RMH - In Floor Heat

I've been looking around for information on RM Heaters that would feed in-floor heat directly. The idea here is a bit different then using water in tubes. Watching many RMH vids and also the idea of a Bell style heater on a RMH stove gave me the idea of creating a bell in the floor of a cabin or house to provide in floor heat.

In my thinking the design would use the floor as the mass. The house structure would have to incorporate a shallow metal box sufficiently supported to be the floor and support 4"-5" of mass as the heat sink. This heat sink could be a concrete slab finished with floor tile inside. The metal box would be a 12' X 20' X 3' BELL, which would be most of the floor in the house. The down feed from the RMH would be ducted (insulated ducting) to the center of the floor box bell to heat the mass, which is also the floor inside the house. Only the sides and maybe the bottom of the metal box would be insulated with standard insulation or air crete (foam and concrete mix) to hold the heat so it would warm up the floor. Exhaust the cooler air at the bottom of the box. The RMH would be built with a damper to this "under floor" space so it could be sealed off when the burn is finished. Thus preventing back draft of the hot air back into the home. The design could also incorporate a below the bell air trap to help prevent hot air from back feeding into the home.

Anyone tried this yet? My idea was inspired by the Walker Stove Bell Bench design.

You do not want the concrete slab to be huge, cause it could crack, unless mesh, rebar, etc.  You also have many turns for the flow.

Are you wishing to implement a Roman hypocaust ? https://en.wikipedia.org/wiki/Hypocaust

To build a metal box capable of supporting a floor would be ridiculously expensive.  Why not just insulate under a concrete slab and run flue pipes through the slab?

To Michael:
The metal box would be supported under the top with metal beams, small open spans in the middle of the beams supported across by posts all under and inside the box. The box would be welded on top of and around the beams. The thin cement floor on top (4"?) would be rebar reinforced. Air would be able to move freely throughout the entire upper part of the box and around the beams inside.

To Graham:
Looking at pics of the Roman Hypocaust - yes similar but using 1/8" metal for the box and sections of beams to span across the posts and support the inside of the box at the top.

To Eric:
A concrete slab with ducting large enough to provide flow might work, but thinking it would be pretty thick. And floor would have hot spots where the ducting is thinnest at the rounded top. This would probably be the easiest to do. Maybe use square ducting, steel reinforced cement between to act like beams and then flooring across the beam structure. Square beams would act like long mini bells within the floor and high density foam under the whole thing for insulation. Likely be limited to 6" square ducting and not too wide so flooring doesn't buckle or collapse. Maybe the "Bell Structure" could just be a layer of cinder blocks sitting on their side to provide ducting but thinking this would also have a hot spot where the RMH exhaust enters.

Your first diagram doesn't show a chimney exiting well above the combustion chamber.  How does the thermosiphon work without one?

So, lots of ancient precedents here.

Hi Kelly

I think your idea should work, but I would put a 'proper' insulated chimney on the exhaust side to ensure you have enough draw for the whole system.

I would also include an area at the base of the chimney exhaust flue where you could light a priming fire to get everything pulling in the right direction, especially when the system hadn't been used for  a while.

Thanks for the replies and feedback.

Graham - regarding the chimney - the drawing was a quick sketch not to scale and just to relay the concept. The outer chimney will be above the stove. I was also trying to show some loops in the design to create "cold traps" so air would tend not to backflow through the system when cold. Also looked at another interesting design called the Ondol - Korean floor heating. Lots of stuff that our cultures have left to the past because of fossil fuel and electric use.


John - Yes, I will incorporate a small burn chamber in the chimney to start the draw, similar to the burn box in a Russian masonry heater. Or might even use a small air pump just to get things moving, it will be quite cool on the chimney side. It would follow the principle of "objects in motion tend to stay in motion". As soon as the entire system is "flowing" in the right direction (air in the system is in motion) the fire can be started. Then the rocket effect takes over and keeps things moving in the right direction, ie out of the house!!

I have been thinking about ways to move heat around to other parts of the house. I use to not like the idea of using water and thought about running air ducts to the far areas from the heater. From all the research I have done, I now believe water is one of the best options. If I was doing a new construction, I would insulated the bottom and sides of the cement pad and bury PEX inside it. Water is such a good carrier of heat and takes a very small amount of energy to move around. It uses less energy then moving air around.

The other factor to consider is Hydronic heating is code and proven if that matters to your area.

One concern with running a heater under a floor would be long term viability on the cement from the heat separating out the chemically bonded water. Also, there would be the clean out maintenance. It all just seems overly complicated for something that could be done with just moving some water around in some tubing.

Still, it is a very interesting idea. I can see the benefit for a primitive era.

