Hydronic heat RMH

I've been testing RMH for about two years now.  My very first was nothing more then 28 gauge Hvac ducts just to see how it worked, and it worked well.  I remember wrapping standard r13 around the j pipe and around the stack.  Within 5 mins the steel collapsed.

I was sold.  

Last night i started my first serious RMH, I designed it to heat water.  The one thing that always bugged my about RMH is there is no possible way to capture all the heat it produces, so I am determined to capture some of it in water.

The main burn chamber is large.  8" wide by 16" tell by 35" long. Overall deminsions are 16wX21"tX42L.  The body is made up of fire clay, perlite, Portland cement, mason lime, and silica sand.  

The riser is 42" tall with a 6" diameter exhaust.  Wrapped around the exhaust is 40' of type L flexible copper.  I then wrapped it with a 10" Hvac duct and encapsulate it with a perlite/refractory cement/fire clay mix.  

I installed a shutoff valve and after the valve I install a compressed air coupling on one end of the copper coil.   I did this so I could remove all water from the coil if the tank get to hot.  

The valve allows me to shut off the inlet so the air is forced in one direction, evacuating all the water left in the coil to pervent stem flash.

I tested the system before construction using cement block and black stove pipe for the riser.  Then insulated and covered in the same 10"Hvac duct.  I was able to heat 35 gallons to 170 degrees in about 60mins but the test system had many air leaks in the tunnel and wasn't well insulated.  

I now plan to heat 250gallons once I build the reservoir.  

Will report back once everything is cured.  To dress it up I'll do a stamp concrete patter over the exterior because these things aren't pretty.  Otherwise I'll show people and they'll assume I'm gonna burn the house down because it looks like Frankenstein in the corner. Yikes.

Any suggestions on a pump?  I was using a Danny mag drive pump rates at 500gph but it isn't rated for these water temps.  It only drew 35w.  That was nice.

Forgot to add that this heater is being used to heat my Wisconsin garage in the winter.  If it turns out to work well for a water heater, I'll drop serious dough to run insulted plumbing from the garage to the house to run hydronic floor heat.  

I'll run PEX inside of a 6" pvc filled with expandable foam.  My garage is attached so the run into the basement would be less then 65'.  

Then break it into zones and heat the first floor.

What is the 6" riser material you are wrapping the copper tubing around? Not metal I presume, as you have done the test to prove that can't work. Water piping close to the interior of the riser is going to cool the riser wall and reduce the internal temperature just where it is supposed to be the hottest to get complete combustion. Also, if the riser does get as hot as it should, the middle of the riser wall thickness will get halfway between the barrel airflow temp (say 500F) and the riser temp (say 1500F), or 1000F. It will not be possible to sustain water flow in the thickness of the riser wall.

The RMH works not by taking the heat from the hottest part of the combustion zone, but by letting it get as hot as possible for enough time to complete combustion, then harvesting the heat.

If you really want a hydronic system, the safe and effective way would be to have an open-air (unpressurized) tank or water jacket being heated, with circulating coils submerged in it to move the heat to where you want it. This keeps the maximum temperature in the tank to 212F, and since pressurized water has a higher boiling point, the coil will never flash to steam.

"there is no possible way to capture all the heat it produces"

I beg to differ... nearly all the heat in a properly designed RMH goes into the space, not up the chimney. If you mean capture it all to save for later or move elsewhere, that can be done without compromising the core function, with water jackets and other means.

I note your description of the firebox, which sounds like a batch box rather than a J-tube. Are you aware that researchers have extensively tested and found the best size, proportions and configuration for a batch box? Your burn chamber is large even for an 8" system batch box, and you may have difficulty handling all the combusting gases in a 6" riser.
http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions

Sorry I forgot a detail . The inside duct is a 6" Sonotube.   The coil is actually wrapped around a 8" stove pipe.  That sits over the 6" sonotube and then the 10" sits around the 8" stove pipe.  All encapsulate with the mix I listed before.

The box will get a steel door that operates as two individual doors.  So that either top or bottom can be opened or closed to adjust burn rate.  

