Evidence in favor of insulated flue pipes

[semipro]

As I mentioned in another recent post after two seasons of burning I recently cleaned the flue that's connected to my Oslo. The flue pipe is 6" dia. round, single-wall solid stainless that was wrapped with insulation at installation. It sits inside about 25 ft. of 12" x 12" ceramic liner with block-off plates at top and bottom.

When I pulled the chimney cap I noticed (with some disappointment) a fair amount of creosote within the cap and at the top of the flue pipe. However, on closer inspection of the flue I found that almost all creosote had formed within about 2 ft. from the top of the pipe. The farther down I looked cleaner the flue looked. I can only assume that this occurred because the insulated flue gets hot enough during operation to burn off the creosote that forms during start-ups and cool-downs. Apparently the flue only stays cold enough at the very top allowing some creosote formation.

I'm concerned about both clean operation of our wood stove and prevention of chimney fires. This experience has given me some reassurance on both counts. I thought I'd share in case others have the same concern and are considering whether its worth it to insulate their flue.

 

[pen]

Well done and agreed. How long and how much wood had you burned since it was last cleaned?

pen

 

[semipro]

This was the first cleaning after two seasons at about 60 hours per week burning. We burn from October to May.
When I installed the insulation on the flue pipe I did it thinking it would in enhance the draft.
I never thought that it would also minimize cleaning and increase safety through less creosote build-up.

 

[Hickorynut]

I have experienced the same as you. An insulated liner and only have creosote and ash mostly at the very top. Gives me piece of mind compared to the mess I was experiencing with the older stove

 

[zzr7ky]

Hi - I have 17' of 6" solid stainles liner that is insulated and my experiance is similar. I did it because the folks here suggested it and I was at the short end of the chimney specification (Manual said 16' min). I am sure is helps establish a nice strong draft sooner on start up, and believe it helps keep the pipe clean as you describe.

Mike

 

[Jackfre]

Regardless of the type of fuel, wood, oil, gas the operation of the flue is dramatically improved be insulating the vent connector. Back in the early 90's Brookhaven Nat'l Labs did a venting study to provide sizing info for re-lining oil flues. Rich Krajewski and John Strasser were the engineers on this project. I had a conversation with Rich and what he said was that regardless of the chimney type, masonry, indoor/outdoor, etc the one thing that consistently had the most beneficial impact on system performance was insulating the vent connector. The report ended up as Appendix E in the NFPA 31, Oil Installation Code book. As an appendix it is not a part of the code. That would have added to much cost to an oil install.

 

[FyreBug]

I would think the reason you are getting creosote form the last couple of feet of your run is because smoke condense around 250f. The last couple of feet if exposed to wind and cold is likely cooler than the gases nearer the appliance

 

[semipro]

I was thinking about this as I drove home tonight and wondered if thermal expansion of the pipe and its smooth inner surface resulted in any creosote buildup releasing and falling down the pipe into the stove outlet area.
Maybe the creosote is falling off instead of burning off?
Either way, the flue is staying pretty clean.
Thanks for the replies.

 

[ScotO]

Fyrebug nailed it. That gas condenses alot worse at the top of the pipe, where it is the coolest. I have the same issue with my cap, especially in the fall and spring, because I am not burning a raging fire......I kinda keep the fire lazy. So, from time to time, I check the cap. It's not so much that you are doing anything wrong, it's just the nature of the beast IMO......

 

[Backwoods Savage]

I was thinking about this as I drove home tonight and wondered if thermal expansion of the pipe and its smooth inner surface resulted in any creosote buildup releasing and falling down the pipe into the stove outlet area.
Maybe the creosote is falling off instead of burning off?
Either way, the flue is staying pretty clean.
Thanks for the replies.

If you are running the stove right and have the right fuel you should not have to worry about creosote. And as others have pointed out, it is always at the very top where problems crop up and it is because that is the coolest spot. But if you have good fuel, even that does not need to happen. For example, after installing our last stove (and we did put up a new chimney at the time), we cleaned the chimney after 2 years and got about a cup of soot and no creosote. We have now gone 3 more years with this stove and still no creosote and we have not cleaned the chimney again. There simply is no need to clean it. The key is good fuel and good burning practices.