constant heat, and really all you have to do is transfer that warm water circulating through the pile, and circulate it through the floor of your building. To do that, it will take very little pumps and controls. Yes, any kind of heating system can power a radiant floor heating system, but to make it work convieniently, it will also take a lot more controls. It has to simply do with being able to over-heat the water, and zone vales is not a good way to control that.

Because compost heat is only 20-40 degrees warmer than what is required to heat the floor (assuming the floor has high mass like concrete, rammed earth, river rock, etc), all a person needs to control it is zone valves and themostats. You would need those with any other heating appliance, but would also need water injection cooling, or a metering valve to prevent over-heating, and a plc to inject the water at the right time, and right temperature. Heck, if you laid out the loops in the floor so that every loop was exactly the same length, you would not evn need flow control vales.

Again, it would be a very simple radiant floor heating system because really you are just transferring the heat from the composting pile, to your floor where you live.

While it is possible to use a rocket mass heater, that would be my last choice because of how it heats in the exact opposite way of how radiant floor heating works. That uses quick, high temperatures to heat up mass, with that mass heating the building over time. It can be done, but I can also walk from Maine to California, but it is not the most efficient way to make the trek. With a Rocket Mass Heater, it is burning quick, short bursts of high heat, to heat up a mass of water, that is then sent into the floor to heat up another mass, that then heats the home. It has a few other steps that makes it a rather inefficient way to heat a home.

I have nothing against rocket mass heaters, and know they are efficient in the right application, but I cannot see how they would be efficient in a radiant floor heating system. IF a rocket mass heater could be automated, my opinion would change. That is because it would then be identical to how my propane boiler works. It too produces intense heat in short bursts so it is efficient, but it can instantly be fired up to provide the btu's needed. It does that by cycling several times per hour, but hand-firing a Rocket Mass Heater that way is just not possible.

A little more info for you

First:  the house is about 6x7m, (something like 20x23ft) with ground floor which will mostly be open plan - there are going to be 3 small closed spaces, 2 for toilet and shower and a larder.  The larder obviously doesn't want heat.  There'll be a superladder (steep open stair) to the first floor, so heat from downstairs will rise naturally.  I plan to insulate the first floor ceiling.

Second:  we're not trying to cope with -40° winters.  In fact I doubt the weather will hit freezing that often.  I'm informed the climate is 9a, which I hope means something to y'all as it means diddly to me
The bigger issue is cooling in the summer but I have plans for that - but the ability to circulate cold water in the floor would probably help.

About cooling; what about sweating? I assume you won't like to walk on wet floor all summer long? You can't have carpets and there will be minimized air circulation- mold (unless you have trench in the middle of the room as mike eohler's underground greenhouse which obviously will remove cool air for the sake of circulation).
There should be great examples of cooling with cool water from persian (developed) and endulüs (perfected) architecture. Maybe they might inspire new designs?

Nicole Alderman wrote:

paul wheaton wrote:
Any chance you have watched the boom squish dvd?

What's the "Boom Squish" DVD? Is it one of the Rocket Mass Heater DVDs?

Yes.  DVD 3 of the first four dvd set

https://richsoil.com/wood-heat.jsp

It talks about using wood to heat water that is pressurized.  

Kyle Neath wrote:I’ve seen a few houses designed with water-based radiant floor heating that was hooked up to a wood boiler, so I don’t see why you couldn’t do this with a rocket mass heater instead.

Those wood boilers are outside.   I always thought it was because they were especially explode-y.  

A basic, simple rocket mass heater in the house is nice and simple and easy.  And not explode-y.   If you like, you can make the floor the mass.   But I think that it will still be easier and nicer to make a basic rocket mass heater.

Austin Shackles wrote:And don't worry Paul, I'm well aware of closed systems that can go boom, I had a while firing steam locos on a preserved railway.  I'm not trying to make a pressure boiler here, any system heated by the RMH/S will be open and vented

Good!

In that case, would you agree with me that most people tinkering with wood fire heating water in a pressurized system have a pretty good recipe to hurt themselves?

As for heater vs stove, Burra wants a stove.  So that part is not real negotiable - and I like the idea; if you're burning the RMH anyway for heating, why not cook on the top of it at the same time?

