Reluctant to rocket

Hello good people

I have built an 8” system in my main living room … my problem is it is very reluctant to (truly) rocket, It has done it *once* … but it required the fire box preheated with a moderate fire for 15 minutes, and then a 75% load of thumb-wrist sized bone dry birch/oak mix to ‘get there’ .. I know it worked because my temperatures, which  stubbornly max out at 350f , suddenly shot up another 125f .. it was very like getting a speedboat up onto the plane. It also noticeably cleaned the soot from the (inside) top of the barrel.

My question really is should it be this way ?   I have to burn the thing *incredibly* hard to get there .. I mean white hot, can’t look at the flames roaring down the tunnel hot.

I was seduced by the various videos on you tube they showed what I would describe as a moderate fire.. which I would say would be 50% throttle on my stove, and purported to be ‘rocketing’ and running (that way) for a couple of hours would charge the mass for 24 hours

That’s just not my experience - at ‘full throttle/load’ this thing *rips* through logs .. like a sack full an hour … I have plenty of draw, but the fuel efficiency is nothing like what I had hoped for, I would guess about the same as burning the same wood slowly in my old ‘efficient’ (Morso) stove  

Is my 8” too big ?

I don’t understand it.. I mean the logs disappear, and the exit chimney temp is *maximum* 150f - so those calories are going somewhere … I suspect it’s out of the chimney as soot (possibly because I am just not burning hard and fast enough)

It’s actually a little worse than all that, because since I rebuilt my riser and removed the metal pipe I used as a form to cast it, I have not achieved take-take off again

Pictures say 1000 words, so let me post some of those, there is also a (fairly awful) video tour shot before I had it actually worked the first time

You will see my barrel is small, and stainless steel .. it was a relief they first time it truly rocketed, because I realise I was pushing the envelope in a couple of respects there, I do have 8(+) inches (worth)  of CSA in the riser, and (believe it or not) 160% of that in the down flow area.

I feel like my mass is dry now, it took a long time nearly a month of daily burns, and a dehumidifier running 6 hours a day .. it still occasionally weeps in odd places, but that’s a bit of an aside because my measure of success is barrel temps.

I have no damper, and no bypass, I only control flow at the inlet (with bricks) … the heater is easy to light, hot or cold and it has a really good draw

I was elated when it worked (once) … if a little horrified at how hard it was to get there, but since then (and since removing the liner) I have had three hard burns and not broken through that 350f barrier

A few technical details, the heater was built to the design on page 100 of Erica and Ernie’s book (which I bought a paper copy of) .. except.. the riser, which was initially an experimental design with porcelain ‘plank’ tiles and  ceramic fire blanket (not a great idea it transpires).. I rebuilt it with advice from Lawrence Fine, as a cast perlite riser.. it was first fired with the internal metal core and that was subsequently extracted.

It has around 4.5 metres of pipe-in-mass … cob and paving blocks (not that that is really relevant to barrel temperatures)

My riser-barrel top gap is was about 40mm .. although draw was never a problem, and it did (fully) rocket with this gap, several FB group members recommended I increase it, so yesterday I lifted the barrel another 20mm on a cob seal.. with no positive effect.

I want to believe. … I want to be a UK evangelist for these things and provide support to others, but right now I am coming to terms with what seems to be an expensive failure.

Ok we can help …..
Looks like you have quite a few potential issues!
A bypass would make a massive difference to the start up as the heat would be directed in a more direct route to the chimney, it would also increase the draw and pull more air through during start up.
Your top gap above the riser is too small, you need at  least 80mm for a  8” system but more would be  better ,100-120 would be good.
The gap around your small barrel looks very tight and restrictive, a full sized barrel would help no end, steel oil drums can be sanded and painted to look nice, red, black or silver are readily available.

Having said all of that I think we need exact dimension of the fire box, burn tunnel and riser before we can accurately asses the issues. The mass pipe work also needs clarifying with dimensions and layout.
Finally your chimney needs to match the system and be tall enough with the chimney sticking above the house roof line?

Lots of question but the info is critical.

Hi Nick;  
Barrel top temperatures do not need to be screaming hot to have a successful RMH.
To start, an 8" J-Tube will only burn for 45 minutes to an hour no matter what, so your burn times are good.
Exhaust chimney temps where they leave the mass should be 140F- 200F, so at 150F they are right in the ballpark.

