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.

Annie -

Another thought that I dredged up from the dark recesses of the old noggin: somewhat in your neighborhood (NZ, anyway), there is a company marketing something badged as the "IntensiFire".
https://intensifire.co.nz/

Depending on the interior arrangements of your stove, something like this may be possible.

As I understand it, the stovepipe is extended down into the fire box, with the lower end protruding into or beneath the coal bed.  With over fire primary draft, the theory is that the partially combusted volatiles are forced to flow through the hot coal bed before exiting up the stack, converting the stove into a downdrafter or gasifier.  Making one of these work with an under grate primary draft would be more monkeying around, since you'd need to convert to over fire draft, by hook or by crook.  You might also need to arrange for a stack bypass for start-up, though proper fire starting technique might head that off.

A DIY version seems doable, though I have a hunch that something as pedestrian as standard black stove pipe - and maybe even stainless stove pipe - wouldn't hold up for very long.  But, maybe long enough to see if it works for you.  Some of the fancy Rolled Alloy steels (I don't know if Thomas can source sheet) would be more durable, if an initial experiment in stainless or Ti sheet proved it out.

I have no idea how well this conversion works in practice, and I can imagine several pitfalls.  But, since it looks like they are just next to you (from my vantage at the Antipodes), maybe there is an installation which you could observe in operation, or a local user you could quiz over a pint or cuppa about their experience with it.

Even with such a combustion enhancer, I think I'd still be entertaining additional insulation inside the fire box to protect the steel/iron stove shell.  Anything you can do to improve heat transfer into the water (or decrease the losses from the water by adding insulation, as was suggested upthread) may also bear fruit.

Hi,  

Thanks for your responses.  Here is some answers to questions and photos:

THE WATER JACKET
I got this from Spiroloc in 2020, but only just trialed it for the first time last week.  It was a one-off he had hanging round and sold me.  Attached is a photo of the top of the water jacket.  It appears to just be two cylinders welded together, and the water fills the middle and no coil inside.
They now make these water heaters: https://spiroloc.co.nz/product-category/wood-fired-water-heaters/   Occasionly photos pop up that shows the inside, and it is a large coil.

I actually bought it to use with this bucket stove/oven ?2019 and only fired it up for the first time last week as well.  https://www.woodburnerstoves.co.nz/product/bucket-stove/
A wood stove shop owner saw it and said it was a great stove to cook on and my water jacket was also great, but I would burn out the stove heating the water jacket, and sold me the big metal stove it is with now for $100. I think the bucket stoveoven is working too well and lacking for secondary air, but that is another post.

I paid a few thousand dollars for the pipes and pumps and controllers to  pump heat under the house, and into a greenhouse, and put 400m of piping under the house  floor before polyester batts underfloor insulation were added under a gov stimulus grant. Then various crises struck, and the parts all sat unused for the next several years.
I am quite happy with the idea of starting again rather than putting more money in stove as it is among the cheapest conponents.  



THE STOVE
I include a few photos of the stove.    It is an old Kent stove, but I can't see a model or year of manufacture.  I It is just a very heavy old rusty cast iron box, now but I am guessing it had a baffle and firebricks at some stage as there is the frame to hold the baffle inside.    It only has air holes above the door at the front that can be dampened down. There is no air inlet or grate at the base.  I think it is a model that works like this:
https://hearth.com/images/uploads/kentbrochsmall.pdf    diagram on page two
https://www.woodmanspartsplus.com/xref/WoodCoal/KentStove/TileFire   (also described here
The firebricks and baffles can still be bought online, but they are in the hundreds of dollars, and there are different models/sizes.

Later models have secondary burn air pipes, but I can see no evidence that mine did.  There is a video here of what they look like:  https://www.youtube.com/watch?v=PWceJwJxm3A
I can't find online anywhere where that shows where  the air comes from for these tubes.

It also seems that the chimney used to have a dampener, but there is now just a metal  hook left.  That hole for the hook lets in a little air, so it may act as a sort of secondary air inlet, but maybe too high to be any use?

MATERIALS
We are not adverse to buying materials, but we are on a limited budget right now.  Our main immediate goal is to stop payng $400++ a month for electricity and still freeze our feet in most of the house, and to start growing seedlings and greenhouse vegies without them freezing and get off disability/unemployent payments.  We don't care if parts of it burn out in a year or so.

