Firewood moisture levels - is my heater very fussy?!

Hi all
I suppose I'm just wondering if other people experience the same thing I am...

To work really well my RMH seems to need the wood to have a moisture content 10% or lower. It seems I can't buy local air-dried wood much less than 20% (although there are laws about how dry firewood should be), and buying kiln-dried wood defeats the point of why I built my heater.
My local firewood is mostly ash, due to dieback.

At <10% my stove is rockety and blazing.. vapourising wood very quickly into loads of brilliant heat.. but anything getting towards 15% and I get smoke, mass not heating up enough and a pretty weak fire.
When I first fired it up I was burning last winters firewood (so down maybe 2.5 years and undercover for probably 18 months) - it ripped!
Now I'm onto new stuff that is closer to 20%. I'm having to split it right down and dry it on the bench and bell top to try and keep the drying cycle going.
Obvs now I know this I'm stocking up and will be able to work a year ahead from next winter.

Just wondering if other people find the same? Realise that the drier the better probably, but maybe my RMH is a bit fussy somehow?

Thanks all!
Pete

To burn wood properly as a dedicated wood stover you need to collect heaps of wood and allow it to dry under cover for a long period.
In Australia its a function of wood diameter for unsplit logs.
[ I just cannot find it at the moment ]
In Australia  the best TIP is : Remember, a perfect log of wood will have been dried out for between 2-4 years (depending on the type of wood) before it is ideal for burning.
If its split and made smaller less time will suffice, but most people do not split green wood.
Locally you will find what works, but I can say you need to think about storing a yeras worth of wood before you use any of it.

15% is really dry. I consider anything below 20-25 plenty dry enough to burn. What you may have is a weak enough draft that as the moisture content gets above a certain point, the latent heat released in condensation at the end of the pipe is slowing things down even more.

How tall is the flue? Is it insulated (double wall pipe) after it exits the building? Can you get some stack temps with a candy thermometer to see how hot the exhaust is?

Can you tell us more about what style of RMH you are using?

Hi all
Thanks for the replies!
I’ve built a Matt Walker batch heater with a bench. Our bench is maybe on the ambitious side.. it’s 10’ long and L-shaped (round a 90 degree corner). Photos attached.
The whole experience was brilliant - Matt’s support throughout has been invaluable.  I did email him earlier and his take was similar to yours John - that wood like ash should really be seasoned for 3 years before it’s optimum for burning. Given that our heater was working really well on the 2 year seasoned it seems it will just be a case of storing enough wood to use in the years ahead.
Phil - I’ll look into measuring exhaust temp. Our chimney is about 13’ of twin-wall insulated flue in total, with about 5’ of that above the roof. There’s a 1.5’ section of single wall flue just after it exits the bell - I’m typically measuring 135 degrees F on the surface of the single wall flue, around 1’ above the bell top.
Thanks again!

I don't have a RMH but I have burned wood all my life for heat. I usually figure out what is seasoned perfect and what has a bit of moisture still remaining early on in the season, and I mix the two. I add some of the less seasoned wood to hold the fire longer and I keep the more seasoned wood to keep it burning hot. Its a blend.

Beautiful setup you've got there!

Pete, that is a seriously smooth looking unit. Well done. Is the entire flue run vertical apart from that little angle dogleg coming out of the bell?

Pete,you seemed to imply that your stove is not a new one? One or two years old?

When was fly ash cleaned last?

Thanks for the kind words!

Phil - the flue runs vertically down to floor level through the bell then 90 degrees to horizontal and then turns 90 degrees and into the first section of bench. It stops about 1' from the wall at the far end of the first bench at the moment. We may extend it round the next corner depending on how it performs over the winter.

SA - oops..  didn't mean to imply that. We fired it up for the first time about a month ago. I had some wood that was a couple of years old (that burnt really well!). I'm planning to open the bench up to reinforce the concrete slab tops with more angle iron (mainly for piece of mind) soon, so will have a look and a good clean out then.

Thanks as ever for all your input!

Well, i would watch for restrictions on a new build, if it is fussy with wet wood.

Hi.. thanks for that.
I'm thinking restrictions are any narrow sections or bends in the system and any irregular shapes inside. Not sure I'll be able to change the shape of it much, but we could block off the second leg of bench at the point the bench turns the corner. That would reduce the overall bench leg by about 1/2 and also take out a 90 degree bend. Would that make a considerable difference do you think?
I'll open it up and make sure there aren't any temporary restrictions (bit of insulation fallen off, a pile of ash, etc) inside too..
Thanks again for your help!

Pete, do you know your ISA? System size? Top gap, side gap, transition area? Photos of the build? Because my own batch isn't too fussy once lit!

Hey Satamax
Thanks for that.
I don’t know the ISA or exact internal measurements (I’m away working atm so can’t measure - can do when I’m back).
Design is a Matt Walker 6” batch heater with a 6” chimney flue.
We started with a single leg of bench heated.. it worked great, so we added the second. I think it still worked well, but quite shortly afterwards we ran out of 2 year seasoned wood and started running into problems.
We could try and go back to one leg heated and see what difference that makes.
I’ve attached some pics of the build in case anything jumps out at you..
Thanks again!
Pete

Hi.. should add that we insulated the chimney flue..
Thanks!
Pete

Ok. Well, if Matt is behind this, what does he say?

What i see is lots of elbows. Did you extend that pipe into the second leg?

