Stack temp high compared to stove temp

Stack temp high compared to stove temp

PostBy: steinkebunch On: Fri Dec 14, 2007 5:41 pm

My homemade bituminous burner is burning at about 200 to 250 degrees today (magnetic thermometer on stove front near top). Just for kicks, I inserted a thermometer probe into the smokepipe about 18" above the stove collar. The dial immediately pegged the thermometer (max on this HVAC thermometer is 220). The way the dial was moving when it "pegged", it was not stopping anytime soon. Wouldn't surprise me if it was over 300 degree inner stack temp.

So although I'm not measuring the actual firebox temp, but just the single-wall stove temp, it looks to me like I have too much heat going up the chimney. As you may know from another post of mine (Smoke rolls out load door - any advice?), my 6" chimney gives me trouble with load-door smoke rollout, so I may not have very good draft (although it can really suck a lit newspaper up the chimney when I warm the flue!).

My question is: How do I keep so much heat from going up the stack? A barometric damper, if it would even help, is not an option since I live in Wyoming and only have bituminous coal (risk of blowing soot/smoke into house through baro during occasional ingition of volatiles in chimney).

Without getting into a battle over dampers and getting this thread locked, would a manual damper help? Is that a no-no with marginal draft?

I have four 1-1/2" square tubes running through the top of the firebox that the stove blower moves air through to get heat off the stove, much like the design of the Harman Mark III (see attached photo for an explanation). I'm surprised the stack temps are so high. Any thoughts? Or maybe I'm OK and don't have too much going up the stack. Looking for some feedback.
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steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Stack temp high compared to stove temp

PostBy: LsFarm On: Fri Dec 14, 2007 5:51 pm

It is not unusual to have temps in the center of the stack in the 4-500* range. The amount of heat coming from the stove is determined by the effeciency of the firebox at extracting heat from the hot gasses before they leave the stove.

I'd get a higher reading thermometer and see what the actual temp in the pipe is. if the outside of the pipe isn't over ~200-250* I think you are OK

Clean the inside of your stove, scrape all the soot off the heat exchanger tubes, and see if the heat output rises. Soot is an insulator.

Installing a manual damper in a bituminous burning stove is not recommended. Forum member Berlin, our resident Bitum. expert clearly say it is a no-no. I agree. Soot will accumulate and block your draft very quickly when burning Bitum.

Greg L
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

Re: Stack temp high compared to stove temp

PostBy: steinkebunch On: Fri Dec 14, 2007 6:02 pm

My heat exchange tubes do need cleaned some. However, I'm finding that our Wyoming bituminous (at least from the Grass Creek mine) is pretty clean burning. I have been burning since early October, and have only cleaned the chimney once. I've been burning the stove hot after loading, and I seem to burn all the volatiles and even my smoke pipe in the house is staying clean. I burn at about 500-600 degrees for an hour or two, and then the yellow flames go away and temp drops some. Seems to keep the soot down very well. All this without secondary preheated air. I was going to add that, but I seem to get a clean enough burn without it.

Also, I have double-wall smokepipe instead of single-wall, so it's pretty cool on the outside of the pipe. Makes it hard to tell what's going on inside the pipe without a probe thermometer.

Thanks.
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Visit Hitzer Stoves

Re: Stack temp high compared to stove temp

PostBy: steinkebunch On: Sat Jan 26, 2008 12:43 pm

After getting the manomter via the loaner program, I learned that I get a draft on my stove of 0.12" when I load fresh coal and get the high-volatile bituminous coal going real good. I think all my posts stating that I have marginal draft are wrong. I have very good draft. It never drops below 0.04" even with a 12-hour old coalbed. Usually runs about 0.05".

I have had some success reducing the primary air thus reducing the draft. But it's a tradeoff, because since I also have secondary air coming through the load door draft slide (to help add O2 to burn volatiles), it just pulls more secondary air when I reduce the primary. If I reduce both, I get lots of smoke out the chimney because I'm not burning the volatiles as well.

I added a MPD a few days ago. Don't think I can use a baro with bit. coal as sometimes it would blow smoke into the room due to occasional explosions in stove from volatile ignitions.

I believe that the MPD has helped quite a bit. I seem to keep more heat in the stove with the damper closed (but still plenty of draft due to holes in the MPD). By reducing the draft and therefore keeping heat and smoke in the stove longer, I seem to get a better secondary burn, and can close the primary air further and still get heat. Also, I load the stove every 12 hours, and seem to have higher stove temps at hour 11 that before the MPD.

I have not measured the draft after installing the MPD - because it's double-walled smokepipe, and the only hole I had to stick the manometer probe into was the MPD axle holes (I had an MPD in the same pipe for a woodstove last year). I hate to drill yet another hole in the pipe. But maybe I should and just plumb in a dedicated manometer fitting. Also tough to measure inner stack temps for same reason.

I think the MPD will stay clean, as the section of pipe I place it in was very clean after 2 months of burning coal continously and not cleaning it.

Still tinkering, but I think I like the MPD. I have 2 CO detectors as well. May purchase a permanent manometer as well. I'll try to post again on this thread when I get some more time with the MPD installed.

Steinke
steinkebunch
 
Stove/Furnace Make: Homemade Bituminous Hand-Fed
Stove/Furnace Model: Prill underfed stoker Model M8

Re: Stack temp high compared to stove temp

PostBy: LsFarm On: Sat Jan 26, 2008 11:35 pm

A Manual damper used in conjuction with CO detectors and a dedicated Manometer can be a good thing.. Your bitum coal is different that what I burned... I could clog a hand damper overnight with my Bitum coal..

If you can keep the damper clean, and the chimney upstream from the damper clean [you will have a slower air velocity] then it may be what your stove needs... an addtional item for fine tuning your stove's burning characteristics.

My only heartburn with hand dampers is when they are in use above baro dampers, or without CO detectors and without a Manometer.. there is too much guessing without the Manometer. I use a Hand damper in my 'Big Bertha' boiler, but it is outside in a remote building, not in my home.

Greg L.
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

Visit Hitzer Stoves