So your water coil is actually in the middle of the riser wall thickness. That would be good, except that the entire riser wall is going to end up at several hundred degrees F at minimum, and liquid water will not be able to exist in it for more than a few seconds (unless it is being pumped through so fast that the riser is being cooled significantly). That much cooling will drastically increase the heat loss through the riser surface and cool the fire.

Everything is still curing, time will tell but it worked when I tested everything before with practically no insulation.  

Granted I'm not using this as a convention mass heater, solely to heat water.   I don't have a barrel anything.  The riser then directly outside.

I poured another 4 inches of concrete around the refractory riser to hold heat for the water to extract.

None the less,  when I tested with black stove pipe and glass insulation I was getting stem and extreme heat.   Maybe cold water will pose a different story and in that case, I'll use my solar heaters to condition the water.

Yep, steam and extreme heat is what I would expect to come out of the copper tubing you describe.

The issue is that when water flashes to steam, it expands about 1600 times, and anything but a short open-ended pipe risks bursting from the extreme pressure buildup before it can get to where it will cool down again. The stronger the pipe, the more dangerous a burst will be, although live steam blowing in your face can be fatal no matter its pressure.

I am afraid of that.  The return to tank is 22" from the main coil and the inside OD is 5/8". The return isn't submerged so I'm thinking minimal restriction is achieved. Also, the tank sits below the coil, and against common practice, I feed water from the top down to even further help promote less restriction if there is a flash. I'm theorizing that if and when I shut the water off, I'll evacuate the water via the compressed air coupling and any residual will flash out via the short return to tank.  

If temps get to extreme for straight water I will look into a propylene glycol/water in a closed system with separate heat exchangers forwarming the water reserv.  One step at a time if I survive.  

At any rate this kinda thing is more or less fun.  I love building and testing in hopes to loosen the death grip of dependency.  

Thanks for sharing concern, I do appreciate that.  Much love.

Did a test fire.   The temp is the top of the riser with water running.

40 gallons of water after 36 minutes.

I need to find a high temp pump that doesn't draw massive electricity.   I'm using a danner mag drive that once powered a dwc hydroponic system, surprised it is still working.

Still have lot of the forms on.  Not the prettiest thing but so far I'm happy.

I added another 2" of concrete around the riser.  Still have to pour the upper half.  

9 minutes later and another 10 degrees.  What's the ideal temp you think?

Top of the exposed riser.  

And then 8" down to the concrete mix housing the coil.

Burn is looking like it should.  

One full hour.  I'm calling it, hot enough.   It's a click shy of 180 f.

Looks like you're doing well. What is the exhaust temperature at the start of the chimney? If it's much above 200F you are wasting significant heat to the sky.

Having the coil open-ended at 22" from the riser is about as good as you can get for explosion avoidance, given the coil location. Good luck with it!

At that start where it begins off the fire box?   I won't ever really know because the concrete is so thick there.  I didn't get over 100 degrees last night.

No, the actual chimney, where its job is to get to the sky.

Well I don't quite have that figured out yet.   Right now the riser is 45" tall and terminates in the garage.  I was planning to route directly out a window located right next to it, but as you pointed out, I realized I'm wasting a ton of energy that way.  

Last night when I tested it I was venting in the garage with the overhead door open and a 1050cfm fan running but the heat dumping out was averaging 575F.

So I went out and bought 10' of more venting i plan on configuring to see if I can plumb it to extract some of that heat before it goes out the window.

Get a couple of barrels you can set over the riser, then plumb the exhaust from the bottom of the barrels to the chimney. With a space two barrels high, the hot gases will have time to radiate and cool before they fall to the bottom and exit. Peter van den Berg has documented a number of these.

Been a litte while since I been around.  I'm still testing out different ideas which has turned my heater into franckinstien.

The water heater works great but I still wanted to extract more heat which I was exhausting away.

So I setup up a mock double chimney.  The main exhaust 6", i then wrapped a 8" duct around it and rigged up a fan to pull air through the 8" around the 6" main exhaust.

Woah.  What a difference.  The air coming out is reaching 250 degrees at times.  

The main exhaust outside is finally not scolding hot which tells me I'm extracting that heat.  

No black smoke, all clean.

Here is my Frankenstein.

The top box is a manifold I suppose.  it's actually made out of cement board and i put particle board on the outside for rigidity.  

Once I get my final design I want to start new with permanent materials.