Question to double skin bell of BBR

[firerob]

This morning the system temp was 30°C/30°C. The house felt definetely warmer (although just one ½ load fire ystday morning). Bell feels warmish. Lots of condensation on the windows.

The ½ load fire this morning started with a perfect draft right of the bat! No flue pre-heating required! Even in this wet state the BBR performes beyond expectation!

(outside temp / inside house: 13°C/18°C)

[martyn]

Sounds very promising, just dont rush the drying stage.
I used to build wood ovens for a living and I have lost count of the cracked ovens I have been called back to!

[firerob]

Thanks martyn, we'll keep this in mind. We thought also, that after all the work we put in, we dont wanna mess it up now in the drying stage.

[firerob]

Oct 22, 2023 3:20:52 GMT -8 martyn said:

Sounds very promising, just dont rush the drying stage.
I used to build wood ovens for a living and I have lost count of the cracked ovens I have been called back to!

What would be your recommendation? How big loads? For how long?

[martyn]

Well i can tell you about curing pizza ovens, the first heat source is charcoal on a raised tray, then small kindling fires building up to a full load over around 10 days.
After the first few days there must be cooling down period back down to ambient temperature so two fires a day then the last few days two fires a day and a full sized load on the 10th day.
However you are trying to dry a lot of brick away from the main heat source so you can follow the above but it will probably still take a few more days…weeks to fully dry the whole system.
Every build will be different.

[firerob]

This morning we made the sixth 1/2 load fire and like in the last days it started nicely. The system temps in the morning are always around 25-30°C.

Top down fires don't seem to start as nicely as bottom up fires - at least not in this wet state of the heater. A small kindling fire in front of the port is drafting nicer right from start and seems to heat up the riser much quicker; with less smoke problems in and out of the firebox.

What is weird: when the rocket is going full power, then there is a sound best described with "pulsating" and I can see the fire pulsate as well with the same frequency; it's like there's less air, more air, less air, etc. in a certain frequency When I open the maintenance door in the back the pulsating stops, same with the front door.

What is this?

[peterberg]

Somewhere, there's what I would call a "kink" in the system. Usually, it's in a spot where the stream of gases is changing direction. Normally, you won't notice it, as long as the stream and expansion is relatively moderate. But at the very moment that expansion and gas velocity are passing a certain threshold, one bend starts to be a restriction and that slow pulse occurs. Opening the door a crack will cool the fire in the firebox somewhat, so gasifying slows down a bit. Bigger logs would help as well. Other than this, there's not much you can do about it now.

One method which could help: insert a pen thermometer in the stove pipe, when this shows 110 ºC deminish the size of the main air inlet a bit. Not much, just enough to let the temperature drop about 10 degrees. The secondary air provision should remain open fully. The roar of the afterburner should go along no matter what, otherwise you are overdoing it. The net result is a slower gasification which deminish the chance of a slow pulse.

All that said, my own heater didn't pulse, not just once during the past eight winters. Knowing about this phenomenon before, I built my own heater with ridicously spacious bends and so on.

Regarding the bottom-up lighting being more succesful in your heater: this is due to the position of the secondary air provision. A P-channel is better with bottom-up, a floor channel is better with top-down.

[amo]

Uzume / Yasin published a chapter about this phenomenon with an illustrative video, on their website. Full bottom of the page :

www.uzume.fr/manuel-utilisation-batchblock

Here's a Google Translation :

My stove makes a train noise! What to do ?
A stove makes a train noise when it overfuels a lot.
This is a situation that can happen to any wood stove when the fire is really too hot. This typically happens when you fill the hearth with twigs.
A lot of hydrocarbons are released at the same time, to the point that the riser is no longer capable of burning them all.
The core therefore gradually fills with flammable gases which explode periodically when oxygen arrives... an internal combustion engine in your living room!
To resolve this problem, you must reduce the air supply for a few minutes to slow the fire. Without oxygen, no combustion possible. The fire will slow down.
To prevent this problem from happening again, read the recommendations for making slower fires.

By the way, on the same page, chapter 2 presents recommandations for the drying phase.

[firerob]

We have had a daily 1/2 load fire for 11 days now with doors and windows open all day. The windows don't have condensation anymore in the mornings although last night was only +3°C outside.

The fires start much easier now and the temps in the bell rise much much faster. In the first week I needed to clean out a small pool in the cubicle under the flue daily, which has now stopped forming.

We are switching to 3/4 load fires now.

The pulsing we had only this one time! It didn't occur again so far.

amo: the pulsing was the same as in the video you mentioned, just with a slightly different frequency.

I am working on a list of all the materials we used. I'll be posting soon.

[peterberg]

Nov 1, 2023 0:55:08 GMT -8 firerob said:

We have had a daily 1/2 load fire for 11 days now with doors and windows open all day. The windows don't have condensation anymore in the mornings although last night was only +3°C outside.

