Matt Walker tiny cookstove build in Portugal

Jay Angler wrote:I had a possibly crazy thought - if you aren't using the oven right now, could you put slabs of marble with gaps for air flow in the oven section to add a little extra thermal mass until you are ready to build the bench? I'm not sure what the cubage is in the oven, so how much difference it would make?

I had a different but similar thought. If I remember correctly, and it's been a while so I might be mixing them up with something else, the Walker stoves are built such that different areas of the cooktop are different temperatures. If so, perhaps you could put a huge pot of water (large canning or stock pot maybe?) with a lid on one of the cooler burners as additional mass. Basically, the idea would be to heat it up, but not to a rolling boil. Similar thing could probably be done with the oven. My thought it that when you don't need it you can use the water and store the pots. Heck, if there's a spot that's the right temperature you might even be able to make something that takes long simmering, like bone broth, some days.

Thyri Gullinvargr wrote:...the Walker stoves are built such that different areas of the cooktop are different temperatures. If so, perhaps you could put a huge pot of water (large canning or stock pot maybe?) with a lid on one of the cooler burners as additional mass. ... Heck, if there's a spot that's the right temperature you might even be able to make something that takes long simmering, like bone broth, some days.

That is exactly how the walker cooktop works, and when the downstairs area is ready to use as a kitchen we will indeed be doing that kind of thing. Hopefully next winter the bench will be in place and enough work done down there that we can experiment and report. I'm looking forward to it!  

But for now I'd like to introduce you all to the puxmi-pulyu!

That's pronounced push-me-pull-you in case you hadn't worked it out. Also puxe is portuguese for pull, which as you can imagine causes endless amusement watching the Ingleses failing to open doors by pushing on ones which are clearly labelled puxe.

This is the tool Austin made for pushing the hot coals to the back of the firebox prior to reloading, and also for pulling the cold ashes out the next day to put on the garden.



This is the business end...



Once the fire is reduced to hot coals, if we want another burn we shove them to the back of the firebox (near the secondary air feed) and lay more sticks on top. They will then burn nice and hot from the back to the front. If all the wood burns at once there is a bit of danger of overburn which is kinda wasteful as even this wondrous device can't handle all that heat at once without letting too much escape up the chimney. The bricks need time to absorb the heat, and the more it absorbs, the longer it can keep giving it out again.



The next morning we can pull all those ashes out of the way. There were a couple of days' worth in there cos I didn't do it yesterday. Ooops...

I might chuck those little unburned bits back in too. Waste not want not and all that!

That thing sticking up at the back is the tube that forms the secondary air feed.



The ashes pull out quite nicely with this little contraption. As far as the start of the air tube anyway...



There's still ash left beyond the start of the secondary air feed tube, but not to worry - turn the puxmi-pulyu on its side and it can fit in the gaps.



There. Ash from either side of the feed tube has been pulled forward a bit and can be pulled right out.



Photo to show how the puxmi-pulyu fits, just in case it wasn't clear enough last time.

Burra Maluca wrote:

Thyri Gullinvargr wrote:...the Walker stoves are built such that different areas of the cooktop are different temperatures. If so, perhaps you could put a huge pot of water (large canning or stock pot maybe?) with a lid on one of the cooler burners as additional mass. ... Heck, if there's a spot that's the right temperature you might even be able to make something that takes long simmering, like bone broth, some days.

That is exactly how the walker cooktop works, and when the downstairs area is ready to use as a kitchen we will indeed be doing that kind of thing. Hopefully next winter the bench will be in place and enough work done down there that we can experiment and report. I'm looking forward to it!  

Burra, I was thinking more in terms of doing this before you're doing more generic cooking as a way to increase mass quickly now, but in a way that is easy to "put away" later once you have the bench. 😉

Thyri Gullinvargr wrote:
Burra, I was thinking more in terms of doing this before you're doing more generic cooking as a way to increase mass quickly now, but in a way that is easy to "put away" later once you have the bench. 😉

I don't have the energy for any more experiments at the moment, especially before the downstairs area is usable. When I am able to be down there for longer periods of time with somewhere comfortable to rest up while I'm down there I should be able to do more. But for now I'm totally up to capacity!

Even my little dragons have noticed I don't have the energy to go outside much and have bought me a little vase which they are putting flowers from the garden in to keep me in touch with what's happening outside of my room...

Burra Maluca wrote:
I don't have the energy for any more experiments at the moment, especially before the downstairs area is usable. When I am able to be down there for longer periods of time with somewhere comfortable to rest up while I'm down there I should be able to do more. But for now I'm totally up to capacity!

Fair enough. 🙂

Burra Maluca wrote:
Even my little dragons have noticed I don't have the energy to go outside much and have bought me a little vase which they are putting flowers from the garden in to keep me in touch with what's happening outside of my room...

Lucky you. It's been below 0° F (-18° C) where I'm at lately. Flowers are but a memory at the moment. 🥶

Well yesterday was the first burn of the season.  I did prime the flue just in case, and I lit it with a regular lighter 'cos I'd mislaid the gas blowlamp.  But she did us proud and fired right up and burned very nicely.  As yet still no bench, that's lined up for building this winter with luck.

Today, didn't bother priming just bypass open and off she went.  Feels like I can shut the bypass sooner but that may just be the wood is nice and dry, so it gets going fast. Anyway, the stove is burning nicely.  It's not all that cold here, mind, but recent rain made the house damp downstairs.  

