Optimum chimney temperature?

Hi All;
As you all know I just completed my new brick bell shop rmh. With its 5 minute riser and ceramic core it rocks!   For the first few days of burning my chimney temps went as high as 150 F but seemed to like running at 130F , I was satisfied with that.  Apparently things were still drying...   yesterday after only 6 hrs my chimney temps were 190 F ! Yikes,  I'm thinking that's to high and I am loosing heat that I should be saving. Anyone know what temp I should be aiming for ? The bell obviously needs enlarging …  I am hoping to build an arched top on a large portion of the bell this summer, but I don't think it will be enough . Might need to raise the walls before then.
Opinions ?

Hi Thomas,   As you know, exhaust temps are an important thing to keep constant attention on when running a RMH (at least for geeky pyros like us). After about the millionth time checking the temp with my IR gun, many years ago I decided to purchase a candy thermometer like the one in the picture below. I drilled a hole about head level in the exhaust pipe and inserted the probe. I have been very happy with its performance -  its quick response, low tech with no batteries and is always there doing its thing 24/7. It really helps me to see how wood quantity and placement can have on the burn (other than the whoosh sound or going outside to look at the exhaust coming out of the chimney to go by) and also how important covering up the feed tube is to help hold the heat in the mass. It ain't no Testo gas analyzer but some things in life may just need to go unknown, at least for now.
Your question has been somewhat of a mystery to me too. What really is an optimal temperature? I would imagine there is a sweet spot for every stove since there are so many variations to consider with each build, but what I have settled on up until this point is what is the lowest exhaust temp I can go and still get good draft with little to no smoke out either end. Typically for me this has been around 225-275F (taken from the center of the gas stream) or about 150F surface temperature. I can go lower to get more efficiency, but it really sucks (no pun intended) during the shoulder seasons when I'm opening the door and window to vent out the smoke with a cardboard box from the shop.
I wold really be interested in hearing others take on this too.

I have no answer. Just wanted to comment and say how funny it is that you say "yikes" to 190 degrees. When we all had regular wood stoves, 700 degrees was pretty normal......we are in a new ball game now!

Hi Gerry;
Here's what Peter had to say    http://donkey32.proboards.com/thread/3471/optimal-stack-temperature
Seems I might should leave well enough alone.
I just ordered a similar candy thermometer to yours, a whopping $7 delivered ! I obviously need mid stream temp numbers not external pipe temps.

I have noticed a nice trait of my baby dragon.    She is leaving NO ash in the feed tube none ! I haven't opened my cleanout door yet , I guess I might find some ash in the bell maybe...but maybe not.


This evening after reading your comment. I blocked off the feed tube for the first time, curious what my bell temps might be in the morning.  

Hi All;
I Have an update this morning.  Checked with the boss last night and she was willing to donate her old analog candy thermometer  to my cause. I'm glad I had already ordered one, as hers tops out at 220F.

Brick temps were the same as before, rite around 51F or so. So covering the feed tube had little effect.  I suspect that the ceramic parts going cold eliminates most draft from the feed tube.
Dilled my chimney and inserted the probe.  Internal pipe temp was 60F before startup.
2 minutes into getting the fire burning , pipe gas temp was over 100F at 5 minutes (dragon was roaring) pipe gas temp was 130F  at 20 minutes it was at 175F and at 40 minutes I pulled the probe as it was just below 220F.... !
I will have to wait for my new gauge to arrive, before I can find out just how hot this baby dragon is getting.
I think this spring she may have a growth spurt...

Thanks for the update Thomas. Perhaps you are correct with the ceramic fiber core not producing draft for a lot less time than a firebrick tunnel and perlite clay riser like I have. All I know is that when my stove is out and I cap up the feed, the temperature guage goes down quite quickly.... In the first minute alone it goes down by about 10 degrees and you can often hear the metal pipe contracting.
I know my bell is undersized for my system but its all the room I have in the workshop. However, I can go upwards so I am thinking of changing out the water tank with a 55 gallon drum and raising it up at least a foot to get the extra space there instead.
One last thing.... about once a month I'll remove the thermometer and clean the soot off the probe with a piece of steel wool just to be sure that it stays accurate.

Ha Ha Gerry;   I think you should build a bigger work shop and a bigger better rocket!
Thanks for the tip about cleaning the probe.

OH YEAH!  I'll take all the encouragement I can get Thomas!
...and thank you for the proboards link.

My extra special, high quality, super duper , new riser temperature gauge (previously known as a candy thermometer) arrived today. Nice and large , easy to read!  I had already quit adding wood to the stove. In fact it had been well over an hour or more and still a chunk sat in the bottom of the feed tube, all by itself glowing away, it would have eventually burned itself down to nothing. Gas stream temperature at that point was at 200F, Added some new wood and gave it a few minutes. (maybe 15 min max) .. as you can see in the photos she roared back to life !  Apx 260 F in the gas stream in 15 minutes with a hot bell.  Bell external brick temps were 135 F - 145F,   barrel top temp, was 880F.  I suspect that if I hand fed her for a while I might have reached 300F in the stack!  I will find that info out in the next day or two.

