Novel, simple, safe idea for a hydronic mass radiant heater (not rocket-y)?

Apologies if something like this has already been shared – the search function on here isn’t always ideal. Thoughts welcomed! The only post I saw that seemed to be getting at anything like this was this one: https://permies.com/t/60386/Radically-simply-hydronic-heat-boom

Background (feel free to skip): I’ve always liked Paul’s tinkering around with more easily deconstructed mass heaters (e.g. pebble stoves). They make the concept of mass heating much more reasonable when there’s a possibility you’ll have to up and move your house, and if the code folks ever come knocking when you’re experimenting with something not in the IRC and it ultimately comes down to them ordering removal/dismantling, it’s less of a sunk cost. Water is the holy grail for easy to obtain and easy to remove, low cost, high mass material, but the main problem (danger) is getting water to play nicely with high temperatures of an RMH.

The original idea – Wood-fired hot water heater: Well, about five years ago I took the plunge on one of those scuba/snorkel stoves that you submerge right in a tank to make a wood fired hot tub (here's the manufacturer I bought from: https://snorkel.com/product/scuba-stove-and-fence/). It had some good times and fun parties while it lasted, but for the past few years its been in storage since we’ve lived in rowhomes, friends basements, etc… Now that we’re finally planning for our tiny little place on our own land, it occurred to me that if I put that scuba stove, which is only collecting dust for now, in a much smaller tank (say 80-100 gal) and insulated the tank well, we’ve got a pretty direct, easy to use wood fired hot water heater. The scuba stove is capable of heating up a 470 gal tank 15 degrees F/hour, so this puppy would require very little firing time, even if I occasionally let the water get down to room temps). In the summer it might be annoying to have the excess waste heat from the stove in the house, but I figure I could find some design way to isolate the tank (maybe in its own little isolated attached mini greenhouse ventilated to the outside) during that season. Or I could run a warm-season only simple solar hot water loop through it and not fire it at all between late March to Mid-November (I’m in zone 8a NJ).  In the winter, the “waste” heat from the stove and chimney would actually be desirable, which leads to the next point…

Adding extra mass: Thinking about that “waste” heat and how it’d be convenient in the winter – especially because a wood stove immersed in water is going to generate a lot more humidity to soothe those nasal passages in the dry winter (one of my biggest challenges with wood heat since I get stuffy really easily) – it occurred to me that maybe I could make the room-facing sides of the insulation (the top and any exposed sides) removable and just let the water’s mass radiate into the room. Sure, it might mean firing the stove again sooner or that my “hot” water gets slightly colder than I’d like, but maybe it’d make for more comfort? Once I started thinking about just letting the mass radiate I started to ponder if hydronics would also make more sense? I could have a shutoff valve that I only open during the winters, with a small circulator pump in the circuit, and have water circulate just like a more mainstream hydronic floor. If I used larger piping, it would up the amount of mass and make the temp swings somewhat more gradual. Though I did the math and if tubing spacing is just one foot, I’d only be adding about 25-30 gallons of mass for our whole 450 sq ft place if I used 1.5” hydronic PEX – so not getting that much volume. Closer spacing would add more water (mass) and improve the heat distribution, but adds $ :/

Other thoughts:
- Why I like this: It’s easy and fast to build (anyone who can operate a drill and a ratchet and put a couple of waterproof gaskets on the tank side can install the scuba stove), can be scaled up or down and customized depending on space and need, adds humidity to normally over dry wood heat, and seemingly very simple to maintain and operate. It also seems very safe because the tank (and therefore the hydronic line running off of it) are ultimately not pressurized and the stove is not relying on heating coils that could get to hot in error or something like that can cause major safety issues. A few issues I can think of with it might be…
- Cost: This is not necessarily a cheap project – the scuba stove is $900 and 500 ft of PEX tubing is $1,000, add in the stock tank, pump, and various plumbing fixtures and I’m probably looking at around $2,500 (if I didn’t already own the stove) for just the heating supplies, not to mention the sand or earthen floor or whatever you’d immerse and support the hydronic tubing in (although if this would be part of your finished floor anyway, that price should only be moderately factored in). But I’m willing to pay a little bit more on the front end for cheap, easy, and reliable. My understanding is that even if you’re pretty miserly about scrounging up materials a rocket mass heater is still likely going to run you $1,500-2,500. An amazing savings over a professional mass heater but still not nothing.
- Durability? The snorkel/scuba stove is designed for being used recreationally in a wood-fired hot tub. I don’t know how it’d hold up to this kind of consistent firing. If it could last as long as regular tank water heater that’d be darn impressive, but maybe asking too much? IIRC from the manufacturers directions, the water helps dissipate the high heats facing the stove, prolonging its life and keeping it from burning out. Might ask the manufacturer about their durability! If I only use it for the hot water heating and drop the space heating idea, I imagine it would only need a very brief heating each day, so maybe that’d prolong it too?
- Stove efficiency? I don’t know enough about stove designs to comment on the efficiency of this design? It’s not a rocket design – actually the feed is a bit higher than the manifold where the stove meets the chimney. I really appreciate rockets for their clean burn so maybe the lack of that alone makes this a less desirable design? Another thing to inquire with the manufacturer? Like a rocket, the feed does go down, and if I understand rockets at all, this is a feature that helps generate that strong air flow, and the chimney is above on the other side of the feed where the chimney can act as a tall riser that stays hot. So maybe it actually is relatively rocket-y? My hunch is that not getting particularly hot is a part of the stoves safety design and therefore combustion might be incomplete? Input from more knowledgeable folks welcomed here 😊
- Amount of storage necessary? I may need a much bigger tank for the radiant idea to make sense (if it does make sense at all) – maybe 300-400 gallons? In which case that seems a bit overkill, although that makes the idea of housing it in an attached small greenhouse even more palatable, where any wasted heat, again, wouldn’t be wasted. I have some calculations from a solar hot water book that could help me here, just too lazy to run them yet

The only thing I see " wrong"  with this idea is the efficiency/cleanliness.
Your firebox will be constantly cooled by the water it is heating.
That will make for a less efficient and dirtier fire.
You will have created an outdoor wood boiler with hydronics and a huge buffer tank.
I would expect a very dirty burn.