Great news, Austin!
I love the fall when the stoves get lit.
With your adjustments last season, dry wood, dry mortar, and of course, holding your tongue just right... She leaped into life!
Congratulations!
Perhaps this winter, after building benches, you will meet the requirements to become a full-fledged Rocket Scientist...
I will submit your name to the committee.
Just to let you know, your dragons do have representation there.
You're off to a great start!
Good Job!

Time for a stove update I reckon.  She's been running well this year, much easier lighting than last year.  I did have an issue recently with some of the firebricks in the core either getting loose (see another post about clay-sand mortar) or in a couple of cases breaking.  

Now, the core gets pretty hot.  However, I would expect fire bricks to cope with that. A bit of the clay mortar that was filling a gap fell out and it's semi-fired the clay so it's almost like stone.

Anyway, lifted the cooktop off and removed enough of the core top to get at the broken/displaced brick which was the top rear one in the firebox which defines the far end of the flue path.  That brick turned out to be in about 3 pieces.  As I didn't have a spare one that size to hand, I glued it back together with "massa refrataria" which is 1500°C-proof stuff in a tube, used to join chimneys onto stoves etc. and glued it back into place with the same.  I also found a couple other cracked bricks to fix while I was in there.  Mostly, though, it's all stayed together.

Next job is to build the bench, when I have time, then we can burn it more than 3 firebox-loads at a time and harvest some more heat.  Without the bench, by the end of the 3rd firebox of wood there's what I consider an unacceptable amount of heat going up the chimney.  3 fireboxes of good wood burnt leads to the whole stove body being nicely hot, so tend to do 2 or 3, then leave it for a few hours so the heat gets used up some, and then burn it again later in the day.

Do you think the broken brick is due to the supreme heat in the fire path Austin, or maybe down to a faulty brick?

Ah well that I can't say.  Since the whole core is made from the same bricks, and only 1 or 2 have issues, I'm gonna optimistically assume that it was a bad brick.  The one that broke is in quite a hot place, but so are quite a few others which seem to be ok.

In a recent podcast Paul mentioned his quest to shut down rocket stoves when you're done burning, so as to avoid the possibility of losing heat up the vertical exhaust.  

This got me to thinking about our Matt Walker stove.  I'd kind of assumed that without the fire driving it, the exhaust would fairly rapidly cool (after all most of it is outside, although insulated) enough to stop it drawing.  Recently however, on checking it some time after the fire was out, the stove was all nice and toasty warm and emitting heat into the room as it should, but feeling the base¹ of the exhaust it was still distinctly warm - and worse, there was a perceptible draft entering the air holes in the firebox door.  Clearly there was still some convection going on and heat was going up the chimney!

So, how to solve that?  I could make a cover for the air holes in the door which could be deployed once the fire was out to prevent air from entering.  But that seems a bit like work, and besides I kinda like the door as it is, aesthetically - a draft blocker would need some kind of slots to fit in which would detract from the appearance.

Now, today I had the fire on in the morning as it's a bit cold and damp.  I did 2 batches (initial lighting and one refill) and then let it die down, as without the bench I still haven't built it's prone to start wasting heat up the exhaust if you burn it 3 times in a row.  Once the fire was down to just a couple of small live embers I experimented with a handy offcut of plastic wrapping, and sure enough there was enough draft happening to suck that onto the air holes in the fire door.

At this point I had an Idea!

At the bottom of the stove is a black "oven" space although with the burning regime I've been doing it never gets all that hot.  I'd already noted that opening the door to that even slightly kills off the draft for the fire quite dramatically.  So, with the fire mostly out, I opened that door.  Immediately, the draft into the firebox ceased, and within a minute or so the base of the exhaust was significantly cooler to the touch.  Looks like I may have hit on a solution for stalling the flue so as to stop heat going up it after the fire is out.  Opening that door can only realistically let heat out into the room, which is no issue.  Unless somehow the exhaust were to reverse and suck cool air back down from outside, but I feel that's unlikely.

Later on I'll experiment with opening the door for some minutes and see what the effects are.  It might be that it can simply be left ajar until next time the stove is lit, or it might need to be shut again for some as-yet unknown reason.  


¹ the first bit off the stove isn't insulated, as it has no need to be and also that allows you easily to check if the flue is getting warm enough when lighting

Update on the previous post:

Last night when the fire was pretty much dead, I did some more messin' with the oven door.  

Opening the door a couple of inches made the exhaust, which was quite warm, cool down within a few minutes.  If I shut the door again, it started to warm up again.  

Of note is that I want to check the bypass to make sure it's shutting adequately; if it isn't then heat could leak out from under the cooktop into the exhaust.  If I'd been on the ball when I was fixing the broken firebrick issue recently, I could've checked it then

As may be, having the oven door opened a bit caused the flue to stay cool so I left it like that, and before I went to bed I checked it again: the flue was quite cold and there seemed to be a slight cold airflow entering the oven and travelling across the bottom of the stove into the exhaust.  Cold air doing that can't really have much effect on the heat output, I don't think, since the hot part of the stove is all above that level.  

This morning when I went down and checked it, the cooktop was still a bit warm to the touch which I regard as a good sign.  As to how much difference this makes overall, that would hard to test - but any obvious heat going out of the exhaust once the fire is out can't be helpful.