RMH Design Review

Long time learner, first time poster, wanted to run the design of a batch RMH I’ll be constructing soon for a home I’ll be building.  The Home is heated by water with Radiant floors, the intent of the RMH design is to be light weight, low mass and not to put heat directly into the home, but send all of the heat into a 350 gallon non pressurized hot water storage tank.  That tank is heated by a large parabolic linear concentrator during the Summer and the RMH during the winter providing both domestic and radiant water heating.

Design Breakdown:
RMH is a 8” Batch Box design using perterbergs dimensions from the available excel file.  I did modify the riser (see photo’s) to help keep the overall height of the assembly down.

The water is heated by a 3/16” stainless steel “pot” that sits 8” above the riser outlet, the pot is filled with water automatically through a float valve and has a large copper heat exchanger submerged in the water pot to send the heated water to the storage tank.

The main shell of the assembly or bell is 1x stainless steel framing as the support structure with ¼” cement board exterior with a thin brick veneer for décor.   The interior of the bell will be lined with 2” ridged mineral board.

The RMH firebox core and riser will be made up of 2” of ceramic fiberboard insulation and the firebox itself will be lined with 3/8”ceramic baking sheets cut to fit for abrasion protection.  The riser zig zags up with a total legth at the centerline of 64.6”

The internal chimney is made out of 2” mineral board which extends to the bottom of the bell with an opening forcing the chimney to draw from the bottom of the bell.  There is a bypass next to the top of the ceramic riser for startup of the draft.

There are two doors on the front of this system, the door assembly as a whole unit is large enough to be unbolted so the entire core and riser can be removed for inspection, cleaning, and replacement as necessary.  The lower main door has a Neoceram glass insert, a primary air intake area of 10.9 sqin and a secondary air intake of 3 sqin with a damper to adjust air intake to the P-Chanel .  The secondary upper door has a Neoceram glass insert for visual observation of flame inside the riser which may need to be covered with 1” of Ceramic Insulation if temperatures exceed the 1292 degree constant rating or to reduce heatloss into the home.  The primary purpose of the secondary upper door is to allow you to place a draft starting Firestarter at the top of the riser on a shelf, secondary would be for visual.

Thoughts, opinions, and questions would be welcomed, just want to make sure there are not any glowing mistakes, this will not be an inexpensive build!  Also, Inspections, Insurance issues ect are not a consideration for this exercise.

Thanks for you time…Mike

Hi Mike;
I like it.   Your water heating design looks good.
I'm not too sure about your zigzag riser performing as you are hoping, but it will work.
So your copper heat exchange is a separate system than the open 350 gal tank?
Or is the copper somehow getting its water directly from the tank?
Your 3/8 ceramic baking sheet as an abrasion protecter for the CF board is interesting.
What is the temp rating on it?  Will it retain its rigidity when becoming super heated?

I do not know if your system will be highly efficient as far as pollutants from the chimney but it should make you a bunch of hot water.

Keep us posted and include photos !

thomas rubino wrote:Hi Mike;
I like it.   Your water heating design looks good.
I'm not too sure about your zigzag riser performing as you are hoping, but it will work.
So your copper heat exchange is a separate system than the open 350 gal tank?
Or is the copper somehow getting its water directly from the tank?
Your 3/8 ceramic baking sheet as an abrasion protecter for the CF board is interesting.
What is the temp rating on it?  Will it retain its rigidity when becoming super heated?

I do not know if your system will be highly efficient as far as pollutants from the chimney but it should make you a bunch of hot water.

Keep us posted and include photos !

The zigzag riser is the main area of performance concern.  
Water is pumped on demand with a differential controller that looks at the temperatures of both the parabolic water heater and the RMH heater coil sending water to the hottest heat exchanger to be heated and returned to the tank.
The 3/8 ceramic baking sheets are rated to 900f, if that doesn't work I may need to look at some of the refractory coatings. but I do believe it will maintain its rigidity but I don't have any personal experience with it in this application.

Thanks for the reply

Hi Mike;
Yikes only 900 F !  I think you might want to start looking before you light the stove off...
When I built my 7" batch I had brand new insulated firebricks! Rated to 1750 F...
They failed completely within 5 days of testing!   They looked like burnt toast and cracked apart.

What you want to look for is sheet metal ( RA-330  ) Not cheap , this is the only metal that can be inside a batchbox other than the secondary air.
I believe Matt walker used 1/16" in his core. Said it has warped a bit but is still in service
Another option would be heavy 9mm stainless grills cut to size . They would provide the abrasion protection you need and would last a moderate amount of time before replacing.
They are also easy to find and not overly expensive.

So your firebox itself failed, or was it the riser?  Good intel on the RA330 alloy, may switch to that just for ease, I have a 5x10 CNC plasma table anything I can cut out on the machine is a time saver.

Hi Mike;  Was the box that failed. Specifically the rear of the box. The 3 bricks that made the port literally fell apart. The others were well on the way.
The bricks at the front of the box were still OK.
The riser was blackened and starting to crack I doubt it would have lasted much longer.

I rebuilt using heavy bricks in the box and a ceramic blanket six minute riser.