Sounds like a slam dunk to me.

I like pressurized systems because they are far more efficient. It is also easier to move heat because they are not subject to elevational changes (they do not have the negative effects of atmospheric pressure acting upon them).

They are not inherently unsafe however because every pressurized system has a 12 PSI relief valve installed, or should, just as every hot water heater has one installed, or should. The cost is about $15, and for extra safety, could, or should be vented outside, in case they do ever go into relief. The cost for that is about $12. In my other home, I had two relief valves in the system, vented to the outside just so there was no question, the system would never be overpresurized.

The potential to go off is real, but only with HAND FED solid fuel appliances (stoves) with manual draft control. It would be possible that a wood boiler gets stoked with wood, then the power goes out, and the circulators are not able to pull the water out of the boiler jacket, and it flashes to steam. In that case, the presure would build to only 12 psi, and the relief valve would go off, safely venting the hot water outside the home. When the power comes back on, any loss in water would be made up by the make-up-water valve.

This is a VERY unlikely situation though.

On a boiler with a power draft (as most are) because the aquastat on the boiler controls the draft fan motor. With the power off, the draft fan motor would not spin, and the fire would only smoulder, and through natural siphon circulation, enough heat would be drawn out of tthe boiler jacket. Circulators are NOT checkvalves, water flow pushed by siphon can flow past a circulator.

In a boiler that is automatically fed, the loss of electricty would stop the feed augers from working, and so the firebox would run out of fuel, and at the same time, the power draft would stop working too, and air to burn the fuel would stop, so ultimately the fire would die. A dead fire would never allow the water in the jacket to flash to steam.

IF a person was still scared that something might happen despite all these redundant safety factors in the system, all they would have to do is get a backup generator, or battery powered inverer set up. With the electricity always on even if the grid goes down, the circulators would continue to pull heat away from the water jacket.

Relief valves, water make-up valves, and aquastats are standard on every boiler system.

s. ayalp wrote:About cooling; what about sweating? I assume you won't like to walk on wet floor all summer long? You can't have carpets and there will be minimized air circulation- mold (unless you have trench in the middle of the room as mike eohler's underground greenhouse which obviously will remove cool air for the sake of circulation).
There should be great examples of cooling with cool water from persian (developed) and endulüs (perfected) architecture. Maybe they might inspire new designs?

That's a good point.  The cooling I have in mind might be supplied by evaporating water like a swamp cooler, and that will be limited.  I'm not trying to build air-con.  I also plan a positive ventilation system which will exhaust hot air from the top of the living space into the roof, and have an inlet low on the north side of the house.  That might be where the swamp cooler style cooling happens, might just leave the under-floor as heating only

Paul and Travis:  Yes, I get the point - I don't necessarily intend a pressurised system - see the other comment about the climate, I don't need an amazing heating system here.  
Further I second what Travis mentioned - gonna make this loud as it's vital for anyone contemplating this stuff:

IF YOU MAKE ANY PRESSURISED SYSTEM IT ABSOLUTELY NEEDS AT LEAST ONE RELIEF VALVE.

Two relief valves are better.  I spent some time a few years ago learning to fire preserved steam locos, and one of the key points is you have TWO relief valves, and when you do the periodic boiler test, the inspector man comes out and watches as you light the fire, being the boiler up to pressure and sees that both PRVs open when it hits full pressure.  This can't be said enough.

So rest assured Paul, I have no intention of blowing up Burra or myself

Austin.

Just my two cent.

Why bother?

You have 46 square meters. Let say you make a Nice cooking rocker with a bench.  Let's  4/5 meters square.

Anywhere in the room, you'll be in direct line of the radiant heat. Pretty much.

Remember that if you want underfloor heating, you will need to insulate underneath, and on the perimeter. That the walls will still act as heatsinks. Conducting the heat back down to the ground.

Another thing i didn't factor in,  the regulation of the heat.  You can't use the floor as a mass per se. As it will be too hot and uncomfortable.  So you will have to have storage somewhere else. Not really space efficient. Nor heat efficient. Since you'd already store heat, then move it again, to use it somewhere else. But in the space the radiant heater will be covering anyway.

Just food for thoughts.

Boom squish.  

I think pressure relief valves can fail.  