The beach is 7km away and river sand and rocks are available free for the taking nearby.
We are in a sandy area, so no easy clay/cob options.    We may be able to source natural clay somewhere, but it is not easy to transport in a car. Pumice stone is relatively cheap to buy bulk locally as a gardening product - $42 for a tractor scoop (?200L) and $210 for a cubic metre, and free trailer use to take it home.  
Fibreglass / rockwool insulation batts is readilly available, and we have a couple of bags of fibreglass batts insulation. Metal sheeting and copper is much more expensive in NZ than the USA.  

The factory that makes this fire morter is actually in our tiny town and sells cash to the public:
https://mastermix.co.nz/product/fire-mortar-mix-concrete/   We can get 30kg for NZ$69

PUMICE FIRE BRICKS/CORE OPTIONS
About 9 years ago we tried to make a rocket stove core with this fire cement and perlite.  I forgot to take into account that 1 cup of perlite and 1 cup of cement will not equal 2 cups of mixture (due to all the air gaps in the perlite), and we did not make enough to fill the mould.  What we did make was so heavy we knew we could never easily move it,  and we went no further.

A while later I was speaking to someone that makes and sells light fire bricks, and sold the Master-mix fire cement, and I am pretty sure he said the recipe for their fire bricks was 9 parts pumice stone to one part fire cement.  I found a few other websites that speak of pumice, and it is meant to be less durable but more insulative.
https://www.hearth.com/talk/threads/firebrick-pumice-lightweight-vs-standard-heavier.128343/

We do have longer-term plans of making a rocket stove core with the pumace that we can carry to the greenhouse and heat up rock/sand beds if we had a week of rain.  It might be better to focus limited time and money just on making that rather than improving the firebox.  I am a maths secondary teacher with 4 years of technical drawing in High School, so I think I could create the mould to make a rocket core if I can find the dimensions. I like the idea of the core going though a box of sand, it was sit up at a show in a day, but I can't find the link right now.

It is years since I researched it, and I never did feel confident making a rocket stove, and wonder if the DVD's on permies.com would be a good investment.  
Most stovepipe in NZ is 150cm / 6 inch, and I hae salvaged a bit of it, so I would like a plan to fit that.  (I guess a 6inch box).


Looking online it seems the baffle in the KEnt stove is a metal plate with a firebrick bolted underneath.  I also wonder if it would be possible to make a firebrick baffle with chickenwire inside it that would last a while at least.  I am wondering if making firebricks for our kent stove would be a good way to trial the process and give us an instant start to warm the house and start growing seedlings (late winter here), and then make a core.  
The baffle firebrick would have to be 40cm x 30cm to fit the metal bracket.  The gap from the bottom of the bracket to the top of the fire is 5-6cm, so if the baffle brick  as 3cm high that would still leave2-3cm for the smoke to move over.  

I think years ago someone else looked at the water jacket and suggested that I put a small length of chimney pipe between the stove and the water jacket to give the fire more time to burn fore the water jacket takes out heat.  It would not be as stable though.  


INTENSIFIRE
I have not ever seen one nor know anyone with one, but knew of their existence.  I had already written to see if it would handle the weight of the water jacket on top, and am waiting a reply.   But NZ$500 is a lot of money for us right now.   I keep hoping a cheap second-hand one will turn up on trademe (NZ ebay equivalent) or Facebook marketplace, but no luck so far. I must confess I don't understand Kevin's suggestions how to make my own equivalent.  

FREEZING PIPES
I am 7km from the sea.  The lowest we ever get is -4C, so freezing pipes are not a major problem.  If it is cold enough to go below zero, it is a clear night, and pretty much always a sunny morning and it is all melted by 10am.  We run rainwater from a tank through a pump and to the house without a problem.    Occasionally 1/2" black Polyethelene pipe running on the ground to a water trough and the top of water troughs themselves freeze overnight.

Annie - I've got an Intensifire. It's been in for two years now and it really makes a difference. Not cheap but I would estimate at least a 25% gain in heat output for the same amount of wood, and zero visible smoke, which is a testament to the efficiency. I have to prewarm the flue when I light on days that it's not really cold out, because the downdraft is working against the weak draw of the chimney. The cast refractory parts are starting to wear a bit and I don't know how long to expect them to last, but Jason did replace the height block for me when the first one failed. The stainless flue extension will probably outlive the firebox.