Wikipedia says Frank Lloyd Wright invented radiant floor heating after experiencing a Korean ondol built in Japan.
The Koreans themselves abandoned the ondol as after the Korean War they lost access to firewood and switched to coal briquettes causing many deaths from carbon monoxide poisoning.
Carbon monoxide poisoning is still a preferred method of suicide in Korea.
Water is so much safer and more efficient.  Air to mass heat transfer is very inefficient.

I don't know if this helps but I was in XinJiang in August and visited a greenhouse.

There appeared to be a whole brick wall which was holding the exhaust of a wood stove.  This wall separated the cooking area from the single bedroom.
I could be wrong but that's what it looked like.

With hydronic heat, the pex is the cheapest part of the system, and the next cheapest part is the boiler. It is the controls that get expensive, but that is the heart of the system and where the efficiency comes from too.

By itself, water is 600 times more efficient then air, yet controls are not 600 times more expensive, so from an investment standpoint, boilers and hydronic heat are the most efficient to implement.

The one thing not cited here is that concrete can only tolerate so much heat before it starts breaking down chemiocally, that is why the water temp in radiant floor heat is pretty low. Yes the "radiator" is the entire concrete slab of the house, but enough pex must be installed so that lower temperature heat is used throughout the entire slab. Even with rebar, a suspended concrete slab that is overheated will crumble like glouten-free banana bread.

With a metering valve (plc controlled circulating pump), a rocket mass heater would be ideally suited for radiant floor heat because only warm water is required to heat the home (100-150 degree water), BUT it takes a lot of controls to get that warm water, exactly where it needs to be, to do the most good.

But if people have a dislike of concrete, just use sand. That is what I used on part of my house.

Graham,

I've thought about doing a mass wall or pony wall shared with multiple rooms. That could also be a masonry wall built as a bell. This might be an easier solution just considering the maintenance of an under floor bell. The bell could also be dangerous as the bell itself would be a potential risk for asphyxiation!! Very little or no oxygen would be down there. Any maintenance plan would involve venting the bell before doing anything down there.

The idea of a bell is just a mass heat sink with an inner open cavity for the air to stratify within. Hot air rises to the top and heats the mass, cold air drops to the bottom where you would exhaust out of the bell.

In the more common series of flue pipes in a traditional RMH setup the stove is always pushing all the air out and hoping most of the heat is absorbed by the mass before the air exits. The exhaust temp would be an indicator that the mass is "heated" and also indicate how much heat is being lost to atmosphere. The mass is hottest near the stove and cooler as the air moves along the duct. In a bell only the coolest air is exhausted out of the system - at the bottom -  and when the fire is out the hot air stays in the bell until the whole system cools. This should make it more efficient. In addition, if the hot air can be introduced into the bell in a distributed manner, rather then one large dump of air in one spot (like RMH's), the top of the bell would tend to heat evenly instead of having hot spots. If the top of the bell is your floor it should be an evenly heated surface and comfortable if you don't fire the stove too long. Would have to experiment with firing time to find the sweet spot for the mass that is incorporated.

The idea of pex in the mass to store and then distribute heat is an excellent idea, would also have to consider the cost of filling the system with antifreeze and keep it filled so air in pipes is not an issue. And I was looking at solutions without too many controls aside from the burn in the stove. The cabin this will be in is an open concept design so am hoping heat distribution won't be an issue. Especially if the bell spans across the few walls that will be in the structure.

I'd guess that the main risk is from carbon monoxide poisoning since you can still survive down to 6% oxygen levels but not below this. But your chimney should be removing these gases unless the fire is out and you've closed the vents down. But at the end of the fire your efficiency is much less so oxygen consumption will be much lower. Anyway，what's needed is carbon monoxide meters.

kelly myer wrote:The idea of pex in the mass to store and then distribute heat is an excellent idea, would also have to consider the cost of filling the system with antifreeze and keep it filled so air in pipes is not an issue. And I was looking at solutions without too many controls aside from the burn in the stove. The cabin this will be in is an open concept design so am hoping heat distribution won't be an issue. Especially if the bell spans across the few walls that will be in the structure.

No, antifreeze is seldom used in radiant floor heating applications. Aside from the added cost, it makes the system 10% less efficient.

Really it is not needed anyway. Once the floor is warmed up, the temperature drop averages only 1 degree per day on an insulated home. Most of the time failure comes from a grid-based power failure, but even last year when I went without electricity for 9 days, I was never in trouble of freezing my radiant floor heating pipes, nor the domestic water lines running through my floor. Even then a back up generator is not hard to obtain.

Air would not be a problem either because the manifolds have automatic air bleeders on them, as does the high point vent, as well as the vent on your your make-up water.

If you really wanted to go cheap, using quarter turn ball valves for flow controls, making your own manifolds, and dispensing with the back up zone valves, and zone valve relay (which are not really needed anyway) would save you quite a bit of money.