I think the DVD does an excellent job of talking about the warnings.  

First:  the house is about 6x7m, (something like 20x23ft) with ground floor which will mostly be open plan

Ah, I didn’t realize it’d be so small! In that case, I think something like the Walker Stove is going to work great. I wouldn’t bother with in-floor heating, maybe just get a little heat-powered air circulation fan or something. In-floor heating really shines when you have multiple rooms, large areas, or sophisticated automation (like pre-heating the bathroom tiles an hour before you wake up in the winter months).

As for the cooling — the reason the mopping works is the evaporative cooling effect from the water on the surface. You won’t get the same effect by running cold water in sealed tubing under the floor. You’ll just get a cool floor, the same as you would if you put tile or stone down in the shade (which won’t affect the ambient temperature in any meaningful way). It does feel nice on your feet though.

If you wanted to leverage the evaporative cooling, you could try running an open trough of water underneath the house (in a crawl space or basement) and install some vents in the floor. If you combine it with your attic fan, this would pull a pleasant stream of cool air through the house all day. We have quite a few old (1890-1910’s) buildings out here that were built on top of creeks for this purpose. One of my favorites even has a waterfall to encourage more evaporation (plus, let’s be honest — it’s rad to have a waterfall in your basement). I wouldn’t do that in a humid climate, but I’m assuming you’re in an arid climate similar to CA if you can mop floors in the summer just to cool down!

Satamax Antone wrote:
Why bother?

Maybe he likes warm feet in the winter?

I think this is a good question. If the goal is to have warm toesies, that's a bit different than wanting to use the floor as a mass, or even wanting efficient heating by having radiant heat coming up from the floor. These things can all go hand in hand, but they don't necessarily have to. So, what is the main goal/reason for wanting underfloor heating?

Kyle Neath wrote:
If you wanted to leverage the evaporative cooling, you could try running an open trough of water underneath the house (in a crawl space or basement) and install some vents in the floor. If you combine it with your attic fan, this would pull a pleasant stream of cool air through the house all day. We have quite a few old (1890-1910’s) buildings out here that were built on top of creeks for this purpose. One of my favorites even has a waterfall to encourage more evaporation (plus, let’s be honest — it’s rad to have a waterfall in your basement). I wouldn’t do that in a humid climate, but I’m assuming you’re in an arid climate similar to CA if you can mop floors in the summer just to cool down!

No basement or even crawlspace in this house.  It's a fairly modern house, brick build rather than stone, but in a fairly traditional Portuguese style with the living space upstairs and the downstairs has been used for storage and as a shed or workshop, with an earth floor.  As per the photo below.



The floor has never been levelled and needs digging out a bit anyway so it's a perfect opportunity for Austin to follow his long-held dream of underfloor heating if we can figure out how to make it work.  Safely.  

Nicole Alderman wrote:

Satamax Antone wrote:
Why bother?

Maybe he likes warm feet in the winter?

I think this is a good question. If the goal is to have warm toesies, that's a bit different than wanting to use the floor as a mass, or even wanting efficient heating by having radiant heat coming up from the floor. These things can all go hand in hand, but they don't necessarily have to. So, what is the main goal/reason for wanting underfloor heating?

There are a lot of reasons to go with radiant floor heat.

They are quiet. There is no noisy fans blowing hot air around when they cycle on and off. Just a few quiet circulating pumps that you can barely hear.

The heat is even. Your kids closet will be just as warm as your living room. In other words there are no cold or warm spots.

The heat is consistent. What you set your temp for, the house will stay within a degree of what you set your thermostat for.

It is extremely efficient. If you do it right, you will end up with a half heating system/half geothermal system that will really let you save money.

You will be comfortable. If you get down on the floor to play with your kids, your dog, or relax your bad back, you will be warm and comfortable.

It acts as a natural dryer. With the floor warmed, within an hour of stepping out of the shower, the bath mat will be dry. So will any spills you make, and if you have any delicate clothes that need to be dried, just set them on the floor. For my wife Katie, she dries her pantyhose and stuff in the boiler room where all the pipes come up through the floor.

Hard surfaces in your home will be warm, not cool, in the winter when you want them to be. Like your bathroom floor when you step out of the shower, but any stone, tile or even hardwood floors.

