Flues

[pinhead]

I've been heating my tiny shop with a 6" Peterberg stove this winter and have noticed a few things which greatly increase the performance of the stove.

One aspect I believe is often overlooked is the effect the flue/chimney has on draft. It's easy to focus on the guts of the stove and forget about the flue.

My flue is 6" steel pipe surrounded by an 8" pipe, with vermiculite filling the gap. Overall height is a paltry 15 feet. Exhaust temps towards the end of a couple of loads is around 128°F, 132°F at the exit of the bell.

I keep looking at the unused 8" triple-wall chimney pipe on our two-story house and wonder how much better the stove would perform in front of that beautiful chimney.

My question is, what type/size/height of flue is everyone using and how much does it effect your stove?

[peterberg]

Gas velocity is key in this stove. The draw of an 8" chimney is much stronger than a 6", so you have to make the stove airtight otherwise a lot of the produced heat will be vented to the birds. During a burn using the larger chimney, the main inlet will probably too large. But the stove will burn like it's swithched into highest gear, that's for sure.

[pinhead]

Jan 3, 2014 2:16:01 GMT -8 peterberg said:

Gas velocity is key in this stove. The draw of an 8" chimney is much stronger than a 6", so you have to make the stove airtight otherwise a lot of the produced heat will be vented to the birds. During a burn using the larger chimney, the main inlet will probably too large. But the stove will burn like it's switched into highest gear, that's for sure.

How does this compare to the chimney on your Batch Box?

I suppose I don't understand what you mean by "airtight," as the stove should be sealed up everywhere but at the inlets of the stove and at the P-Channel.

In watching your videos, it seems that your prototypes were able to burn "wide open" - that is, without any inlet restriction - without smoke-back. As I understand it, the stove is more efficient with the inlet at 20%,  however, the fact that your stove can run "wide open" would indicate that draft and gas velocity is much higher than in my stove; mine will smoke back if the door is left wide open. I also assume you also have higher exhaust temperatures, probably >212°F/100°C?

I know that running at maximum efficiency is a balancing act between having enough EGT to produce sufficient draft, while also limiting the mas flow out of the chimney. Which of these two factors has a larger effect on overall efficiency?

[grizbach]

Pinhead, I also wonder why draft/cfm is not mentioned also. It is a major piece to the puzzle.
I came to the same conclusion last year and made this thread donkey32.proboards.com/thread/781/draft
We common people don't have the instrumentation to measure cfm to compare accurately.

My chimney is stainless triple wall about 5' higher than the roof. It had a 9" ID. This fall I put an insulted sleeve down it to bring it down to 6". This made a small improvement.

When I was running my first test fires in the stove I exhausted right out the chimney. I was able to have a fairly small primary opening. Once I connected my bench to it, It ran terrible. I had to open up the primary to counteract the extra resistance of two extra bells. My bench/bell being horizontal, has more resistance than the vertical ones I have seen on this site.

[grizbach]

Pinhead, There is glass blocking the opening in those videos of Peter's stove.
Peter has mentioned he runs about 300C on his exhaust.

[matthewwalker]

All I can add is agreement here Pinhead. I've done too many experimental tests and don't have a permanent install to give you data from, but I have for sure seen the stove stall when I added bells without sufficient chimney. Adding insulated chimney changed things dramatically. I would think that plugging into the larger, insulated chimney would give you a stove that runs great under a variety of conditions, with the downside of blowing a lot of heated air out the roof. You would have to tune the intake to balance the system for that stack, but I think it would give you a wider "sweet spot" with regards to fueling, load on the stove (mass or bells), and the like.

[grizbach]

Another factor to consider is how wet your wood is. Everyone knows water is the enemy when it comes to fire, but with these batch stoves i think it is more pronounced.
To start this winter season I had some pretty dry wood, and the stove preformed great. Now I am burning wood that was split and stacked 5 yrs ago in a field. It burns alright, but not as good as the dry stuff.
Even though I'm getting a clean burn, the temp isn't as high. Water expands to 1640 times it's volume when vaporized. This is a lot of volume that is fighting with the exhaust to get out. If the moisture takes up half the flue volume, that's half the flow!!

[pinhead]

Jan 3, 2014 9:55:38 GMT -8 pinhead said:

Jan 3, 2014 2:16:01 GMT -8 peterberg said:

I know that running at maximum efficiency is a balancing act between having enough EGT to produce sufficient draft, while also limiting the mas flow out of the chimney. Which of these two factors has a larger effect on overall efficiency?

I guess Peter already answered this question in one of the plethora of extremely helpful threads that he has contributed to:

Feb 5, 2013 7:48:21 GMT -8 peterberg said:

Remember, air is composed of a lot more than oxygen alone. In fact 79% of it doesn't contribute to the combustion at all. On the contrary, it cools down the fire, often leading to incomplete combustion. Efficiency is dependent mainly on two factors: end temperature and excess air. The last one is making a far greater impact than temperature.

