Water based thermomass thoughts/ideas

I currently have an "air tight" style wood stove with the ash pit style clean out under the burn box and has an air jacket around the burn box. Currently I have blocks around the outside to act as a thermomass which seems to be working great. I've been liking the idea of having two separate burns in the wood stove to save a little wood and have a heat sink so the house doesn't shoot to 90F during a good solid efficient burn.

I'm planning most likely some bricks on top and build some sort of small oven and figure out a way to control the temps inside with the air jacket fan system and switch it between just pumping heat off the stove vs keeping the "oven" around the correct desired temp. I'll have to dig into that more later.

Anyway, I've read a bit about using water and the best solution I've seen people actually use was 2 liter bottles filled with water lined along their walls on the floor (like behind the couch and such). I understand that steam + pressured (closed) systems is a science I'm not looking to explore. However, in terms of effectiveness, wouldn't it be more logical to place the water on shelves along the tops of the walls to absorb more heat from the wood stove? The down fall with that idea in a passive sense is the fact the heat won't want to go down, so the top of the house will stay warm while the bottom/floors will cool down.

Another idea I've been pondering is to soak up excess heat with a water jacket (copper tubing) and keeping it a open system with a large vent encase the water gets the boiling temps (say power outage). The main mass storage of water in this type of system would be best placed under my house on the cement slab in the crawl space. I'm thinking it might be safer to use kind of a two loop system, basically a large vented loop right by the stove that uses a little water (say 6-7 gallons), then when it is warm enough, mix it into the storage tank. Logic behind it would be the vent would be sized for the 6-7 gallons and the main storage tank wouldn't ever have a chance to directly interface with the wood stove heat. Does this seem like an acceptable idea, or is the double loop idea just a bunch of wasted effort?

Down the road, I could tap into the storage tank, and pump the water though pex tubing in the floors.

I'm more or less thinking out loud and looking for some feed back. I'm a hobby programmer of around 18 years, and probably would be looking to use a raspberrypi to control everything even though I've never used one personally before. Website linked below. I also attached a paint diagram to better illustrate the idea since I'm not always the best with words. My wood stove already has a copper winding for water.

https://www.raspberrypi.org/

Basically, you are making something that is like the condenser of a still only instead of just dumping the heated water you are using it to heat your house. You might find it useful to look at some condenser designs for your project. I would probably just use a single loop system. In the event of a power outage, the pump would stop and the water inside the mass would vaporize/boil. As it vaporizes, it would push out any liquid water from inside the mass, and the mass would just get much hotter than expected.

If you want to try water thermal storage, I would suggest a 20 or 30 gallon tank supported above the woodstove, with the largest copper coil you can manage (like 1") to collect heat and allow it to circulate with minimum friction, and rather than a pressure vent, simply have the tank open to the air. Add a makeup float valve like you find in toilet tanks, and you are safe whether there is power or not... almost nothing to fail, and safe even if it does. Then you can add a thermostatically controlled pump to move the hot water to bulk storage in the crawl space and circulate it in pex.

The best system would have the whole water storage tank above the stove with just one gravity loop, but that would take some structural engineering and occupy living space, so probably not going to happen.

Even though you are being very clear about avoiding the risks of a closed system that involves water and heat by using a large open vent, I think it's worth reminding everyone that errors in design or implementation risk the "boom! *squish*" problem whenever heat, water, and complex plumbing find themselves part of the same system.

I'm not saying not to do it, I'm just saying it raises the stakes to a whole new level.  

The wood stove has couple of copper pipes sticking out the back which are quite small, 3/8 to 1/2 inch which is why I'm kind of expecting to use a pump. Thermosiphon based water heating would be really neat to do, I just fear that I'd be boiling the water too easy when I start burning well dried oak. Currently I'm burning old standing ash trees that are dry enough to burn alright, but are not as dry as I'd like. Every once in a while I'll get a log that sizzles. When I find a couple really dry logs the room's temp shoots up quite quick and the fire box with the blower on hits ~400F (front face above the door) and exhaust I try to keep around 350-400F but have had it up to around 550F to keep the build up down. The thinking behind the pump also went along the lines of using the controller to monitor the wood stove's temp so it only kicks on when there is heat to extract and when the stove isn't fully up to temp, it keeps the smaller loop warmer so it doesn't suck as much heat off of it. I'm not sure if I could make the controller run the pump at lower speeds, or if it will be a simple on/off relay style. I guess there is always the option of different flow rated pumps for different "speeds". I don't know how many loops the copper line has, but I suspect it is only one or two. I'd have to pull the air jacket off to see it better.

With my thinking on the project (not saying I'm right by any means), encase of power failure and I'm not around to drain the small tank for some reason the whole 6-7 gallons would more or less be boiled off since the tank would be higher than the wood stove (if it can get the water that hot with an unattended burn which I typically damped down more than ideal burn). The vent idea would probably just be a simple metal flap so it slows the evaporation, but still allows air flow in/out kind of like the exhaust ends that are put on tractors. Non running, it is closed to keep rain out of the engine, and running it is open from the exhaust pressure and counter weight.