Adding on is easy. Just add another heating loop to the system. In my home, I even add radiant floor heat to an existing part of the building that had a concrete slab for a foundation. Any type of building can have radiant heat added.

They can use small heating units. Because you are heating mass slowly, at low temperature, they do not need high btu heating units to operate them. My boiler is smaller than a microwave, is wall hung saving space, and heats a 2500 square foot home, and can easily heat a 3 car garage if I had one.

The heat lasts. Because it is based on "low and slow", and uses mass, if the power goes out, you will still have heat. My home losses about 1 degree of temp inside the home per day every day the power is off. By the time I even start to feel a difference, the power is back on. Or you can get a back up generator, or battery bank inverter system too. My system uses (1) 15 amp circuit. Circulator pumps do not use much electricity.

You can use any type of water heating sytem to power them: solar, oil, propane, wood, compost heat, regular water heaters, etc. I could heat my home with propane, wood, or coal; all using the same radiant floor heating system.
,,,

The reasons for radiant floor heat are many, and after having radiant floor heat, and now living in a Tiny House that does not have one, I really miss it. So much so that eventually this Tiny House will have radiant floor heat. I will never live in a house again without it.

There is a wonderful bit in the medieveal farm series with Ruth Goodman where she shows the design of old dairies.  A north facing and an east facing window, unglazed tiles on the floor, a bucket of water spread over the floor and the temperature stays ver low regardless of the season.  I love Ruth.  Am thinking of a new forum - the Ruth Goodman Fan Club.....

Nicole Alderman wrote:

Satamax Antone wrote:
Why bother?

Maybe he likes warm feet in the winter?

Well that is certainly a point in its favour  Getting out of bed on a cold winter night and putting bare feet on a cold tiled floor is not high on my list of good life experiences!

As for the whole mass/heat storage thing - if you look back at the initial post my point was that the specific heat capacity of water is very high, better than air or rocks or soil or whatever else; so my idea is to use an insulated water tank as a heat store for the Walker stove/RMH.  While the floor itself will also store some heat, that's not the major part of it.  I'm in an excellent place to build this (see Burra's pic) as there IS no floor there right now.  I have to dig some of the earth floor out in order to place a concrete slab, and in that case it's pretty easy to insulate under the concrete before laying it.

I can also feed heat into that from a solar water panel, even in winter if it's a clear day - I've had solar water heat before and it's well suited to the slow, non-intense heat you want for under-floor.  

Satamax Antone wrote:Austin.

Just my two cent.

Why bother?

When you get older, and you begin to suffer from arthritis, rheumatism and old codgerness, not to mention a lifetimes of broken bones, keeping the lower part of your body warm is of utmost importance. No point breathing in nice warm air when your feet are freezing. Living in old farms, as both Burra and I do, those lower barns are particularly chilly ans stone floors really sap the warmth from your body in cooler weather. I suffer from sciatica and a cold floor can be crippling.No, extra socks do not help (just to save time).
The other advantage of UFH is no radiators. Yiu can put your furniture where you like! AND it is great for seed starting if you dont have a heated greenhouse AND for drying beans and seeds gently  in autumn.

Austin Shackles wrote:As for the whole mass/heat storage thing - if you look back at the initial post my point was that the specific heat capacity of water is very high, better than air or rocks or soil or whatever else; so my idea is to use an insulated water tank as a heat store for the Walker stove/RMH.

Heating with water is incredibly effecient because it is 600 times more dense than air. It is also easier to move around because it is more condenensed and there are more controls made for directing it to the right place, at the right time.

One mistake a lot of people make is in adding antifreeze. Not only is it toxic and expensive to buy, it automatically makes your system 10% less effecient.

Austin Shackles wrote:A little more info for you

First:  the house is about 6x7m, (something like 20x23ft) with ground floor which will mostly be open plan - there are going to be 3 small closed spaces, 2 for toilet and shower and a larder.  The larder obviously doesn't want heat.  There'll be a superladder (steep open stair) to the first floor, so heat from downstairs will rise naturally.  I plan to insulate the first floor ceiling.

Second:  we're not trying to cope with -40° winters.  In fact I doubt the weather will hit freezing that often.  I'm informed the climate is 9a, which I hope means something to y'all as it means diddly to me
The bigger issue is cooling in the summer but I have plans for that - but the ability to circulate cold water in the floor would probably help.