How new is this RMH?
Not only do you need to dry it out, but after that, you must build up the mass temperature.
This takes some time. when you are burning every day the heat retention is cumulative.
Do you have a laser heat gun?   What are your mass Temps?  Close to the barrel and again at the far end where you make the 180 degree turn.
How well-insulated is your home?
How large an area are you wanting to heat?
What kind of "mass" are you heating?  Large rocks bedded in cob? Or perhaps all cob?
What kind of day/night temperatures do you have?
Not every home is the same, some RMHs need to be lit all day and let go out at night.
I promise that your old stove running shut down all night was creating massive amounts of creosote.
A chimney fire waiting to happen...
An RMH burns up all the bad particulates, there will be no creosote.
Have you checked your ash cleanouts yet?
How long is the roof of your burn tunnel?
It is critical to have consistent sizing throughout the J-Tube.
What material did you build the core with? Firebricks?

Thanks for your reply

The firebox was built to the dimensions in the Wisner’s book … page 100 .. I will post an image

The Wisner’s recommend a 5cm gap .. I had around 4cm when it worked and increased it to 6.5 … which (by my reckoning is 819cm2 over 200% of the systems cross sectional area

I have no issues with starting, it burns well from cold, I have a good chimney 10” square to 5’ above the ridge line

I am attempting to upload a video of ‘sub rocketing’ performance .. where things get stuck , you can hear from the audio that it draws pretty well, it never smokes back into the room.. but the temperatures get stuck at about 180c/350f .. it seems very hard to push beyond that .. I do seem to get some “combustion instability” as I try and push through that temperature the flames seem to move back and forth in the firebox..

I speculate wildly, but it is as if the secondary ignition, blows itself out .. maybe that *is* due to the smaller barrel, or an inadequate air gap

Should a fire of the intensity in my video create/sustain secondary combustion ? .. if that were happening and I was getting the extra 100 degrees I know it produces, I would be a happy bunny

I would really like to make it work with the small barrel, for several reasons, longevity, clearances, aesthetics, and (wanting reactively low radiant) heat output to name a few … pig headedness to name another

I know I am sailing close to the wind  (the Wisner’s speak of downflow areas of less than 150% being problematic) I have 160%

Let me try and post this … nope forum doesn’t like mov files … will get it up on YT

Hi Nick;
My apologies I had not watched your video before replying.
You will want to finish covering your mass pipes. you are losing heat to fast with them exposed.
As Fox mentioned increasing your top gap over 60 mm preferably to 80mm.
Even though you have a good draft, I am concerned with the amount of room inside that beautiful stainless barrel, it looks tight.
You could free up some room by changing to a 5-minute riser made with Morgan superwool.

Hi Thomas .. nice to ‘meet you’

I will try and reply to your points although some may be better answered by my other replies


“Barrel top temperatures do not need to be screaming hot to have a successful RMH.”

Sure .. but they are my metric of a secondary burn, which i fon’t think I’m getting


“To start, an 8" J-Tube will only burn for 45 minutes to an hour no matter what, so your burn times are good.”
Didn’t know that, I have successfully burned for up to two hours, but pretty sure it wasn’t truly rocketing.

“Exhaust chimney temps where they leave the mass should be 140F- 200F, so at 150F they are right in the ballpark.”

Yep, with a moderate fire I can hold my hand there.. full throttle, no way but that’s when I measured 150f


“How new is this RMH? “
1 month old, burned daily for 3 weeks, 2x1 hour a day

“Not only do you need to dry it out, but after that, you must build up the mass temperature.
This takes some time. when you are burning every day the heat retention is cumulative.
Do you have a laser heat gun?”
Yes, and a thermal camera

“
  What are your mass Temps?  Close to the barrel and again at the far end where you make the 180 degree turn.”

Pleasantly warm 1 hour after a burn .. 120f maybe -lukewarm coffee temperature -  … but it dies away over 4-6 hours back to maybe 80f .. and ‘room temperature’ by morning

Not massively stressed amount the poor mass temperatures, I can imagine it is still ‘sweating’ a bit, it seems to be improving over time, there is a another layer to add, and I don’t yet have any cushions, blankets or insulation.

Because of the incline of the tubes there aren’t really any hotspots, the hotest parts are deepest, it takes longer for the heat to come through but it is never uncomfortably hot


“How well-insulated is your home?”

The room in question ..  excellent.. the rest of the house (I had hoped to heat) double glazed, 9” glassfibre loft insulation, averagely airtight


“How large an area are you wanting to heat?”

