Thermal Mass

I took the easy way out, I bought a rocket heater. Now I need thermal storage. I have a 3000 gallon metal water tank. I was thinking to route the flue through the tank. Is anyone doing anything like this? This would be for heating my shop.

Hi Dean;
Welcome to Permies!
So you bought a rocket stove, that's great!
What type of rocket stove did you buy?  What size?
Water is one of the best masses available if you can make it work.
How are you thinking of plumbing this?

be aware that if the water is to cold in the tank it can cause condensation that can run down and put your fire out.  So if you do this be sure you have a path for water to escape away from the fire.

On my long term goals list is to build a barrel for an RMH.  Spiral the 1 1/2" or 2" pipe and piece of flat sheet up together and weld into on assembly and us that to convection heat a storage tank.

I bought a Liberator RMH2. I'd like to pipe the exhaust right through the middle of the water tank, which would be positioned within a few feet of the heater. Im talking about a 7' diameter, 10' tall tank, so it would separate from the heater. When I say through the middle I mean side-to-side, but down low. I'm assuming copper pipe will hold up, given it will be submerged. Any thoughts? The outlet of the heater is 6" so I either have to use 6" copper, which would probably cost more than the heater, if it even exists,  or I have to use 3 runs of 4" pipe to equal the flow of 6", or so I believe.  I assume that as the exhaust gas cools, it shrinks in volume, and if that's true, then I can reduce the size of the pipe coming out of the water tank. Is that true?

More...
I'm wanting to heat a 6000 sq ft shop. If I could maintain it at about 55 degrees I'd be happy. I'm assuming I'm going to need more than one heater/thermal mass unit. And I'm wondering if the 3000 gallon tank is overkill. I build the tanks for a living so I can build one from 100 gallons, up to 3000 gallons. I'm in Virginia, 7a, near the Virginia/ North Carolina border. Thanks for any input.

Hi Dean;
A liberator is a good starter model rocket stove. A well-built, UL-certified, mini mass heater.
It can push 12' of horizontal pipe thru a mass.  An 8" J tube can push 50'.
I will say, you have a lot of water in a 7x10' tank, and I'm not sure you will be able to get it heated up.
Might want to start out with half a tank of water and see how well it does.
Trying to run a 6" pipe into three  4" pipes will be an issue.  Much better to stick with a 6".
No need for copper pipe, plain steel pipe can be used. It is underwater with very little oxygen to form rust. Barnacles might grow on it...
Not sure how you plan on entering and exiting the tank.

It is true that cooling smoke shrinks in volume. However, in your case, I would not attempt any restrictions at all.
I suspect you may have draft issues with that much cool mass and you want no slow-downs on the way up and out the chimney.
,

Thomas

Is there someone who offers an 8" j tube heater, or does it have to be built on site?

Hi Dean;
Here is the only one I know of.
https://rocketstovecores.com/    Sadly he is out of stock for this year.

Building a J tube yourself with brick is not hard at all.
Check out Glenn's post on this thread   https://permies.com/t/180036/Rocket-Mass-Heaters-Ukraine-Eastern
He shows plans for a quick build RMH with a bell to help folks in Eastern Europe stay warm this coming winter.

Hey Dean;
Matt Walker is selling plans for his 8" core on his site.
https://walkerstoves.com/super-hot-ceramic-fiber-cores.html
Check it out, Matt is a super guy and easy to work with.

A 6000 square foot shop is more than likely going to need a lot more than a radiant heat supply. Radiant heat works well in a well insulated fairly confined space. Radiant heat transfers at the rate of the fourth power of the difference in the two temperatures. That's the good news. Unfortunately it diminishes at the square of the distance. That means unless there is an extreme difference in the temperatures the distance becomes the more important factor.  The room sized Liberator will do little to affect the comfort level in your shop. I recommend you look up Eric Hroboni's massive rocket furnace, I think he calls it huge, that heats a 3000 square foot uninsulated greenhouse in a cold part of New York.