Hi Satamax
Sorry for the delay in getting back on here.. busy times!
I've messaged Matt about it and his take was to definitely leave wood to dry for multiple years (minimum 3) and possibly removing some insulation from the chimney flue where it runs near to the heat riser to help with smoky start-ups. He also gave me ALOT of info on different ways of measuring and issues around putting a number on wood moisture. Of course there's lots more to it than I understand!

I do want to try and keep the system retaining as much heat as possible, so I haven't removed chimney insulation.. instead I bought a 1/2 pallet of compressed sawdust heatlogs as locally as possible and paper wrapped. They work really well. Our smoke problems are pretty much gone (now just puffs a tiny bit from the kindling when it's starting up from really cold).  

I didn't extend the chimney flue round into that 2nd section of bench, and the back wall of that bench is a solid internal wall of the house (bathroom on the other side). It works really nicely as a very low level radiator in the bathroom to take the edge off the cold, but I realise I might be at the limit of what I can expect from this configuration.

I did wonder - I should be trying to burn as quick and hot as possible right?  As I understand it, hottest fire combined with coolest chimney = most efficient use of fuel.
With the door on now, and using the heat logs, it is possible for the fuel to burn slower and cooler.. I guessing that might cause build up of nastiness in the system? Anything I really need to watch out for here?
I'm hoping to open it up in the coming weeks to see what's happening inside...

Thanks!!
Pete

You definitely want to be burning as hot as possible, and generally that means fast. A batch box is designed to burn at a steady rate while combusting as completely as possible; Peter van den Berg's tests have generally run for around an hour for a 6" unit.

I don't think I would extend the exhaust duct any farther into the bench cavity. Flow will be sedate enough at that distance from the core that I think you will get good stratification in the bell and maximum heat extraction without more duct.

Glenn Herbert wrote:You definitely want to be burning as hot as possible, and generally that means fast. A batch box is designed to burn at a steady rate while combusting as completely as possible; Peter van den Berg's tests have generally run for around an hour for a 6" unit.

I don't think I would extend the exhaust duct any farther into the bench cavity. Flow will be sedate enough at that distance from the core that I think you will get good stratification in the bell and maximum heat extraction without more duct.

Well, looking at how Fraser Stewart has done it.

https://permies.com/t/40/149538/Tips-Cob-vertical-bell-barrel#1341318

I think to gain another 6" of stratification,  that idea is the right one. And it might solve some of the issue, since the end of the slit will be closer to the vertical chimney, Than the actual opening.

Pete, if you have access.

Block the end of that pipe in the corner of the bench.

And cut a slit on the bottom.   Of something like 70x4 cm.  Like in the pictures in the thread above.

That's better than the pipe's end in the corner imho.  Which gives less stratification, and may be more friction.

Back to your burning problem.  Can you tell me if you have water/sap boiling off at the door end of the sticks ?

How have you tested for 20% moisture content?

I wonder if your moisture content is not far higher. I have had problems in the past burning my big batch rocket. But with wood wet to the touch, when you split it.

I fully concur with the slit idea - anything to get the exit lower in the cavity and possibly reduce friction. The more height above the exit, the more effective stratification is likely to be.

Pete, very nice build! My sole heat source is a 6" batch-box (built in 2015/2016) that employs the all brick combustion unit as per Peter van den Berg, pushing 26 feet of 8" dia. horizontal exhaust in a 10-foot long mass bench, to a 15 foot tall 6" Class-A chimney. My favorite firewood is ash, and there are tons of it still remaining around East Tennessee, beetle killed though. When I first built my stove, I ran it with 3 year seasoned ash, and man that stuff burns hot! Even with my previous "box stove" I found that ash does need about 3 years of drying for the non-beetle killed wood, and about a year for the beetle-killed ash.  To get my RMH quickly up to temperature from a cold start,  I'll load in a layer of finely split (1/2" ~ 1"dia.) seasoned poplar, or cedar, or arborvitae cuttings on top of paper and kindling, then a couple of layers of 2" dia. ash sticks on top of that. Then the reload/second batch will be 100% ash, or ash mixed with whatever else is in the wood shed.

For the reload/second batch, I've tried under-seasoned ash and my stove just stalls, loafs along with a slow fire not putting out much heat, and making a smoky exhaust.

Thanks so much for the replies and kind comments!
The slit along the bottom + blocking the end for more stratification sounds like a great idea.. I'll def have a go at that when I open it up.
I've got alot more to learn about testing the moisture levels too - Matt said moisture levels using a meter are taken by splitting the log and testing the inside. I was getting 20% on the outside which means the wood is wetter than that in real terms.  
I've split a couple of loads up (mostly ash) and will leave it stacked under a roof for next year.
Using the heatlogs is quite different. They're really dry, so burn well and no smoky start-ups now, but they're designed to have a really high BTU, so I can only really put a few in at a time.. don't think I could fill the batch as I was with wood - it feels like that would be too much heat - we can get the batch about 1/3 full max.  We do then also end up dropping one or two 5" heatlogs on to the embers of the last and keeeping it ticking. This feels a bit more like conventional woodstove use, and although the body and bench do get really hot and stay hot, I know that's not how these systems are designed to run. Although the heatlogs are also designed to smoke as little as possible, I'll need to open it up and see how dirty it is inside.
As soon as I do that I'll report back...
Thanks again for everyone's feedback!

Pete,
I see that you are drawing the from the end of the bench. This is interesting as it is not in the plans. Does this help keep the bench at even temps along its length?
Kevin

Hi Kevin.. yes, it should be pulling more heat down the bench. We were going to try and carry the pipe on round the bench corner, but more turns and longer flue pipe would reduce the draw too much.
We used a design from Matt Walker (who was brilliantly helpful all the way through!).. Best.. Pete