Trust me, Austin, cooling will not be a problem. Your place looks like ours and in the high 30s low 40s inside is fresh and cool as the stone walls absorb the heat and realease over night very very slowly. Our coldest night rarely go below - 2 or 3, and the ground never freezes. We are also 9a ish.

Mandy Launchbury-Rainey wrote:
Trust me, Austin, cooling will not be a problem. Your place looks like ours and in the high 30s low 40s inside is fresh and cool as the stone walls absorb the heat and realease over night very very slowly. Our coldest night rarely go below - 2 or 3, and the ground never freezes. We are also 9a ish.

Well, I'm basing on experience with the house we live in now, which IS stone.  Managing the heat is quite important and a week or more of high 30s °C causes the house to gradually get warmer than you really want especially overnight.  There are a few issues with this place, though, which I intend to address on the next place - most notably the ability to manage airflow better.  

Austin Shackles wrote:

Nicole Alderman wrote:

Satamax Antone wrote:
Why bother?

Maybe he likes warm feet in the winter?

Well that is certainly a point in its favour  Getting out of bed on a cold winter night and putting bare feet on a cold tiled floor is not high on my list of good life experiences!

As for the whole mass/heat storage thing - if you look back at the initial post my point was that the specific heat capacity of water is very high, better than air or rocks or soil or whatever else; so my idea is to use an insulated water tank as a heat store for the Walker stove/RMH.  While the floor itself will also store some heat, that's not the major part of it.  I'm in an excellent place to build this (see Burra's pic) as there IS no floor there right now.  I have to dig some of the earth floor out in order to place a concrete slab, and in that case it's pretty easy to insulate under the concrete before laying it.

I can also feed heat into that from a solar water panel, even in winter if it's a clear day - I've had solar water heat before and it's well suited to the slow, non-intense heat you want for under-floor.  

Well, if you really want it!

Remember that keeping water hot is complicated.  Somewhat.  And that it is prone to legionella.

Your slab should be in the 5cm range. i think, for a 200/250 mm rocket.  That's already a lot, 5.52 tons.  My 220 mm is at 4 tons, approximately, of real mass. https://permies.com/t/44806/Cobbling-workshop-heater

I don't know if you go thicker, if it won't be too much.  And i don't know what i would use bellow, besides polyurethane panels. Sand, or clay balls, or even perlite concrete i guess all would be a heatsink.   Remember that under those products, you need a waterproof tarp, so moisture doesn't go up inside the insulation, and change the insulation value.   As i said to Burra. Use a batch, with the oven. You won't be sorry.

I have tossed plenty of drawings of cooking stoves in sketchup here.

https://permies.com/t/38889/Firebrick-riser-Esteban

Travis Johnson wrote:

I think the absolute best sustainable way to power a radiant floor heating system is compost heat..

I was just reading through a study on this and was surprised to learn that you would get almost the same about of btu's from wood that is composted as you would burning it. The trick is to keep all the heat created from your compost insulated so it doesn't escape to the outside air.

If I had the space, I would build a large enough shed that was insulated all around, fill it with the biomass needed for a whole winter then add a heat exchanger inside up towards the top where all the heat would be. Circulate the water to the house from there.

It has been done and it apparently works well.

"why is it so cold in here?"  "Oh, the underfloor thing broke again.  I think we are finally going to give up and just do something else."

People love it.  Until it breaks.

And the thing about boom squish and pressure relief valves.   I don't know how many times I've heard "In my area, you have to test it at least once a year - because they get sticky and can fail."

I think somebody who is extremely savvy with pressurized hot water will be fine.   There are several possible issues that need to be considered - and they know those issues.   But, I strongly suggest that the novice steer clear.  

That same novice will have no problems build a regular j-tube style rocket mass heater.  

I lost count on how many times the original poster stated that he was NOT making a closed system.  And then even more telling him NOT to make a closed system.  If you don't trust a relief valve, then put in two of them.
The odds of a double failure at the very same time, is well.................. pretty darn hard to calculate.

