Integrating two heating systems

Can you use a rocket mass heater/stove to provide the heat for an in floor heating system?
Historically the Chinese and Koreans used an in floor heating system using a fire outside the home that heated flues under the home.
I’m speaking more to either making the rocket mass heater also heat water that flows under the flooring or…..something.
It may be completely inefficient or just silly but I would like to know.

Hi Anna
It could be done, but there are inherent dangers with heating water with an RMH.

Take a look at this thread of my  current build  https://permies.com/t/360535/Build-Generation-Batchbox-Double-Brick#3499176
Note the gap between the brick walls.
On my build, that gap is apx. a 1/4" , that gap can be enlarged to fit water pipes into.
The inner brick skin gets progressively hotter the higher up you go.
I'm sure there is a safe level where your water pipes would not get heated to the point of turning to steam.
Steam is a killer; first, your pipe collapses, and then it bursts, spraying scalding hot water. (Known as Boom Squish)
Yes, an inline pressure relief valve can stop that.
Finding the safe spot inside a double bell would be the problem.

Hi Anna,
Could I ask what prompted this question? What made you want to combine these two systems?

I think it could be done... but it would be very complicated to do safely. I have seen some people create RMH's that were lowered... so the bench/bell that most people have, was at floor level... essentially creating a heated floor.

I wonder if a different technology might be able to provide the hot water. There are some really good solar hot water heaters.

I think it is difficult to do as a passive system (so completely fail safe) but have seen (online) an acquaintance wrap pipes around his barrel and then to a manifold which distributes heat round his floor...I'll see if I can find the video....

As I said - not simple!

If it were me, I would build the rocket heater to heat a large volume of water with an open(ish) top. Something along the lines of a 200 gallon box with a large opening in the top so that there is no way to build pressure. Then I would run a separate coil inside the vat of water to heat the flooring. There would need to be fail safes of course, just like any other system, but this could mostly eliminate a lot of the worry. Only downside is the need to add water as you cook some off.

 Using water to move heat from one room to another, or from outside to inside, seems like good fit with a rocket heater, but a "conventional" rocket mass heater design is meant to comfortably heat at least the room  it's located in.

If I were to add hydronics to a rocket mass heater it would be in the form of heating a tank of water.
Copper coils can be expensive.
Derelict water heaters are cheap, and a liquid thermal mass makes distribution of heat more flexible.
In a conventional hydronic system there is sometimes a moderately sized buffer tank used to store heated water.
I would use multiple smaller tanks, chained together, to keep down costs.


It's important to not locate that mass anywhere in the flame path that will negatively affect combustion.
In a batch box that can be anywhere after the riser, but it could also be the ceiling of the burn chamber.
I conclude this because some builds use un-insulated  glass or iron/steel as the ceilings of their burn chambers, and even boil water on that surface.
Iron/steel normally decays at the expected temperatures, but close proximity to a mass of water should keep the temperature of the metal much lower.
I've used this water cooling effect to prevent back smoking on a J tube rocket, with mixed success.

Can you use a rocket mass heater/stove to provide the heat for an in floor heating system?
Historically the Chinese and Koreans used an in floor heating system using a fire outside the home that heated flues under the home.

The systems you mention here are broadly following the lines of a roman hypocaust. Essentially a hollow floor where the gas stream to the chimney flows, (or chimneys, like hypocaust) through channels or under the entire floor. All systems like that has a raised floor.

Streching this idea a bit, you would need a pit, cellar of basement where the combustion core is located, feeding the hollow floor. This could be done, with the use of a reversed Shorty core. This is a latest-generation batchrocket core sporting a very low riser that won't exit at the top but at the front or back instead.
Have a look at the workshop report from this spring in France. The core that was used there happened to be a reversed sidewinder, and it worked flawlessly.

thomas rubino wrote:Hi Anna
It could be done, but there are inherent dangers with heating water with an RMH.


