'portable' RMH: contained water instead of rock/gravel? Help ASAP? Thanks!

I'd like to build a hybrid 'portable' RMH based on Chris Burge's mini mass design - that he placed in front of his fireplace - and the wood-cased gravel-filled design.  We are leasing, so cannot build anything permanent.  

We're in our 70's, and facing another winter of $350 to $400 monthly electric bills - and even then we still 'layer' or wear sweaters, heavy socks, etc, to feel 'totally' warm.  Since my husband has severe arthritis, I'll probably do most of the design and work.  I'm a retired lady home builder, so I'm very familiar with woodworking, etc.  I 'know' about masonry, though I don't have any hands-on experience.  (No problem - I'm a fast learner).  

My major concerns are:  1) Building a very 'rockety' mini mass heater that can hold or maintain heat overnight,  2) be made of materials that I can lift and carry, and 3) be built on a extra-heavy-duty platform with locking casters.  I got the DVDs - and was disappointed.  Yes - there is a lot of information in them - but I wanted THE FINAL BUILDS and explanation (as well as 'plans' and parts list) - instead of so much 'trial and error'.  They were informative - just not what I needed.

Working from the bottom up:  I'd like to build a wooden box with a 3/4" to 1" plywood base, attach heavy-duty casters that lock, and put cement board, or something similar, in the bottom of the box (if needed) to protect from excess heat.  (The locking casters would be used to hold the box in place while exhaust pipe is in the chimney.)  I understand how to build the manifold (as in the portable mass heater videos), and that the flue needs to be near the manifold... but there seems to be some disagreement on how the way the heat-absorbing pipe should be placed in the remainder of the box.

Perlite with a bit of refractory cement 'dust' would be easy for me to handle; gravel might be a problem by the bucket-load (though large rocks/gravel are good for heat transfer).  Water is a very conductor of heat....so... WHY NOT OBTAIN METAL CONTAINERS (like paint thinner cans) PLACE THEM IN THE BOX AROUND THE PIPES, THEN FILL THEM WITH WATER???  You can buy new metal "F" containers in bulk.  They are oblong shaped, so would fit in and around the pipe.  You could FILL IN AROUND THEM WITH PERLITE, to complete the 'mass in a box'.

If you need to remove the "ROCKET WATER MASS HEATER", you'd just pull out the containers, pour out the water, and replace them (empty) for moving.  When you set it up again, a carefully applied water hose, or even a long-spout watering can, could be used to refill them, as they are put back in place.  (You could even 'pre-heat' by filling them with HOT water!)  ;D  

I am concerned about how much steam might be produced...and what it's effects might be.  The containers could have a small opening (like a beer-can opener) in the top so steam wouldn't build up in the can....or the lid just left loose so steam could escape around it's edges.  I don't know how much heat the 'box pipe' would produce - and this effects a) how much stream might be produced, and b) the type of container that could be used.  If the pipe temperature was 'reasonable', you could even use heavy-duty 'plastic' containers instead of the more expensive tin.

If there is any steam production, it obviously would effect the wood box, particularly the under-part of the box's top.  Perhaps I could level the perlite, etc, and place a layer of ceramic tile on a mesh base - just 'lay it on', rather than making it permanent.  It would give a bit of additional mass, some steam to counteract hot dry air, and look better than the 'mass' innards.

I am concerned about 'appearance' - and would probably paint the plywood box base and 50-gal barrel to match - or perhaps consider 1 x 6 or lapped siding, stained to coordinate with the painted barrel.  Is there a high-heat paint that comes in beige instead of black???    ;D     It just has to 'look nice' if the landlord makes an appearance - so it looks fairly innocuous instead of a fire-breathing dragon that could burn down the house.

In addition, I'd like to increase the height of the feeder tube to handle LONG pieces of wood.  We are in the middle of the Ozarks, surrounded by woods.  The feeder should be of sufficient size to allow long branches that are "reasonably" straight, but have some kinks, to feed properly.  I don't want to rely on wood pellets for overnight heat.   I'd also like to know how to circulate more hot air - are those wood stove 'fans' worth the $90 they cost?   Someone mentioned a fan with a Stirling engine....but I don't have an extra $300!

