single-wall flue: how hot?

[Ashful]

So, achieving light-off with my historically-marginal wood has, in the past been a balancing act between overheating the stove, flue, and achieving light-off. Things seem to be going much better this season, with my 1-year seasoned wood. I also already have my 2014/2015 wood stacked, and am now working on 2015/2016 wood, so things will only get better, from here on out.

However, I'm wondering how hot you folks let your flue get, preferably as measured on the outside of your single-wall. It's not uncommon for me to see 600+ on the outside of mine, with the IR gun, prior to engaging the cat. After cat engagement, it quickly drops below 300F.

 

[HotCoals]

I don't do a start up that high.
I may see about 300-350 10 inches up or so.
In cruise mode usually around 200 with the air shut off.

Right now 210 stack..475 stove top(near cat) and maybe 2/3 on my cat gauge.
Fire is maybe 3 hours into it...or should I say the orange glow now from the cat..lol.
No visible flames.

 

[oldspark]

Well I dont have a cat stove but on start up on the summit I get it to about 500 to 550 or so (can be 600 sometimes), it seems to work well that way.

 

[Ashful]

Cool. Just wanted to make sure 600F wasn't crazy high / gonna ruin the pipe territory.

 

[begreen]

On start up I might see +800F in the flue for a few minutes until I start turning down the stove. It depends on the wood and how the fire is set up and how quickly I respond to turning it down. I have some super dry old poplar and fir I'm using up right now. It takes off very quickly which can lead to some warm flue temps. +600F flue gas temp is pretty normal for our stove on a reload or startup. As the primary air is reduced and the secondary air kicks in, the flue drops down to about 450-500F.

 

[oldspark]

BG is that single wall pipe you have?

 

[begreen]

No.

 

[oldspark]

So internal temp vs surface I guess, the temps are way different, did not want OP to lose sight of that fact.

 

[begreen]

Good point. That is why I posted - "800F in the flue". All my readings are with a probe thermometer in double-wall pipe. 600F surface temps are high and should be brief.

 

[oldspark]

Ah did not see the "in".

 

[Todd]

I cut back the air on my Keystone once I hit 500 external temp while in the bypass mode. I figure internal temp is about double external and I don't need to send too much heat up the stack. After engaging the cat she settles in at 300-400 depending on the burn.

 

[begreen]

Ah did not see the "in".

No problem, if you didn't see it, someone else probably won't either. It's good to clarify.

 

[Ashful]

Okay, guys.... so I've been doing similar, cutting the air back to half throttle when I see flue temps close to 600F, even prior to engaging the cat. I'll continue to do that, if not a little earlier.

Then my question becomes, should I go back to full-open on the air, immediately after engaging the cat? I think the cat sometimes needs all the help it can get to light off, being a downdraft design, and my wood still not 100% optimal this year. My exterior flue temps quickly fall in the mid-200's, immediately after engaging the cat, probably independent of where my air control is set.

 

[Highbeam]

600 surface temp is too high. Your class A, if you have any, is only rated for 1000 continuous and 600 outside temp is 1200 inside temp. Continuous flue temps of 1200 are above the design limit for your class A. Now you are not running it their continuously so a brief jump to 600 is fine but get her back under 500 soon. If stainless class A is only rated for 1000 then I can only imagine the mild steel single wall is lower. Internal stove parts are even higher temp to get your EGT up over 1200.

I would engage the cat at full throttle. No sense in turning it down and then immediately back up. Are you in the situation where your flue temps are super hot but your cat temp is below light off temp?

 

[Ashful]

Interesting thoughts, Highbeam. Stove is connected into a blanket-wrapped smooth-wall flex liner via maybe 6 feet of telescoping black pipe, and the temp's I'm giving are on the surface of the black pipe less than a foot above the stove. I will have to check for numbers, but I know the temps are much lower up at the transition to flex liner.

Being a downdraft cat, the cat probe usually reads 75 - 100F prior to engagement, whether the stove is a 200F or 900F. I usually try to get the top-load door (hottest part of the stove) to at least 550F prior to throwing the bypass damper, as that's what I always found my minimum temp had to be for achieving cat light-off, but perhaps with dryer wood this year I can push it a little earlier. The issue is that the one stove (new install, tall mo'fo' chimney) drafts like nobody's business, so flue temps shoot up pretty fast when in bypass mode. This is not bad, as the stove is completely controllable with the cat engaged, but again... getting the stove body (or even top-load door) heated up while maintaining flue < 600F external is a challenge.

 

[oldspark]

If you have ever run a IR testor up and down your stove pipe you will see how quickly the temperature drops just a short distance up the pipe, so 600 should not be a problem what so ever especialy if 18 inches above the stove . Coupled with the fact its only for a short period I dont think there will ever be a problem.

