How hot a fire do you need to make a thermal mass bench?

This is a stove we have bought for a greenhouse lean-to.  https://www.woodburnerstoves.co.nz/product/bucket-stove/   Its purpose is to heat the greenhouse, provide free cooking, and also to be a back-up to heat water for an underfloor hydronics when there is no sun from DIY solar hot water panels.  I plan to heat water in two ways - a heat exchange coil in a pot on the stove and a water jacket flue like this:  http://www.axeman-fireflue.com/  

I have also wondered however, about building a gravel box thermal mass with the water pipes build in.  I have two questions - how hot does a fire need to be for a thermal mass bench to work (ie. draw the smoke down and out)?  Secondly, will the water jacket draw most of the water from the chimney anyway? I also have thought of building a thermal mass of bricks round three sides of the stove box, as I am going to put cement boards  round it anyway to protect the plastic walls  from the fire heat.  I could also catch extra heat from the chimney by putting a sideways u-bend in it above the water jacket before it exits the lean-to.  The top of the lean-to is 3m high, so there is a bit of room.  Would this also slow the flow a bit and give me some more heat in the water jacket?

I also have the frame of a second-hand 300m2 (3000 foot square) greenhouse awaiting the money to have someone help errect it, and the firebricks to build a batch rocketstove inside it and take back to the house, so this is really just a temporary solution to get through winter (which has just started in New Zealand) as our indoor wood stove/boiler cracked a pipe in an earthquake and had to be taken out.  I am expecting that this cook stove may only last a few years of constant use as well.  I am effectively a sole parent on acreage, so time and money is quite limited to experiment, but wood is free for the splitting from rounds.

Kia ora Annie. Are you thinking of hooking the flue of this wood stove to a thermal mass bench? I don't think it will work. The design of a rocket mass heater relies on a combination of factors which all work together as a system to produce the super-efficient burn and draft. A wood stove needs the stack effect to produce and maintain its draft, and this requires the exiting flue gases to be 200-300 degrees to keep the upward motion strong. Very different from the operation of a normal RMH, which is using the barrel as a sort of pump to push the gases along. This push is strong enough that we can wring out most of the heat in a mass of earth or stone and by the time the flue gases go up the stack they might be as cool as 40-50 C, with precious little stack effect left.

The mass around the stove is a really good idea, though. You can probably get away with a short horizontal run to wring a bit more heat out of the stack, but be prepared to abandon the design if it doesn't draw well. I don't think it will add any heat to the water jacket, because that's produced closer to the firebox if I understand the stove design.

I've got a little J-tube RMH in my glasshouse and it keeps things from getting too cold. If you can build a decent batch version it would be an awesome addition to your greenhouse.

Hi Annie;
That is a very nice looking cook stove you have purchased.  Unfortunately asking it to run horizontal and heat a mass is most likely to fail.
It will work wonderful for heating your water , cooking on and keeping the room warm. But it must be vented properly or you will have problems.
Trying to slow the exhaust flow will create creosote , this is not good.
Rocket stove's need exact sizing and are run wide open to create the extreme heat that can travel horizontally before rising and leaving the building. That is how they are designed.
Your cook stove is intended to cook on and provide nice room heat. Trying to ask it to perform like a rmh is like asking a VW beetle to compete in an off road rally race with awd turbo Subaru's...
In the future when you build your batchbox you will experience the incredible roaring fire that is created with Peter Bergs design... there is no comparison to any other wood burner, they are a breed all alone!
 
Using cement boards or sheet metal to protect the plastic is a good idea.
Adding some brick is OK... adding a lot of brick too near your stove may not be such a good idea.
Metal stoves are designed to shed their heat quickly, they will quickly warp if overheated.

The Axeman fire flue is a really neat product! I have never seen one before.  It should do the job admirably.
Would love to see pictures of your water heating setup after you have it up and running.


 

Hi again Annie;
It sounded like you have already purchased your cook stove.
But I wanted to tell you about Matt Walker's riserless  cook stoves. They use a similar principle as the batch stove but in a cook stove arrangement that can safely heat a mass.
Matt sells detailed plans that you purchase material locally and build yourself.  He offer's outstanding support during construction.  

Here is his website)http://walkerstoves.com/index.html

Hi,

Thanks for your replies.  I have decided that rather than put bricks directly against the stove I would make a Gabion wall enclave for it.  I already have the wire mesh to do it, and river rocks are free for the taking.  (https://gabion1.co.nz/prices/  this is what are called Gabion walls in NZ)  It will not touch the stove but will hopefully still absorb quite a bit of heat, and should give sufficient protection from the fire heat to the wall behind it.

I have also found someone in NZ that has a 130mm water jacket prototype sitting on the shelf that they will sell me for lest than half the cost of the Australian one.  The first break in the stove pipe is 60cm / 2 feet up.  I was thinking of putting the water jacket above this, and of putting rockwool insulation around the flue below this, as well as putting a piece of rockwool over the stovetop when I am not using it.  Would doing this help  increase the combustion of the stove, and would there be any danger in doing this to the stove?

The burn chamber itself is a 28cm diameter x 35cm high bucket, with a 6 cm diameter hole in the middle of the bottom of the bucket that sits on a 5cm hole through the bottom of the stove (with a damper there).  Then it moves up and sideways under the stovetop to the chimney. The top airflow damper to the chimney on full is 3 x 6cm diameter holes, so the smallest point in the whole system is the 5cm hole at the bottom of the burn chamber.  Because the round lids on the stove top are not sealed, you probably always get a small secondary airflow just before the flue, and could increase this by lifting a lid a bit.

Annie

Here's a conventional woodstove pushing exhaust through a masonry thermal mass.

https://www.permaculture.co.uk/articles/turning-wood-stove-mass-heater