Our house is unusual.  It is a rectangle, with a 2-story 12 x 12 'fireplace' living area (downstairs) that becomes a large opening upstairs. (There is a larger living room to the right of this fireplace 'room').  Bedrooms surround the 'balcony' on the second floor.  The fireplace is located in this 2-story stone wall.  Of course, heat will rise up the stone wall and 2-story opening, to help with bedroom heat.  Perhaps leaving the bedroom doors open, with fans, might help in heating this area.  BUT - this large 2-story opening might create a problem of 'funneling' too much heated air upstairs, when we need it downstairs.  I've considered closing this opening off with Visqueen (clear 'plastic') - and could only partially close off the upstairs from downstairs by leaving openings in the Visqueen to allow some heat to rise.  SUGGESTIONS PLEASE?

I checked for info at http://www.engineeringtoolbox.com/ - but was a bit overwhelmed.  I really need HELP as soon as possible - we've been lucky so far, and have had no real cold.  But, in mid-Missouri, it's not going to stay 'pleasant Fall weather' for too long!  Thanks in advance!

I don't have time to address all of your questions right now, but basically, the water mass idea should be sound as long as you do it right. You would need metal containers, and the bigger the better. If you need to use containers that are rectangular, putting the short side next to the duct would be best, so there is the greatest amount of water mass per heated surface as possible. Smaller containers that get hot enough to boil will not just put lots of humidity in the air but empty themselves fairly quickly, and refilling would be a pain.

You do not want to put perlite around the mass containers; they would add no mass, and keep the heat from getting to the room. Fine gravel would be about the best option; you can carry it in small buckets instead of big bags or pails if that is an issue.

You need to make the box floor noncombustible. A layer of cement board will not keep a plywood base from getting hot enough to char. One possible way to support a floor would be steel studs (you can get them at bigger building supply stores at least) cut to fit the box width and set in flat side up, open side down (you'll understand when you see them), side by side along the bottom of the box. They will be strong enough to support the load, and a sheet of cement board will keep all the gravel etc. in place.

You can get "stove paint" in numerous colors and heat-resistant grades from several different manufacturers. Ask at a hardware store, or look online. For the barrel top, you would want something that can stand 1000F, but the rest is much lower stress. If the barrel is currently painted, you need to burn off or otherwise remove the paint. Painting over it with stove paint will not help.

I don't think there is going to be any safe way to make a long feed tube that will not smoke back or risk hangups, and a portable unit is likely to be inherently less failsafe than a built-in unit. Besides which, the mass is supposed to be what does the heating overnight, not a continuous fire. The woodstove mindset is tenacious, and I understand it is very hard to break the idea that you need an overnight fire.

Is the fireplace stone wall facing most of the lower floor, or does it divide off a significant living space? Any room on the other side of that wall will get little heat from the RMH. But you will not get so much of the plume of hot air rising strongly and leaving the lower floor cold as you might from a woodstove. It's not possible to tell from here how effective the lower floor heating will be in your situation. Only experience will tell, though I suspect that a RMH small enough to be portable will be more of a supplemental heater than a whole-house heater.

After doing some research, it occurred to me that for 'water containers' I could use PVC pipe (sized to match the available spaces) with end caps.  They could be filled with water, stacked in place around the pipe, then small gravel used to fill in the spaces.  I think the PVC, at fairly low cost for smaller sizes, would be easiest to use, and least expensive, allowing more 'customizing' of hot water storage inside the wood box.  

I don't think PVC or other plastic pipes will work at all. The pipes may hold their shape while there is water in them, but they would be so small that the water in the ones closest to the duct would quickly be heated to the boiling point, and when the water is all boiled away, the plastic next to the duct will start to soften and melt.

Ooops - I think I was thinking about the 'hot water hose' that is used for hot water heaters and other HOT liquid applications.  Back to Google to do some more searching!  Thanks.

Aside from avoiding any plastic which could be dangerous if run dry, you want the biggest possible individual containers so heat can be distributed effectively from the duct. Heat will not transfer very fast from one container to another, so with a lot of small containers, you would have very hot, maybe boiling, water nearest the duct and still cool water farther away.

Some people with greenhouses have made containers lined with plastic (like EPDM roofing or pond liner) on top of or surrounding the ducts. This gives one continuous water vessel so heat is distributed to the whole mass at once, and no part ever gets close to boiling.

