Bricks or insulation - What should I do to this firebox

I am trying to make this set-up (in picture) that has the primary purpose of heating water.  We are wondering if the best thing to do is to wrap the fire box in rockwool batts and then a metal shell like the rocketovens on this website, (make the shell purely insulative) or to build up bricks around and top of it (so there is also some thermal mass - but would this reduce the burning efficiency).  We are 7km from the beach with very sandy soil. so cob is not an easy option.

It is a purpose built water jacket we bought, and it has the chimney heat going through the middle of it, and water pumped in the bottom and back off the top.  We did a trial in an open 140L metal tub of water, with no attempt at insulation.   In the first 15min of burn it took the water from 9C to 19C, by the end of a 1 hour burn it was at 38C.   So I reckon it will give 5-10KWH of heated water with proper insulation of the water pipes and storage container.
This stove is not meant to be our main heating source.  It is meant to be the back-up after passive solar heating and thermal mass and solar water panels storing heat in water and pumping it under the floorboards are not adequate.

We live in Temperate NZ where our mid-summer highs are an average of 23C.  Warm enough so that we don't want it indoors, but cold enough that we don't go swimming in nature and would like to make a heated spa and/or pallet pool.  We are thinking of making a small outdoor bath-house and boiler house shed and putting it in that.  We run the house on rainwater, but have unlimited high-iron bore water (that stains the bathroom fittings) so we could use this in the out-door bath house.

Separating the heat extraction from the combustion zone, as you are doing here, seems like the right approach (rather than having a water back directly in the firebox).

Is this stove made from welded plate steel, with a fire brick lining?  That's what I'd expect in a US context, given the photo.

How is the primary draft configured (i.e. is there a grate, or is the draft over fire, with a hearth)?  Does this stove have a secondary combustion air inlet?  A catalytic combustor element?

Is freezing of your water loop a possibility in your climate?  Is this water jacket basically a double wall chimney with water inside the enclosed annular space, or is there a coil of tubing wound on the interior of what we can see in the photo?  Or, perhaps something else?

Could you purchase fire clay, perlite, refractory cement, etc. - typical stove building and mending supplies, or are you trying to be self-sufficient in this endeavor?

My first inclination, without knowing the answers to the above, would be to improve the firebox insulation within the metal shell (thus protecting the shell from high temperatures) to improve combustion efficiency.  Adding preheated secondary air, or a catalytic element, to effect more complete combustion may also be low-hanging fruit, again depending on the answers to the above.

In the long term, it might even be worth considering building a rockety combustion system for the water jacket, and using this stove for other purposes.

Since this is for intermittent use (not your primary domestic heating), the payoff for squeezing the last scrap of efficiency from it may be less critical - but only you can judge that, based on  your physical ability to collect fire wood (now in in the foreseeable future), the availability of fire wood, etc.

Hi Annie;
Interesting setup for water heating.  Like Kevin, I am curious what the inside of the riser looks like.
Here is the problem with your idea.
If you wrap the box stove with insulation or cover the outside with bricks, the metal of your box stove will warp.
It might take some time before it becomes a problem, but it will happen.
Leaving it as it is will probably last for years.
Perhaps lining the inside with firebricks would help, but it reduces the size of your burn.

I would use it as is and begin experimenting with bricks to see if a Batchbox design or even a simple J-Tube design is a better alternative.
Is there a commercial name for your water heating riser?  I would enjoy seeing what they have come up with.

Annie Hope wrote:I am trying to make this set-up (in picture) that has the primary purpose of heating water.  We are wondering if the best thing to do is to wrap the fire box in rockwool batts and then a metal shell like the rocketovens on this website, (make the shell purely insulative) or to build up bricks around and top of it (so there is also some thermal mass - but would this reduce the burning efficiency).  We are 7km from the beach with very sandy soil. so cob is not an easy option.

I think general consensus for the great efficiency of RMH's is the fact that they burn so hot in the burn chamber and recover so much heat with the mass. Yet still letting enough warmth move through the flue as to NOT STALL.  So your question about reducing efficiency by the addition of mass would not be valid, but also, if I am reading this right, your wanting as much heat collected for your water as possible? A worthy goal.

Annie Hope wrote:
It is a purpose built water jacket we bought, and it has the chimney heat going through the middle of it, and water pumped in the bottom and back off the top.  We did a trial in an open 140L metal tub of water, with no attempt at insulation.   In the first 15min of burn it took the water from 9C to 19C, by the end of a 1 hour burn it was at 38C.   So I reckon it will give 5-10KWH of heated water with proper insulation of the water pipes and storage container.
This stove is not meant to be our main heating source.  It is meant to be the back-up after passive solar heating and thermal mass and solar water panels storing heat in water and pumping it under the floorboards are not adequate.

Tom as always had some really good thoughts and questions, and one of mine, that was not mentioned, would be the need to keep the flue gases from getting too cold (thus potentially causing a chimney stall or a lot of condensation)   but it sounds like you have been playing with it and getting along. But something to keep in mind, is the possibility if you perfect the collection of heat to much, you could rob the flue of necessary flow direction (up)  of flue gasses.  This is perhaps a minor thing, and easily monitored by a chimney prob.   I does sound like a great project!

Best of success.
Scott

I think there are good suggestions here.
I would only add that building a boiler shed with good insulation and lots of shelving could also boost your efficiency.
The hot air trapped around the firebox and  riser will be insulative, and the shelves can be loaded with fuel, which will burn better because it will be dryer.

I agree with Thomas and Kevin.
An option could be to insulate the jacket itself and the flue pipe.