By the way, every single water heating system, here in the states has a "relief" valve.  Yes, we are not heating water to steam levels.  But you think about it, you could get paranoid about "What if the gas valve never shut off and the heater continues to burn" or "what if the electric element never shut down and continued to heat" and so on.

Friends, while I feel it is grand to calculate what may work and what may not.... in this case when it would be so permanent it would seem prudent to go with a known working system and then NOT change a thing.

Some of the best guru's in the world concerning Rocket stoves/heaters have made little changes in attempt to make a better stove with  disappointing results.  I love the fact they tried, failed and then explain what went wrong.  Thus we all learn. Some of these things are simple items like "just a few feet to long of heat run"  or "one little restriction here"  or  "when warm it works well, when cold it don't work at all"   yet these very same folks, can undoubtedly state-  This system HERE, works well.  That my friends, if you don't have the resources to do it all over again, would seem the prudent way to go.

We all learn from others mistakes, I will give you that, but in this case, I would be most interested in learning from someone that has done exactly this method of heating, in this climate, with these resources to go forward with a permanent build.  I don't mean to throw water on the fire...but rather slow down some of the  "what if's" "maybe if you do that" and " I think that perhaps" ideas.

Mike Dinsmoor wrote:

Travis Johnson wrote:

I think the absolute best sustainable way to power a radiant floor heating system is compost heat..

I was just reading through a study on this and was surprised to learn that you would get almost the same about of btu's from wood that is composted as you would burning it. The trick is to keep all the heat created from your compost insulated so it doesn't escape to the outside air.

If I had the space, I would build a large enough shed that was insulated all around, fill it with the biomass needed for a whole winter then add a heat exchanger inside up towards the top where all the heat would be. Circulate the water to the house from there.

It has been done and it apparently works well.

I have read about this as well.

The system I heard about, and thought about replicating was; a guy took a shipping container and put it in the ground and buried it with soil with only the rear doors open to the air. He then took tubing and lined the top of the shipping container with it, closely spaced. Then he filled the shipping container with compost.

As the compost heated up, that heat rose, but was insulated by the soil surrounding the shipping container. Water circulated through the tubing, got warmed, and then helped heat the home.

The beauty of this was, the man never had to fuss with tubes running through his compost. It was overhead, and out of the way so when he loaded, or unloaded his compost, he could sit on his tractor and just drive. And so I deduced out of ever compost heat idea I have heard about (to date) that was the best idea I have heard of. It might be somewhat inefficient because tubes were not directly in the compost, but apparently it worked well enough to heat his home nonetheless, and was super easy. Considering a shipping container costs about $2200, that is far cheaper then buying a boiler. The tubing you would have to buy no matter what you had for a compost pile system, and being out of the compost pile, a person could probably get by with Pex instead of copper tubing. That would save some serious money.

I have not ruled this out in my situation, but the only reason I have not done this is because I have a lot of logging equipment. Again this is me, and my situation only, but I have a super-insulated home, so I only need a few cord of firewood. In less time than I could build a compost heating pile, I can cut enough firewood to go me all winter. But for people with very little woodlot, or no logging equipment, for sure compost heat is a very viable option!

Travis Johnson wrote:

Mike Dinsmoor wrote:

Travis Johnson wrote:

I think the absolute best sustainable way to power a radiant floor heating system is compost heat..

I was just reading through a study on this and was surprised to learn that you would get almost the same about of btu's from wood that is composted as you would burning it. The trick is to keep all the heat created from your compost insulated so it doesn't escape to the outside air.

If I had the space, I would build a large enough shed that was insulated all around, fill it with the biomass needed for a whole winter then add a heat exchanger inside up towards the top where all the heat would be. Circulate the water to the house from there.

It has been done and it apparently works well.

I have read about this as well.

The system I heard about, and thought about replicating was; a guy took a shipping container and put it in the ground and buried it with soil with only the rear doors open to the air. He then took tubing and lined the top of the shipping container with it, closely spaced. Then he filled the shipping container with compost.

As the compost heated up, that heat rose, but was insulated by the soil surrounding the shipping container. Water circulated through the tubing, got warmed, and then helped heat the home.