Well, I had naively hoped to heat the whole house, or at least 2 or three bedrooms and a bathroom as well as the room in question .. a total of maybe 1200 sqft (9600 cubic ft) .. I was going to actively shift warm air between rooms through ducts in the loft (once the fire is out)



“What kind of "mass" are you heating?  Large rocks bedded in cob? Or perhaps all cob?”
Dense, concrete block pavers, embedded in cob (cob is the thermal interface to the pipes) .. I think the pics say it all

“What kind of day/night temperatures do you have?”
Right now.. mild .. nights around+5c (41f) and days around 10c/50f
Midwinter nights might -10c/14f … we might get daytime cold spells around 0c/32f for a few days at a time
I am in the UK .. around the middle, inland .. our lattitude is a bit deceiving because we are warmed by the jet stream..  basically our climate is mild - by most North American standards

“Not every home is the same, some RMHs need to be lit all day and let go out at night.”
Understood .. planned to burn once or twice a day, I am here all the time, but I have stuff to do

“I promise that your old stove running shut down all night was creating massive amounts of creosote.”
I never ran it all night .. cleaned my chimney once a season.. I would get a pound or two of soot.
I burn(ed) a good mix of stuff, soft and hardwoods, some not as dry as it should be, quite a lot of paper and card ..
but I understand the rocket might be more fussy.


“A chimney fire waiting to happen...”
Well, I’m not blasé about it, but it’s been waiting 20 years sofar

“An RMH burns up all the bad particulates, there will be no creosote.”
I get that, and I am (now) a true believer, having had it work once .. the soot you can see in the inside of the riser in some of my pics was *completely* burned off, the tube came out spotless and the top/inside of my barrel had also been significantly cleaned

“Have you checked your ash cleanouts yet?”

Yup … It was very black in there (consistent with no secondary combustion ?) .. now there’s more ash than I might have expected.. but it has been using a lot of fuel and I have been naught and burned some cardboard
See attachment …that’s the real unadulterated first look in 2 weeks



“How long is the roof of your burn tunnel?”
4 bricks … just like page 100 of the Wisner book

“It is critical to have consistent sizing throughout the J-Tube.”
Yup, 18x19cm … into a 200mm riser

“What material did you build the core with? Firebricks”

Marshmallows ;o)

.. only kidding .… Vitcas high silicate firebrick https://shop.vitcas.com/products/fire-bricks.html
I used the 1300c ones for al but the tunnel roof where I used the more expensive 1430c rated ones

The thing is.. I have seen it work properly .. and that’s what I want, working like that it would fulfil the promise(s) .. but I can’t believe I’m supposed to have to crank it that hard.

Again thanks for the reply - I don’t want to come across as arrogant or ungrateful, and I am open to suggestions, but I have tried a lot of (expensive and time consuming) things to get to here and some of the advice I am getting is contradictory, counterintuitive or has been tried - so I do have to filter

Hi Thomas .. replies crossed - I think I addressed some points (mass performance etc)

This is my second riser .. built because my first experimental, porcelain riser (allegedly) did not have enough insulation to work …so I “went large” on the insulation .. making the clearance tight .. but keeping  160% of CSA in the down draft area

See images

I could try a 5 minute riser .. but finding/buying materials is going to take substantially more than 5 minutes

It’s also frustrating to have spent  upwards of $100 and 3 days cutting, welding, mixing, packing and firing a new riser (which took 2 attempts) to have to bin it … I guess this is the price I pay for going ‘off piste’

Let me put the video of it burning up on YouTube, I’d really appreciate it if you could take a look/listen and see if it seems nominal to you …. My concern is that if it isn’t burning clean, I am not getting the efficiency AND I am creosoting up my pipes

Thanks

Nick

Ok, so here it is jammed up against its top temperature … like I say *one time* (whilst the galvanised tube was in the core of the riser) it ‘pushed through’ and surged another 125f (100c) … but it took a 75% full firebox of 1” kindling to do it

https://youtu.be/NafXx_gRxDM?si=xylQvHXD4p7YLABr

Should I not have secondary combustion at this point ? … pretty sure I don’t.

I have a crazy idea (easily tried) to put a ‘tumbleweed’ bundle of nichrome wire  inside the riser to act as a ‘glow plug’ and try and initiate/sustain the secondary burn .. it’s easy and I’m lazy so I will report back on that shortly

Update … I removed the barrel, and measured my top gap again, I have 65mm

Also this (see picture) is after about 4 hours of burn time … although I am not seeing that magical temperature surge, it looks to me (from the cleanliness of the riser) that I have a good clean burn now

I’m going to try my nichrome tumbleweed idea anyway - but now I’m starting to doubt my own doubts

Hi Nick;
As much as you like that stainless barrel, how about temporarily placing a full-size barrel over your riser and giving it a try?
I still think you have a choke point slowing things down.
Your idea of placing nichrome in the riser is easy to try but I do not expect any improvement, rather more of a slowdown.

My 8" J-Tube in the photo ran 700-800F easily all the time and when pushed I could get it to 1100F with a glowing orange circle 8" in dia.