The exhaust from a rocket mass heater passes through the mass after it has been thoroughly combusted at a high temperature.
I also prefer water for mass, but clean combustion is both efficient and responsible.

Hi William,

Just to be clear, I'm also willing to entertain the idea of keeping this inside, or at least in a semi-inside/semi-outside state that I could vary with season by opening a door or something. Your points on the efficiency of the stove are the ones I'm most interested in - and I have some questions since I'm not all that knowledgeable on this front:

1) Air moves away from your wood stove more rapidly than water would, correct? So the stove losing heat to the water its heating is similar to a woodstove losing heat to the air its heating? I guess the difference here is that water is a much better conductor of heat?
2) If the unit were inside (or at least semi-inside, as in a partially insulated glazed attached area), the lowest the water temp would reach would be room temperature. If the home is as well-sealed, well-insulated, and passively solar warmed as I intend, I'm hoping the lowest that that would get in the winter would be 65-70. The water wouldn't be lower than ambient air temps, so less of a big deal, right? As with insulation, the heat loss is greater the greater the temperature gradient, so if I were trying to consistently heat cold or near-freezing water I'd think it'd be a bigger issue, but less so with room temp or higher water?
3) If my tank were larger, say more in that 200-300 gallon range, it'd take longer to warm up but would also take longer to lose warmth - and that larger capacity would give a bigger buffer when new water has to be added to the tank to replace hot water I've used. I'd also like to be creative with how all of my potable water is stored in general, maybe somehow within the insulated envelope of my building so it has time to warm up passively and the water going into my tank never has to be heated up that much in the first place.

I've been frustrated because books on wood stove and fire efficiency seem either too general audience oriented with no technical info, or oriented toward mechanical engineers with very little hope of me understanding. Structural engineers have made things pretty easy with span tables and simplified calculations, but understanding all this heat mechanical stuff has been a rougher go! If you have suggestions for education sources I'd love them!

I'm definitely no expert, so take this with a grain of salt, and hopefully someone more knowledgeable will chime in.

Lu Bivona wrote:Air moves away from your wood stove more rapidly than water would, correct? So the stove losing heat to the water its heating is similar to a woodstove losing heat to the air its heating? I guess the difference here is that water is a much better conductor of heat?

That, and it also has a very high specific heat capacity, meaning it takes a lot of energy to heat a kilogram of water one degree, compared to a kilogram of many other substances. The energy is obviously not gone (since that's physically impossible) and it will heat the surroundings eventually, but like William mentioned, it draws heat away from the combustion zone. Its high specific heat capacity paired with high thermal conductivity and the fact that it's liquid, which gives rise to convective heat transfer, means that it would cool the combustion zone quite a lot, compared to other types of mass. In a rocket, if I've understood it correctly, you want to keep the mass away from where the combustion actually takes place (i.e. place a whole bunch of insulation around the firebox and heat riser) in order to achieve a high temperature and a clean burn. The heat would be extracted into the thermal mass after the combustion actually takes place, so it doesn't lower the temperatures in the combustion zone.

Riiiiight, heat capacity! Thanks Eino. I always forget about it at my own peril when we talk about thermal mass :p

Okay, so to continue along this train of thought, another question: let's imagine I really over-insulate this thing and am only using it for the water heating (take the heat loss associated with the radiant out for the moment), to the point of say R-40 insulation all around, and it retains its heat incredibly well. So when I wake up in the morning, having heated it the previous day, the water temp is 105, totally reasonable for a hot water heater. If I fire it to run it again at that point, the ambient temperature around the stove will be higher than the ambient air temp around any  typical wood stove, because we're never trying to get the air in our homes that hot. Do we think that means it can actually run hotter (and hence burn more efficiently) than that identical wood stove surrounded by air instead, or does the fact that the water its surrounded by could still continue to go so much higher (all the way up to boiling) mean it'll just continue dumping energy into the water at a relative (but diminishing) cost to the temperature in the combustion area?

I guess I'd add that the snorkel stove is only dispersing heat to the water through its surface area and two closed holes in the body of the stove that water can flow through it. While that's a good amount of surface area, I imagine it doesn't offer the best possible transferance.

My understanding is that more consumer-available EPA certified wood stoves have quite good efficiency thanks to things like chambers for preheating the combustion air, secondary burning, catalytic converters, etc. If those tech were in my lil scuba stove I wonder if that'd bode better for the idea? Or perhaps the fact that the combustion area is not separated enough from the mass area is just too big a design flaw, since that's sort of a lynch pin of RMH's efficiency
EDIT: Maybe put air intake pipes in a circuit running between the tank and the insulation so that that air could get pre-heated by the water as its coming in and improve combustion efficiency? We're starting to complicate the design  now haha

The water touching the surface of a (metal) combustion chamber is going to be a killer to efficiency no matter what. To get full combustion, you will want at least 1000 F, so there will be a massive temperature differential no matter how hot the water gets.

What you really want is a standard masonry J-tube (for simplicity) core, which exhausts into a chamber surrounded by water or containing big circulating water tubing connected to a big non-pressurized tank.