OK, it's getting drier in the house now, good sign.

Nov 1, 2023 0:55:08 GMT -8 firerob said:

The fires start much easier now and the temps in the bell rise much much faster. In the first week I needed to clean out a small pool in the cubicle under the flue daily, which has now stopped forming. We are switching to 3/4 load fires now.

All expected behaviour, you are slowly getting there. Go on, your heater appears to be correctly dimensioned. In a couple of weeks the whole thing will be at the stage where it will stay all winter. The temps in the bell rise faster now because most of the heat was used initially to evaporate water.

One last remark: first fire in every autumn will be a little bit reluctant to get going properly. During summer the bone dry heater will attract some moist which have to be driven out, one full fire should be sufficient to do just that.

[firerob]

With the 3/4 load fires the heater is now always pulsating from the moment it's going full power to the moment it starts to go down (so just during peak fire).

With everything normal open/closed (the way it's supposed to be), the pulsation pulses smoke out of the main air inlet sometimes and just a tiny bit.

With the maintenance door in the back open, the pulsating stops!

With the front door cracked open, the pulsating also stops. 

With just the slits in the front door open, the pulsating is less but not gone.

With the primary air inlet slightly closed, the pulsating is really bad and it's actually pulsing smoke out of the main air inlet.

So, it seems like there's not enough primary air going in!!??

There's no difference in pulsating with 3m flue or 4m flue.

Also, when is pulsing, there's a bit more smoke coming out of the flue than usual (but not pulsing out of the flue top!).

Don't know yet how it will be with full load fires!!

Here's a picture from the flue outside the house. Above the flue is a clearance of 1,5m (straight line up). Could this be the cause of it?

(We built under a roof because it's impossible to get a building permit here, even on your own property!)

Otherwise I can only think of a "kink" in the cubicle where the gases exit the bell, find themselves in another "small bell" and need to "find" the flue hole at the ceiling there.

[fiedia]

I am not sure if I understand well. 

Does your pipe go through the upper roof?

Is your shed closed (walls on 4 faces + roof)?

[firerob]

Nov 4, 2023 8:34:59 GMT -8 fiedia said:

I am not sure if I understand well. 

Does your pipe go through the upper roof?

Is your shed closed (walls on 4 faces + roof)?

No, there are 1,5m between top of flue pipe and upper roof. The big shed has only two walls the one on the picture and the other is 60m away at the other gable. Front and back walls are open. The roof's pitch is not steep.

[josephcrawley]

Nov 4, 2023 22:58:46 GMT -8 firerob said:

Nov 4, 2023 8:34:59 GMT -8 fiedia said:

I am not sure if I understand well. 

Does your pipe go through the upper roof?

Is your shed closed (walls on 4 faces + roof)?

No, there are 1,5m between top of flue pipe and upper roof. The big shed has only two walls the one on the picture and the other is 60m away at the other gable. Front and back walls are open. The roof's pitch is not steep.

I would definitely run the flue through that upper roof. It seems like there would be all kinds of negative pressure issue under the upper roof on windy days. Plus all that smell condensation dripping off the upper roof on cold days.

What a wacky world that you've got to hide a house in a building. It would be the ultimate magic trick to one day turn the barn into that much smaller inside house with a midnight demolition.

[peterberg]

First rule for chimneys: must end as the highest point in the vicinity. No other buildings, trees and so on, within a range of a 45 degrees angle up, measured from its mouth. So, it should be lengthened through that upper roof anyway. I second Joseph Crawley on this.

Apart from that,I am not convinced that roof is the cause of that pulse, so there should be something else in my opinion. Analising the build by the pictures is not easy, but all the boxes seems to be ticked. Bar one: the main air inlet is under a brick, forming a small tunnel just behind the door. In effect, the air inlet is in reality not at the front but about 10 cm behind the door. There's no example and no drawing from the website that shows this.

Also, it is mentioned somewhere to keep the fuel at least 5 cm away from the door/air inlet. I am assuming you are loading the fuel in the firebox right against the air inlet inside, maybe even on top of that brick. In case that assumtion is correct, cut the fuel much shorter than that, keep it away from the air inlet AND the rear wall of the firebox and try again. When this won't cure the pulse, make the small "air tunnel" in the shape of a funnel, much wider at the inside to the sides and up. It might be necessary to dismount the door. Again, the fuel should be away from the air inlet in all circumstances.

If you don't want to redo things on the heater because cold is setting in, run it with the door open a crack. More air is flowing over the fuel then and will solve the tunnel venturi effect at the base of the fire. Just a thought: having the door open a bit and the air inlet blocked completely shouldn't result in a slow pulse. In that case, here's your "kink".