Made it up on the roof yesterday while we were between storms and got a chimney cap and a sealing collar installed on the stack. No more rain cascading down the chimney!  Good thing, as this morning new snow covers all. Kinda slippery up there with snow.....

Wanted to update on how well the ceramic blanket is sealing to the masonry and the metal barrel.   Many thumbs up !!!   Not one leak (so far) at all ! Not at the bricks or at the hardy board roof ! Time will tell but if it was going to fail I would have expected it to happen before now.
As I mentioned earlier I still have no ash build up in the feed tube or burn tunnel AT ALL !!! This thing is burning nuclear hot no ash seems to survive,   of course I still haven't opened the cleanout door yet ...

OK Thomas, now your all set to see how much more heat you can squeeze out of that bell !
Good thing you've got a 'convertable' where you can easily pop-the-top and play around in there...perhaps with another column, wall or decrease the size of the outlet?
Sorry for my eagerness, its just been a while since I've wanted to experiment with my own bell but can't at the moment so I'll have to vicariously learn from your build. Not so bad though !

I was just up on our cabins steel roofs cleaning the chimney stacks the other day and know what you mean by slippery. Nice to have that part done I bet.

I know for my bell, a thin layer of ash accumulates there, but mostly it tends to build up in the manifold.

Thomas, on the workshop heater 220mm batch, 11m² double bell. 80/120 if i do three burns.

On your black stove pipe chimney, you make a barrel bell, with a plunger tube. Pish easy to do. And you gain another 1.86m² of ISA;  And quick heat, but not too much, since the temps are lower than on the heat riser. If you don't want the quick heat, you either fill it with bricks, or cover with brick.

Uhhg you guys.....can I play?  My mass is not quite complete. Several thousand pounds to go. I've seen my top barrel temp as hot as 815 degrees. I usually run it to about 600 and let it burn down to coals at about 350 to 400 before restoking the fire.  I feel for my space between 400 and 600 degrees is optimum. I kind of have to baby it to get above 750.  Usually the stack going to the roof I can leave my hand on.  Today I have had it going for several hours and was really ripping it. Barrel temp was 560 ish at the top. Could barely leave my hand on the chimney at 165 and internal pipe temp was 220 before I pulled my thermometer out for risk of hurting it. The thermometer caps at 220.  It was getting pretty slow when it approached those temps.  Honestly, my chimney temp was lower before I started cobbing more of the barrel.  The cob retains more heat in the chimney because it cant shed its heat as fast.  Its seems there's a fine balance point between exposed metal to shed enough heat for chimney temps and thermal mass to store it

Hey Eric;  Yes, makes sense. Your cob is drying and its holding more heat so your stack is getting more hot air fed thru it.
  I'm new at this monitor the internal gas temp rather than point my IR gun at the pipe.... things were simpler then but not very accurate.
I have not put a thermo in my greenhouse rocket exhaust yet so I have no idea what it is really running. I can put my hand on it, but not for long.

We use these on our flues.



They're easy to read simple go or no-go conditions.

Hi Greg;  
I have one of those on my house wood stove.  I like it.  It is reading surface temperature.
This is important with a conventional stove, as a high pipe temp could start a chimney fire. In the creosote conventional stoves produce or ignite nearby wood framing. RMH's do not do either.
You would be surprised to see the numbers if you were able to read the internal gas temp.

What we are trying to do. Is maximize the heat held and keep chimney temperatures  warm enough for convection and to resist moisture buildup in the stack.
But not so hot that I am "wasting" heat out the chimney.

Yes, this is me being a geeky pyro … but I like it !

thomas rubino wrote:Hi Greg;  
I have one of those on my house wood stove.  I like it.  It is reading surface temperature.
This is important with a conventional stove, as a high pipe temp could start a chimney fire. In the creosote conventional stoves produce or ignite nearby wood framing. RMH's do not do either.
You would be surprised to see the numbers if you were able to read the internal gas temp.

What we are trying to do. Is maximize the heat held and keep chimney temperatures  warm enough for convection and to resist moisture buildup in the stack.
But not so hot that I am "wasting" heat out the chimney.

Yes, this is me being a geeky pyro … but I like it !

Same here Thomas! (lol)
I love fussing with the fire and the controls to keep our stove operating within its optimum range. It's not a rocket, just a conventional cast iron stove. This indicator is located at the hottest spot on the stove surface.



I used an infrared thermometer as a reference to adjust the thermometers so they'll give more accurate readings.