 

[Todd]

Yeah, 600 temp at 12" probably is ok for a short time, the temps drop off as you go up. I have my therm right at 12" like Condar suggests and the red too hot zone is about 500 so I like to keep it under that.

Those down draft cats are a different animal compared to my WS so maybe it's better to engage with more air for a bit before turning her down.

 

[Backwoods Savage]

Okay, guys.... so I've been doing similar, cutting the air back to half throttle when I see flue temps close to 600F, even prior to engaging the cat. I'll continue to do that, if not a little earlier.

Then my question becomes, should I go back to full-open on the air, immediately after engaging the cat? I think the cat sometimes needs all the help it can get to light off, being a downdraft design, and my wood still not 100% optimal this year. My exterior flue temps quickly fall in the mid-200's, immediately after engaging the cat, probably independent of where my air control is set.

We tend to aim for 500 and ours is measured on a horizontal single wall about a foot from the stove.

As for playing with the draft after engaging the cat, that just does not sound good and I'm sure they would not have designed the stove so anyone would have to do that. Sometimes some stoves tend to do better by cutting the draft some when engaging then after about 10 minutes cut it back to the normal setting. Most definitely the fuel is still the big factor. If you think you see a difference in one year seasoning of the wood, just wait until you try 2 or 3 years or even more. It will amaze you the difference it makes.

It is great to hear you are working on your wood now for the winter of 2015-2016. It will pay great dividends. We'll be burning some wood this year which was cut during the winter of 2008-2009. It was split and stacked by early April 2009. Ah, that is some sweet burning wood. Have some of it in the stove right now too.

 

[Ashful]

We tend to aim for 500 and ours is measured on a horizontal single wall about a foot from the stove.

It is great to hear you are working on your wood now for the winter of 2015-2016. It will pay great dividends.

Thanks, Dennis. Do I read you correctly, in stating that you use 500F on the outside of the single wall as your primary guide for when to engage the cat? If so, I may start doing the same.

Yes, I'm slowly getting to the 3-year mark on my wood. I plan to set the Thanksgiving weekend aside for wood processing, and do the majority of my 2015/2016 splitting that weekend, but hopefully without repeating the excitement of last Thanksgiving's splitting party: https://www.hearth.com/talk/threads/five-cords-one-finger.95375/

 

[jharkin]

I'd tell you what my flue temps are, but the rear exit of my stove goes right into the tee and up through the block off so i can't measure it. I know its not good when I hear lots of creaking and expansion noises

As far as engagement temps you are on the right track. On small loads I can run wide open and close the bypass damper as soon as the griddle hits 500. On full loads I will have to throttle back and let it run a while to 550 or so to get the whole load charred without overheating the pipe.

 

[EatenByLimestone]

Interesting thoughts here on high singlewall temps. I've always figured when they get that high they approach where the smoke dragon folks get theirs daily to clean the chimney out. I never worried as I figure I'm well below the melting point of steel. There are a couple different grades of singlewall out there though. I found my local hearth shop sells a thicker gauge than the stuff Lowes does. That would certainly affect safety when burning on high temps.

Matt

 

[Backwoods Savage]

Thanks, Dennis. Do I read you correctly, in stating that you use 500F on the outside of the single wall as your primary guide for when to engage the cat? If so, I may start doing the same.

Yes, I'm slowly getting to the 3-year mark on my wood. I plan to set the Thanksgiving weekend aside for wood processing, and do the majority of my 2015/2016 splitting that weekend, but hopefully without repeating the excitement of last Thanksgiving's splitting party: https://www.hearth.com/talk/threads/five-cords-one-finger.95375/

Yes, we use both the stove top and the flue temperature as guidelines. Flue minimum is 400 and stove top is 225-250.

I remember that thread and still wince thinking about it.

 

[Ashful]

Interesting thoughts, Highbeam. Stove is connected into a blanket-wrapped smooth-wall flex liner via maybe 6 feet of telescoping black pipe, and the temp's I'm giving are on the surface of the black pipe less than a foot above the stove. I will have to check for numbers, but I know the temps are much lower up at the transition to flex liner.

Checked on tonight's start up. 500F on single wall connector near stove, 300F up near block off / liner transition.

 

[oldspark]

Checked on tonight's start up. 500F on single wall connector near stove, 300F up near block off / liner transition.

Perfect, no problem with that temp, as you see the 600 will not be a problem either, metal glows at about 925 degrees so should never get it that hot IMHO.
My older stove was run by flue temps with 500 about max and its over 30 years old and it is going in the shop for some more.