T6/Secondary Combustion stoves, tall chimneys, and cold temps

This is going to be a long one so bear with me here but I want to post this in hopes that it helps at least one other person. These results are specific to my stove only and may not produce the same results for others.

Considering this is the only time I have had a cold snap like this in the 4 years I have had my T6 I am just learning along with others. I have had some issues with getting really deep coal bases and felt like I should have been able to really pump more heat out of this sucker. Well after much pndering about this situation it dawned on me that when the temps are in the thirties outside when I shut the stove down I always have decent secondaries but I also have nice flames coming off the front of the logs through out the whole burn cycle as the wood burns from front to back. I started realizing that with the cold temps in order to keep my flue temps down on the probe I had zero flames on the wood in the firebox and the only thing going on was secondary combustion from the top. It hit me like a freight train a few nights ago that I had a big problem. it seems what was going on was in order to keep the flue temps down I was shutting the air down quicker and killing the active burn that supplements the secondary combustion through out the burn cycle. The load was then burning from the top down and leaving me with huge piles of coals.

I had two thoughts:
1) I could open the primary to get active flames and let my flue temps run up to 7000-800 through out the burn. My issue with this is that it felt as if I was just blowing heat up the chimney and the loads still seemed to burn down to coals more quickly. I decided I didn't like this option and started thinking about other options
2) I decided I could try the key damper that I have had in the stack to see what would happen. (I put this damper in years ago and have yet to ever use it as I was afraid some day my wife might need it for some reason.)

Going with option two I closed the damper slightly and weirdly enough my flue temps climbed up 100-150 degrees. This had me slightly puzzled as my initial though was it would make the flue temps fall. I then noticed the secondarys started to drop in activity but just a bit. I played withi it a bit more and realized that if I closed it all the way and then opened the primary air about 1/4 of the way I was getting very nice active flames on the front of the wood, great secondary action, and my flue temps settled in at 600 degrees per my probe thermo.. During all of this I had the blower turned off so that I could see what the acutal stovetop temps were doing and before the damper was closed the stove top would match the flue. After closing the damper the stove top started climbing. I ended up and went right up to 750 with ease and low and behold the flue temps never budged over the course of the load I watched everything closely and realized that even as the flue temps jumped fell or rose the stove top temp would follow but always be 100-200 degrees higher than the flue temps. On top of this I had great flames on the front of the logs, and my secondaries slowed down to nice lazy secondaries instead of blue flaming jets.

At first I thought that this might be an anomoly but I have now burned 3 loads like this and in exactly the fashion described above and every load has burned down the same way. I am also now getting a burn that actually burns from front to back, burns hot for the whole load, and leaves way less coals in the back of the stove.

I think that my chimney which is 27 foot of class A through the middle of my heated house just drafts way to hard in these temps. I have never really had this issue before as we have never had cold snaps that last like this in the previous 4 years. I will need to play with this to see at what temp I don't need the key damper but so far in my situation I think I need it in cold weather like this.

My new reload pattern in this weather is open the key damper/draft all the way and burn what little coals there are left, pull the coals forward, reload, with damper and draft on high, as the flue temps approach 600 I close the key damper let it settle just a bit and then start dialing back the primary until I have nice flames on the logs and active lazy secondaries. so far it is seeming like 1/4 primary air has been the sweet spot.

This last load I was running a 550-575 flue temp on my probe and 700-750 stove top temp on both of stove top thermos.

Anyways hope this helps someone. I will update with any changes.

 

Nice write up. It makes perfect sense.

 

That sounds like you are on to something Cert. I know these cold temps make a very good draw on the stoves. I can burn with very little air feeding the fire and still keep my glass very clean.

 

I just added a key damper to my set up a few weeks ago and have similar experiences. NC30 in the basement on a 35 foot unlined internal masonry 7" chimney. The thing drafts like crazy all the time. Now it is routine to close the flue damper all the way as soon as things are hot.

 

We have been having these crazy winds lately and the stove is acting weird due to the extra draft, last night the flames in the stove were kind of pulsating and the stove top temp and the flue temps both seemed lower then normal.

 

Great finding, I will play with mine tonight.

Its been known that really tall insulated flues pull extra air into the stoves. Being that the secondary air is not adjustable the only way to control it is with a Flue pipe damper.

But what you are actually doing is making an adjustment that favors a little more air coming in the front promoting a better front to back burn. Which is how my stove operates is a front to back burn also. As these stoves are usually designed that the primary air is actually the glass wash air and the air comes in just above the door and window and flushes down over the window then into the front of the stove plus there is usually a doghouse air at coal level bringing in just a little air there also.

Thanks.

 

Cert, this is the same problem I'm having whenever the temps get under 20°. My set up is only 18', but is drafts like it has an exhaust fan on it. I've tried many ways to get a full burn to the back with nothing working other than raking coals to the front and opening the primary. It requires a lot of babysitting. I had never thought of a damper being necessary to help this situation, but you proved it can work. Please keep us up to date on how this works out for the rest of the season.

Many thanks!