I'm sure there is a safe level where your water pipes would not get heated to the point of turning to steam.
Steam is a killer; first, your pipe collapses, and then it bursts, spraying scalding hot water. (Known as Boom Squish)
Yes, an inline pressure relief valve can stop that.

To elaborate more on this subject, lets examine a bit more what happens with water when it converts to steam.  The shear volume which occurs from almost hot enough to actual steam,  causes a expansion of the rate of 1 to 1700 volume wise- thus the ability of a steam engine. This expansion factor remains the same, whether imperial or metric numbers are used. And it can happen fast.

When considering this, a point that should be fully understood, is simply " CAN YOU HANDLE" this expansion if it occurs?  But wait- there is more.  Every hot water heating unit has a sensor ( or two ) to not only allow blow off of this expansion, but perhaps more importantly STOP the introduction of more heat being added to the water. With a few exceptions this is done electrically. -- i.e.  Stopping the influx of more heat, weather gas fire, oil fired or electric, and this is done before the problem happens, otherwise known as a high limit switch.

So where does that put most rocket mass stoves that are trying to tweak out every ounce of heat or transfer this heat to a larger mass zone via water? Non-electrically I might add?  If considering, please ask your self, " what if for some reason I have to shut down the fire, for what ever reason, and do so if I am not here, and most likely with NO electrical inputs"    If all of this can be incorporated, I am sure you would have a large audience in many forums.

The second part of the equation, is the math, as most of the latest generations of rocket mass stoves are calculated for ideal heat extraction, as Peter has done so well.  there is almost always a ISA ( internal Surface Area ) number that is calculated for the size of the rocket mass system. Large extractions of heat from the system, changes many dynamics, not saying it can't be done,  .  Going from a precalculated rocket mass bell system that is absolutely known to work, to a system with so many unknowns, could get to be a challenge as well as expensive, if it don't work out well.    All things can/could work unless they don't!

Best of success to all!

I've been thinking and planning, hoping to build in a few years, and am leaning towards radiant/hydronic heat (as a backup/supplement to Passive Annual Heat Storage), primarily sourced through solar hot water panels, with a rocket water heater as backup/supplement.

I will likely start off with something similar to the Geoff Lawton and Tim Barker design of water heater, pumping hot water to a large insulated buffer/storage tank, and feeding the radiant system with a thermostat controlled pump.

Matt McSpadden wrote:Hi Anna,
Could I ask what prompted this question? What made you want to combine these two systems?

I think it could be done... but it would be very complicated to do safely. I have seen some people create RMH's that were lowered... so the bench/bell that most people have, was at floor level... essentially creating a heated floor.

I wonder if a different technology might be able to provide the hot water. There are some really good solar hot water heaters.

So I don’t actually want it to heat water for use. I’m thinking of a circulating system that would solve the problem of it being colder further from the stove. And as mentioned in my question, I became interested in maybe trying to figure out a floor heating when I learned how some Asian countries used an outdoor fire pit and blues to heat their floors. That idea is a little more scary to me, but I’m also coming to see that the potential for disaster here is high.
I wonder, though, if you could use flues under the floor with a RMH. Might have to scrap the whole idea.

Nancy Reading wrote:I think it is difficult to do as a passive system (so completely fail safe) but have seen (online) an acquaintance wrap pipes around his barrel and then to a manifold which distributes heat round his floor...I'll see if I can find the video....

As I said - not simple!

I’ve seen people use this same type of system for a water heater for their home, but I’ve never seen anyone use it for in floor heating. Although I’m not sure why it wouldn’t work. But as you said, not simple and I don’t believe that’s something I could DIY. Thank you.

'I’m thinking of a circulating system that would solve the problem of it being colder further from the stove'
I had the same problem , my room was way colder than where the stove is , my solution was a 200 mm pipe with a fan blowing air from the top of the bell where there are 26-30 degrees C  2 meters through a hallway into my room , which is now heated furnace style, the air returning through a grill low in the rooms door