While your idea is indeed novel, it is also unproven and seems very expensive compared to building a traditional rocket heater as the boiler part of the heating system, and including a plc controlled metering valve to keep your home nicely heated.

Travis Johnson wrote:With hydronic heat, the pex is the cheapest part of the system, and the next cheapest part is the boiler. It is the controls that get expensive, but that is the heart of the system and where the efficiency comes from too.

By itself, water is 600 times more efficient then air, yet controls are not 600 times more expensive, so from an investment standpoint, boilers and hydronic heat are the most efficient to implement.

The one thing not cited here is that concrete can only tolerate so much heat before it starts breaking down chemiocally, that is why the water temp in radiant floor heat is pretty low. Yes the "radiator" is the entire concrete slab of the house, but enough pex must be installed so that lower temperature heat is used throughout the entire slab. Even with rebar, a suspended concrete slab that is overheated will crumble like glouten-free banana bread.

With a metering valve (plc controlled circulating pump), a rocket mass heater would be ideally suited for radiant floor heat because only warm water is required to heat the home (100-150 degree water), BUT it takes a lot of controls to get that warm water, exactly where it needs to be, to do the most good.

But if people have a dislike of concrete, just use sand. That is what I used on part of my house.

Reviving an old post!

Just spent a day researching my heating vs budget came across Ondal heating, searching for likely other names for the same thing j came across the Roman's Hypocaust,  then Japanese and Korean versions... FOUND THIS THREAD Haha great minds and all that! 😆😆😆😆

Anyway, would you kind kindly elaborating on how you have used sand?

I'm in the uk living in a small Tipi 3mx4m single skinned and cold in the morning  after the fires gone out!

My restraints are no permanent structure (poured concrete base) Minimal digging

So the current plan is below



                    /\
                  /    \
                /        \
              /            \
Thin layer of fiber insulator (to level the floor out and make it a bit gentler on the knees)
Sand to act as a levelled
Cinder blocks {}{} and rocket stove at same height
Insulation of some kind
Pallets

Then I thought  about cinder block,

{}{} represents one two holes cinder block

           [Inlet]
         /          \
     /                  \
  /                        \
{}{} {}{} {}{} {}{} {}{} {}{}
{}{} {}{} {}{} {}{} {}{} {}{}
{}{} {}{} {}{} {}{} {}{} {}{}
{}{} {}{} {}{} {}{} {}{} {}{}
{}{} {}{} {}{} {}{} {}{} {}{}
{}{} {}{} {}{} {}{} {}{} {}{}
{}{} {}{} {}{} {}{} {}{} {}{}
\                                  /
    \                         /
        \                 /
              (Flue)

Thoughts or ideas or criticism?

I have looked at a pebble style mass box to put the tipi on top of,but flue pipe and ducting pipe is well beyond my budget and sourcing any smooth bore is seemingly futile.

Kelly,

I don’t know why I did not see this particular thread when it first started, but personally I think the idea of having thermal mass in your floors is wonderful.  Heating air does just that—it heats up air that rushes out every time you open the door, and seeps out through every little crack and seam in your house.  But heating the mass inside the house really heats and stores the heat and releases it slowly.

An RMH is a great way to heat a house, but it is not for everyone.  I think heating a floor is a great alternative, especially as you plan to put in some heat storing mass in the floor.  Further, I love the idea of a thermal wall, especially a shared wall.

Even better, but far more complicated, in my opinion would be to somehow make that mass water.  Water has such a great specific heat capacity that it stores heat extremely well and releases it slowly and gently.  Now I fully appreciate the fact that water+houses=bad things happening if the water is not 100% contained.  I also appreciate that not everyone will like the idea of even more opportunities for water leaks.  And if you don’t like the idea of using water as a heat storing medium, I completely support your position.  But just for a moment consider that plain old water had one of the highest specific heat capacities of any known substance.  Even better/crazier would be to use pure ammonia as it has a better specific heat capacity than water (if memory serves, the space shuttle used ammonia as a coolant because it carried a tremendous amount of heat and would not freeze).

I just checked and most stone/masonry materials have a specific heat capacity of around 0.8 J/gk.  Water has a specific heat capacity of just under 4.2 J/gk.  Ammonia ranks at 4.7 J/gk.  If one was absolutely nuts and really wanted to push the the absolute limits of specific heat capacities, hydrogen has perhaps the very best specific heat capacity at a little over 14 J/gk.  But of course hydrogen is out for the question for all sorts of reasons.  The fire hazard is obvious.  It has next to no mass.  It does not like to stay in place, as atoms of hydrogen will slip right through the most dense materials.  And the list goes on and on.  And for the record, I would be very hesitant about ammonia for several reasons—it’s expensive, highly noxious if released, and only slightly better than water.  

All of this may just be me pipe dreaming, but back to your original post, I think that using a thermal mass as a functional component of your home makes a lot sense!!

Eric