With a single loop setup, the tank would be at the same level as the wood stove which is in an addition that was built with a cement slab on the bottom. It's not built perfect (floor isn't sealed or insulated), but it was built in a good fashion for a wood stove to be in it since the top of the door way is the same level as the ceiling to allow the hot air to flow up into the house. I don't think the tank would be high enough to use thermosipon so it would rely on the motor moving the water to keep it from boiling, and it would be a bit of a pain to check the fluid level and top it up unless I ran some piping into the wood stove room and basically setup the second tank with out hooking it up to the stove to have a nice visual reference for the water level, but venting would be more difficult.

Another option that I'd think would be safer but less effective would be to put the water coil on the outside of the air jacket. The stove's fan current turns on when the out side metal hits 180F and turns off at 160F, so while I have power and the fire isn't roaring out of control, it "shouldn't" get to 212F to boil standing water. This setup might work better with the double loop system and use thermosiponing instead of a pump for that loop. I might have to mess around and do some more research since I don't really know much about thermosipon, just the basic idea behind it.

For the bulk storage tank, I have a couple 250-300 gallon fuel oil tanks I could use, but it would have to be outside next to the house. I could insulate the lines and tank, but isn't ideal since if it gets too cold (say I go on vacation for a week or two) it could freeze. Don't really have the money to invest in anti-freeze like the outdoor wood burners use. The insulation I have is R-8.5 styrofoam, not the best stuff but isn't bad. Two layers would be R-17 which might be enough? I know there is a post somewhere the gives the math behind the thermal mass's ability to store energy, but I haven't been able to find the correct search terms to bring it up again for cob and such. I'm not really sure how much mass I even need. I'd think 300 gallons is probably a bit more than I would need since it stores around 2500BTU per degree increase (if I did the math correctly).

I guess I should give a bit more details about the house. It's around 620 squre foot, addition room is about 9 feet tall, and the rest of the house is the standard stud size (86 or 92 inches or something like that). It isn't exactly a huge house or anything, but it would be great to burn a hot fire for say 5-8 hours twice a day with a fair break between the burns, kind of like the typical rocket stove setup except with a longer burn time.

I am a little new to wood burning, been using this wood stove for about a week or so now and helped load the one at my dad's for the last 5 years or so, but I never really knew about how to burn wood efficiently back then, actually that was around the time I was just getting into how to drive a car efficiently (40mpg or so out of a 97 corolla with 300k miles when the EPA says I should get 31mpg on the express way and 26mpg city). My dad goes though a lot of wood per year, so I'm hoping to use a lot less than him which shouldn't be hard since his setup isn't exactly ideal. His stove is at the far end of the house and is ducted into the furnace ducting with a large blower. He has to keep the wood stove room at something like 80-90F to keep the far room at 70F. My setup is a bit more centralized and with out forced air my furthest away room is 4 degrees cooler than the mid section of my house.

Anyway, I'm going to research thermosiponing a bit to better understand it and see if I can think of any way to use it with the bulk storage tank since I think that would be ideal. Thanks for the replies so far!

The wood stove is in a slab-floored addition with a 9' ceiling? Is it prime living space, or a utilitarian heater space? If esthetics are not critical, I think you could move one of the big tanks in next to or above the stove tight to the ceiling on steel legs with sway bracing. Steel angle spreader beams on the floor could spread the load enough for the slab. With added larger coils for heat collection, this could make a simple foolproof and failsafe gravity system. It would be work up front, but benefit you ever after.

I've thought about it a bit, a smallish tank would be doable, but I'm limited on space pretty badly. The room is only around 9x9 feet with steps, swinging inward door for the back door, estimated 2-3 weeks worth of wood, washer, and hot water heater. I have about a 3 foot wide path though the room lol. Directly above the wood stove is quite a lot of space for a tank and above the washer which is in the opposite corner. I attached a photo to give an idea, of course it isn't drawn to scale.

I do have several 55 gallon barrels that used to store paint that would probably work quite well for a tank. I suspect I'd need more mass than that though, probably two 55 gallon barrels. Might be possible to stuff two over the wood stove side by side and make plenty of support for them (~930 pounds). The cement should be fine for strength, it's a rather small slab and is over 3 inches thick. I could probably make a copper coil directly above where air ducting would normally go on the stove and not hamper the fire in any way while soaking up the heat coming off of it, and even more heat when the fan kicks on. I suspect the blocks I have stacked up on each side of the stove would work well to be the base of the stand, probably should make it two blocks wide to be extra stable though. Trying to keep things easy to move out for if and when I go the rocket stove route, which would also be in the same room and efficient space usage for the mass will probably push me to re-use the water tank system. Maybe some sort of heat exchanger in the cob bench or around the barrel.

Does thermosiphon require a sealed heat exchanger on the stove? Just for playing around I was thinking of putting a pot of water on the stove and a couple of pipes at high/low levels to another container a few feet higher to see/feel how the system works. Would the the water flow to the second container this way though?