The beauty of this was, the man never had to fuss with tubes running through his compost. It was overhead, and out of the way so when he loaded, or unloaded his compost, he could sit on his tractor and just drive. And so I deduced out of ever compost heat idea I have heard about (to date) that was the best idea I have heard of. It might be somewhat inefficient because tubes were not directly in the compost, but apparently it worked well enough to heat his home nonetheless, and was super easy. Considering a shipping container costs about $2200, that is far cheaper then buying a boiler. The tubing you would have to buy no matter what you had for a compost pile system, and being out of the compost pile, a person could probably get by with Pex instead of copper tubing. That would save some serious money.

I have not ruled this out in my situation, but the only reason I have not done this is because I have a lot of logging equipment. Again this is me, and my situation only, but I have a super-insulated home, so I only need a few cord of firewood. In less time than I could build a compost heating pile, I can cut enough firewood to go me all winter. But for people with very little woodlot, or no logging equipment, for sure compost heat is a very viable option!

https://permaculturenews.org/2011/12/15/the-jean-pain-way/

I didn't see anyone addressing the fact that he said the house is in zone 9a, which should only need minimal heat.  It should have far more of the year in need of cooling, in fact.  It's fine to play with something labor-intensive and failure-prone like under-floor heating if you really want to mess with it 'just because,' but in your climate, honestly, I wouldn't.  

If I was building a house in that climate, and planning to use my heat source for cooking (presumably year-round), I think I would build a sun room -- a screened porch in hot weather, but close in the screened windows with glass for the cooler parts of the year.  Attach that to the house, and put the heater/cook stove in it.  With such a small house, all you would need to do to heat the house is open the door between the two spaces when you need heat, but in summer when you absolutely don't need extra heat in the house, close the door and keep the cooking heat out in the screened porch.  In fact, unless there's a lot of cloudy weather during the coldest parts of the year, the sun room would probably provide most if not all of the heat necessary for such a small house.

Kathleen Sanderson wrote:I didn't see anyone addressing the fact that he said the house is in zone 9a, which should only need minimal heat.  It should have far more of the year in need of cooling, in fact.  It's fine to play with something labor-intensive and failure-prone like under-floor heating if you really want to mess with it 'just because,' but in your climate, honestly, I wouldn't.  

If I was building a house in that climate, and planning to use my heat source for cooking (presumably year-round), I think I would build a sun room -- a screened porch in hot weather, but close in the screened windows with glass for the cooler parts of the year.  Attach that to the house, and put the heater/cook stove in it.  With such a small house, all you would need to do to heat the house is open the door between the two spaces when you need heat, but in summer when you absolutely don't need extra heat in the house, close the door and keep the cooking heat out in the screened porch.  In fact, unless there's a lot of cloudy weather during the coldest parts of the year, the sun room would probably provide most if not all of the heat necessary for such a small house.

I've been living in this area for 15 years already, and the climate isn't just 9a, it's also mediterranean, which means 'cool, wet winters'.  We generally get one or two six-week sessions of solid rain at some point during the winter, where we will absolutely need heat both to keep us warm and the building dry.  I've been dreaming of a rocket mass heater for so long simply because it's a perfect solution for this climate.  It's likely that a single burn of around an hour a day during wet or frosty weather will be all we need so long as there is some way to store the heat and give it out gradually over the next 24 hours. And as Austin has been dreaming of under-floor heating for years and years, we'd really like to try to combine our dreams into one functional reality.  

I totally agree about cooling in the summer, though I've never had air-conditioning and am fairly well adapted by now and have got pretty good at managing air flow to keep the temperature indoors relatively stable.  We will have gas cooking stove, designed to be swapped around so that during the heat of summer it will be in a screened porch, as you suggested, but inside the main house during the cooler months.  During the wettest, coldest periods we'll cook on or in the rocket mass heater as much as possible.  

we have a ground-source heat-pump which heats our hydronic floors from a well in our in-town Missoula well. In-floor heat is very comfortable. In the summer heat we circulate well temperature water through the floors to keep the space comfortable, which is very important if the Missoula air quality gets bad from forest fires. Closing the windows and running 55 degree water through the floors is an effective way to hold off those awful smokey periods.

ALSO: what is not talked about with Rocket Mass Heaters/Stoves is the necessity to be there at all times. There is no way to take off for a few days, or else everything freezes. I have the mind-set of embracing technology and install solar panels that power the electrically sourced hot water.