Hi All;
 Just an update on the change I made inside my bell, I added a 4.5' tall, brick column next to the original.
Today I got to spend a good 4-5 hrs puttering in the shop. I kept the dragon well fed. 18F outside and over 60F inside YEE HA  tee shirt temps!(admittedly that was 60 near the stove not over by the doors)
After 4 hrs burn my exhaust gas stream temperature was over 320F! Almost half way to that 700F Eric!  Seems I can go bigger yet.   I do have 16' of chimney with 13.5' inside, that helps.
Currently I have over estimated my ISA at 86-88' ,  true ISA  might be +80' maybe ?  Still a ways to hit the 95' an 8" batch can push.
Considering adding a brick or 2 to block off some of my exhaust outlet and see what that does to the final temp.

Interesting watching the temp gauge when making changes at the feed tube.  Simply poking the fire , hear it roar louder and then watch the temp gauge drop 15-20 degrees !  Add wood and watch it drop 50 degrees ! Who would have thought that big a change.

Finally starting to clean things up in that corner.  Next I need to replace that rickety old chair with a nice recliner... you know lay it back to add more wood :)  Get a nice electric splitter …. oh yeah, that's right... we make our electricity, hmm not really enough, to power a log splitter.  OK maybe hire the neighbor kid to come split it for me !  

If your flue temperatures are this high, perhaps you need to use less wood?

Hi Graham;
In an insulated building that would be so.   In this building it means I can have a bigger bell!   The amount of wood burnt was nothing compared to what the old double barrel stove would have used.

thomas rubino wrote:Hi All;
Interesting watching the temp gauge when making changes at the feed tube.  Simply poking the fire , hear it roar louder and then watch the temp gauge drop 15-20 degrees !  Add wood and watch it drop 50 degrees ! Who would have thought that big a change.

Yup! A person can sure learn a lot from that little gadget even if Peter says they're not too accurate.

thomas rubino wrote:Finally starting to clean things up in that corner.  Next I need to replace that rickety old chair with a nice recliner... you know lay it back to add more wood :)  Get a nice electric splitter …. oh yeah, that's right... we make our electricity, hmm not really enough, to power a log splitter.  OK maybe hire the neighbor kid to come split it for me !

Who needs a TV when you have a recliner and a RMH! ... well...perhaps some popcorn and a beer and you'd be all set!  :)
We have an electric 6 ton log splitter that is 15Amp 120V  1-3/4 Hp. Unless your going through knots and/or twisted grain, your actual power use is maybe 3-5 seconds per log as the return is spring actuated so I would think that using it with solar is very doable. I tell you, it has been a life saver!
I rigged ours up so there's a foot petal and stands up on its end so its much easier to operate. I also welded up an extension on the cutter blade as well made a bigger platform/pusher so it will also make nice cedar shakes.

Just got a new candy temp (T&G dragon breath monitor) gauge to check the gas temperature in the studio RMH.

Its been running since 7 am, outside temps up from 10 F to 22F . room temp is 74F , READY... stack gas temperature 2' above the mass is  over 400F ! seemed to settle in around 420 F or so !  OMG I had no  idea I was loosing that much heat !!!  Hmmm  maybe a bell at the far end …  (wife will strangle me if she reads this ) She hates when I interrupt her happy artist life … with trivial things like heat!

I informed her the other day, I intended to install a 5 minute riser in that stove this summer... After the lecture … I will be shielding my work area from her work area during that project.

thomas rubino wrote:Just got a new candy temp (T&G dragon breath monitor) gauge to check the gas temperature in the studio RMH.

Its been running since 7 am, outside temps up from 10 F to 22F . room temp is 74F , READY... stack gas temperature 2' above the mass is  over 400F ! seemed to settle in around 420 F or so !  OMG I had no  idea I was loosing that much heat !!!  Hmmm  maybe a bell at the far end …  (wife will strangle me if she reads this ) She hates when I interrupt her happy artist life … with trivial things like heat!

I informed her the other day, I intended to install a 5 minute riser in that stove this summer... After the lecture … I will be shielding my work area from her work area during that project.

Poor, poor Thomas..... I feel for you man! The R&D of our new venture is taking its toll. Perhaps we should have opened up a warehouse or something to do further testing with the T&G DBM before we go public next time?  ;)

7AM 20F  greenhouse studio 48F . Snowing heavy , new snow 10"+ and gaining.
Chimney gas temperature before firing was a solid 150F .  Within 2 minutes of starting fire, really just the kindling was really going good it was at 250F ! That fast.  I sure if I go check it now it will be over 300F.

Wow ! World of difference between a solid rock mass and a large brick bell.

EDIT)   30 minutes into the burn, gas temp 340F   Added what wood would fit in feed tube … temp rose to 350F ...