 

I still pull the coals forward at the end and burn them down a bit but it's actually burning to the back so well that if I leave them pulled forward for very long at all they burn down so quickly it's hard to get the next load started. Before I was trying to burn coals down for the bettter part of 1-2 hours.

 

Update: Burned two more loads this way and it's still working like a charm! This morning it had burned down so well I didn't even have coals to burn down. The key damper made a huge difference with the 26mph hours winds and 18 degree temps. I actually had to shut the damper a bit earlier as the wind was wreaking havoc on gettting it settled in this morning. I'm definitely liking the outcome so far.

 

I had removed my key damper late last year to put a flue probe in the opening. I am able to access the area through the top loading door. Once this years cold temperatures got steady, I noticed I was burning too much wood and that the flue temps were high. Put the damper back in and drilled a new hole for the probe. The results were exponential in terms of higher stove temps and heat output. Like you describe, I close the damper all the way once it is rolling and adjust the primary air. The Hickory loves it and its just great in controlling a sustained, hot burn...yes sir.

 

Update on today...... I loaded at 6:30 this morning and got home just now (5pm) to stove top temp of 350 degrees. The house is at 74 degrees and toasty warm load was burned down perfectly with a few perfect amount of hot coals to reload on. The key damper s making a huge difference. Wish I would have tried this ages ago!

 

That is an awesome burn. No way I'd get 350° on the stove with over 10 hrs of burn time.

 

Here is a slant on this. I noticed that raking coals forward or shoveling out coals so only enough left to have coals in the front 4 inches of the stove, this allows me to load the back of the stove all the way down to the bottom of the stove and not on any hot coals. Then the front of the stove on the hot coals my last row right in front of the door I give up putting big size splits in this last row closest to the door. What I do is load this row with small split kindling made from good dried oak split down to around an inch size. I load as much kindling on the hot coals as I can. What happens is the kindling will start burn hot and fast building heat. The kindling up front build heat up in the stove and you dont burn up the big stuff in the back. As if i load a bunch of big stuff only I keep the door open and it gets all the wood burning and charring before I shut the stove down. Seems like the small stuff up front allows the stove to heat up and I dont have to crack the door as the small stuff burns quickly with the door shut and even lets the input air to be closed sooner as we all know as we close the input air down reduces the heat that gets wasted up the flue and keeps that heat in the stove for even a faster build up of the heat. I kind of think the fast burning kindling is robbing all the oygen from the bigger pieces as another reason the big stuff just sits back there not burning while the small stuff burns so hot and fast. But your getting the heat built up in the stove so fast you can get things shut back down quickly for a long over night burn with lots of your main load still there. I have in the past used alot of my main load just to get the heat up to operational levels.

But I mention all of this as this also creates a nice front to back burn set up. So if guys dont have a damper in they might try this until they try a flue damper.

I am a real believer in using good dry small split high btu kindling as in these type stoves its all about building the heat up fast in the stove.

 

Great find cert!! I just loaded mine up about 45 min ago so I'm gonna give it a try. Damper is closed about 90% and air is 1/4 open. I def notice a slow down in secondaries pretty quick. I'll let you know how it looks in the morning

 

Reloaded last night at 8 pm got it going and shut it down. Had the flue temps running a bit higher at 650 wi lots of active flames on the logs as I wanted to make sure it was pumping heat. Got up at 2 and opened the primary to make sure it burned down good by this morning house was at 74 degrees first floor. At 5:30 the stovetop is 400 and first floor temps are 69. Nice little pile of coals to reload on and stove will be up and going in the next 15 minutes. O yeah it's a balmy -11 on the outdoor thermometer and it was 0 last night when I put in the night load which consisted of red oak and hickory packed in as tight as I could get her.

 

I want your stove!

 

This how it normally burns when it's in the twentys outside and I'm not using the key damper. If its in the thirties during the day and dropping into the low twenties or teens at night I would never bother opening it up at night and it would usually go from 8pm until around 10 am before my wife would reload.

I am really glad to figure this key damper thing out because I was starting to get frustrated with it. I knew I wasn't getting the max heat out that I should have been. All the cast iron that this thing is wrapped with really makes it retain heat an radiate warmth for a long time. I am sure without the cast iron I would have a lot more temp swing and fluctuating stove top temps.

I should add the guys with the blaze kings are probably laughing right with how short a burn time that was. If you want a super long burn time check them out as they are getting 30+ hour burns.

 

Cert, if these winters keep moving in the direction of being colder, then I will consider a larger and more efficient stove. I'm not complaining with the Lopi, unless the temps are in the teens or lower. Even pretty much shut down it can chew through a load in three hours before it all turns to coals when it's colder out. Also, having a house built in '59 doesn't help.

 

Nah, twelve hours is about it, lately. Considering the difference in fire box size, I'd say we're burning at about the same rate. The pizzer there is that my house is half the size of yours. I'm trying to warm it up out there

My draft is not really affected by cold temps. High winds will make it pull a little harder. It's actually kinda handy, as I need more heat at that point, anyway.