"Water Mass" RMH

My greenhouse is build over a septic drain field, so I don't want to do excavation, or move a lot of solid mass into the area. Are there designs for a RMH using a tank of water for the mass? If so, where?

If repairs to the drain field are necessary, I could more easily remove the mass and disassemble the RMH apparatus, for access to the drain field.

I've been thinking about this myself. Water has the highest Delt G of any substance but for some reason, all the RMH designs seem to prefer cob or something very much like it. I have also been thinking about using AAC (aerated autoclaved cement) which is used for building walls with R40 ratings...not bad,huh? Well my guess is that one of these two items should be useful for this type of construction.
I'll bet there is someone out there with some engineering experience who can tell me why both ideas are not good, so I don't waste my time using those materials. I really want to know if using water for the thermal mass is possible or would it take TOO LONG to heat the water with the given amount of fuel?

Problem of water is risk of leaks, explosion, rust of the container, too much moisture in the air, legionella etc! Complexity of the system to keep the level of water even with an open to the air tank.

Do NOT use AAC for your RMH!

You actually NEED the large thermal mass to make the most of your RMS, you don't want insulation there. Use the insulation on the outside walls of your house, but keep the large mass on the inside, to store heat.

If you were to use AAC, you'd find that your RMS didn't store nearly as much heat as the design promised, and you'd need to burn much more frequently, & use much more wood.

Water is a very nice material. But it lacks in the reliability department.
Cob does not flood the house (possibly with hot water) if something fails, nor does it explode. Also while water is a natural resource the things that can hold it are not so much.

If you want to build a water based system, you likely need special parts and materials. In a water system it is an advantage to have the water in few large tanks, to avoid parts of it boiling, while the rest needs more heat. This poses a problem, since we usually dont have a container that fits just right for the space and form we want. We are forced to put together the thermal mass from pieces we have access to. Its not a problem in a greenhouse, one just needs one or two big tanks. But putting the same in a home poses practical and aesthetic limitations.

The answer is: most people have such homes, and skills that a cob mass seems more practical.

What is "AAC"?

The 2nd post in the thread brought it up, but it is Aerated Autoclaved Concrete.

Concrete that has been cured with lots and lots of bubbles inside it for air pockets and it does make for much lighter weight than normal concrete, and much less thermal mass. It is also structurally not nearly as strong as normal concrete, although a RMH isn't normally a structural member.

check out the hot tubs at www.snorkel.com - you'll see that one could put a barrel on the chimney and convert it into a RMH really easily. I have one of these in my greenhouse and am switching it over with a big lump of thermal mass added in this spring. The snorkel technique and then you can submerge all the stoves in water - even if the water temp is 45F, it is very warm compared to the 20F air surrounding it in the winter...greenhouses with large and low areas of water absorb so much cold in the winter protecting the higher trees. Id love to see a 42ft greenhouse dome with a big swimming pond and a cottage inside.

http://snorkel.com/hot-tub-info/snorkel-and-scuba-stoves.php that gives a good idea...so by adding the RMH concepts and thermal mass the design is improved upon 1000x