You've made another good point.  Do you have any idea about how hot the pipes get?  Using a pond liner type 'rubber' or 'plastic' would leave an open top so a lot of steam would escape into the house.  That would be great for a greenhouse (we used to raise orchids) - but a little too humid inside a house!   Maybe I just need to go back to the rock and gravel fill.  

Could you break down how to build the box?  Here is how I visualize it:  a 'coffin size' wooden box that is mounted on locking casters....so they keep the box 2-3" above the floor.  You mentioned using cement board (you used another name) - on the bottom of the box.  You also suggested having a metal frame built to 'hold' the cement board - WITHOUT plywood.  Is that correct?  Would one thickness of the cement board be adequate, since it will not sit directly on the floor?   Would you also use cement board for the sides and ends?  The product I'm familiar with is 'breakable' - though it requires some pressure, compared to plywood.  

That said, I suppose the rest of it could be constructed like the gravel/rock unit that was built in Paul's house.  I'll review the video again.

Someone mentioned a possible problem with running the flue up through the chimney.  Since the pipe will be connected, all the way to the roof, I don't understand a possible smoke or draw problem.  What am I missing?

Again - thank you for your help.  It's starting to get chilly here - so I need to get busy!

Your thinking is similar to my own and I've done a lot of designing and pondering about it but never had time to actually build it. Someday. So of course I want to learn from any mistakes you make

PVC is a bad choice, not least because it's a symbol of all that is evil and unholy in the world. It's just not meant for temperatures above about 140F. Copper piping is just as easy to use once you get the hang of it and can handle far higher temperatures without degradation. In my own designs it's treated like a radiator rather than individual vessels. About the biggest you can realistically get is 4" so that approach just made sense to me, especially from the heat transfer point of view. Oh, but in my designs (for a greenhouse actually) I employed a pump and/or gravity feed to circulate the water through the piping from a central tank.

Now on the steam front, I wonder if you have thought of a closed system? You need only at that point a pressure relief to get rid of excess pressure once the water heats up. That way you are not continuously generating steam into your home.

Hi Eric -
Hmmm... a closed 'hot water system' - using 4" copper pipe?   Wouldn't I have to sell a kidney to buy that much BIG copper pipe?     ;D     As a retired lady home builder, the only experience I have had (that involves 'pipe') includes the old plumber's adage:   "Just remember that _____ doesn't run uphill."   Not very applicable for a 'steamless' system.  Using the video of Paul's gravel/rock system (that was built in his house), how would I a) install the pipe (i.e. the 'design' it would be laid in), b) attach it to the barrel and seal joints?  How would water be added before the system was 'closed'?   Maybe it might be easier to get an old water heater, for the 'water mass', place it on it's side in t he box, and fill around it with gravel.  What about a drain for the system, when it needs to be moved?    

Good grief!    This all seemed so simple when I started!    ;D  Thanks for your comments!  Maybe we can work this out after all!

Well you don't need 4" copper. It's also not that hard to find old copper and piping, despite people literally ripping out wiring from houses to sell it.

Caveat: I was only suggesting a design that happened in my head and given that I have not yet built any kind of RMH, I could be blowing smoke. In fact I'm really on this thread to hopefully validate my thinking through the experience and expertise of others.

My design, which I believe is sound but more complicated than most uses a series of tubes to transfer thermal energy to the water, which is recirculated into a tank. So imagine if you had something like a car radiator buried in your RMH, then imagine water circulating through that to pick up heat. The pipes are all 3/4"-1" copper pipe and are connected to the tank in the same way you do any plumping. In fact  one of my designs uses an old water heater, of which I seem to have too many. If you used a barrel or something much smaller then you could braize/weld/solder a bulkhead into the tank. Same thing for a drain.

If I get some time today I will try to draw this out. It's really a lot simpler than you might be thinking. I hope.

For your case, the pond liner would also go on top as a lid so that the system was not open to the air (not sealed, but not freely ventilated.) I would make a sturdy plywood lid with the liner glued to the bottom. Extending the plywood an inch all around with a wood flange on the bottom would lock it in place.

The water tank set on its side next to or on top of the duct could work. A duct that goes down one side of the box and back the other would cradle the tank and give it better heat-absorbing capacity.