This batch box rocket is able to burn very clean and have very low excess air at the same time. The lowest Lambda figure I've seen is 1.38, 1.5 is coming by almost every burn.

If the CSA of the batch box is the same and the air velocity is lower, one would assume that less air is flowing through the system. This would point to less chimney-induced draft being contributing to higher efficiencies. Judging by this metric alone, my poor-drafting stove should be more efficient than Peter's. I seriously doubt this is true, however; Peter has mentioned air velocity within the box being critical to combustion efficiency - and I have no way to test the contents of the exhaust unfortunately.

Jan 3, 2014 10:05:53 GMT -8 grizbach said:

Pinhead, There is glass blocking the opening in those videos of Peter's stove.
Peter has mentioned he runs about 300C on his exhaust.

300°C is a pretty big jump from my <100°C exhaust. This may be the differing factor after all - with higher EGTs come higher chimney draft.

[peterberg]

Jan 3, 2014 10:05:53 GMT -8 grizbach said:

Pinhead, There is glass blocking the opening in those videos of Peter's stove.
Peter has mentioned he runs about 300C on his exhaust.

Yes, I've mentioned that. Using one barrel as the bell, in June or July last year I would think. After that, I've changed to two barrels and now I am using three of those. No channels, no second bell, just an open cylinder with the combustion chamber sticking out the side. Resulting in very low internal friction.

Maximal running EGT is about 80°C (176°F) when started cold. With each next batch the temps does rise, max 120°C (248°F) when running the third full batch. The chimney is a straight 180mm (7.09") square brick channel of approximately 9.5 m (31') high. Experienced a smoke-out when I tried to run the thing with the temperature outside some degrees higher than inside...

[Dan (Upstate NY, USA)]

I have an 8" J-tube system with 30' of 8" pipe with 27' of 6" ID triple wall stainless steel exterior chimney, goes about 2' over the peak of my roof. This setup Keeps the 30' of 8" pipe in the heat bench at a slight vacuum compared to the internal house pressure. No worries about CO coming in the house... average exhaust temps are 135F without any condensation (once I insulated my cleanout T's with rockwool).

[Robert]

Jan 5, 2014 0:12:27 GMT -8 Dan (Upstate NY, USA) said:

(once I insulated my cleanout T's with rockwool).

hey wolf. thats kind of interesting. you mean the cleanouts in the bench? is that makes really a difference? i was thinking that its good they are just covered with the cob, so it will be heated. but you just wrote something that its interesting. can you elaborate on that a little?

[Dan (Upstate NY, USA)]

OK, my heat bench is made of reinforced concrete (to support its own weigh on a 150 year old wood floor. There is a full basement under my 2 ton stove). The pipe inside was steel with just plain steel caps or plugs to block the cleanouts (I count 7). Before I stuffed rockwool inside them the first two would be too cool and would allow the water vapor in the exhaust stream to condense and leak all over the floor. After I added rockwool inside the caps and plugs all my condensate problems went away. It also made the flew cracks that I still have that join to the exterior pipe draw incense (indicating) smoke harder. I guess I could make it even draw harder if I wanted to seal up every crack that I could.

As long as you have cob over your cleanouts without any condensation coming out I wouldn't worry about it. I guess I could have made concrete caps but that would make my yearly cleaning more bothersome.

[pinhead]

Some details of my stove:

6-inch Peterberg Batch Box made of plain-old red brick with proper dimensions and P-Channel.
48-inch tall heat riser.
Double-barrel primary radiator. Top-gap is ~12 inches.
Low mass Dual Bell bench in the matthewwalker style - two 55-gallon barrels split length-wise, positioned end-to-end. The half-barrels are separated by a bell wall - with the opening on the lower half of the partition wall (opening has >30in2 CSA). Clay/sand is ~4 inches thick around the bells.

I ran the stove in my shop all day yesterday, through probably 5 full loads of split and dried hedge. That stuff burns WELL in this stove! The temperature outside when I went to the house late last night was -3°F with a light north wind.

At the peak of the final burn my IR thermometer read 850°F about 3 inches from the top of the primary radiator. The roof of the stove body was about 200°F, the surface of the first secondary bell was ~180°F, the surface of the tertiary bell was ~160°F. The temp at the exit of the second/final bell was ~135°F.

Unfortunately the only insulation my shop has is double-sided radiant barrier on the underside of the roof. The walls are un-plastered limestone with quite a few leaks. Fortunately, on the other hand, the Batch Box was able to keep the shop comfortable enough for jeans and a coat. A box fan in the rafters helped to move some of the heated air down to the inhabited zone.

It seems that my stove has enough draft to "work" sufficiently. I don't have an EGT analyzer but when I "analyzed" the flue gasses I detected mostly steam with a very slight "sweet" smell.

I suppose the only "complaint" I currently have which would be cured with a stronger draft is the smoke-back that occurs when I open the door to load more wood. As long as I allow the previous load to burn down to the coal stage it isn't a problem but when the temperature is below 0°F outside I like to keep the first couple of loads burning as hot as possible.