Still getting creosote in flue

So I swept the flue today and got over a cup of black shiny creo chips. This is from just 3 weeks of part-time burning of 3-year C/S/S red oak and other wood (ash, maple, cherry) C/S/S over 1 year. In prior years, I blamed the wood not being well enough seasoned. The flue is 25' vertical single-wall stainless 6-inch rigid pipe surrounded by thermix in a masonry chimney. With the fuel seemingly seasoned well enough, our burning habits could be at fault. The picture of the liner inner surface shows where I scraped away some creo with a screwdriver before poly-brushing it. At least the creosote was brittle and easier to remove than before. We burn weekends and evenings (overnight) but not weekdays. I've stepped outside a thousand times and viewed the chimney to figure out how to tweak the burn for heat waves but no smoke. Top-down startup fires get a little sizzling but no bubbling. Just can't figure it out.

The only wood here that hisses is the skinny kindling used for starter fires. The fat splits used for real burns never sizzle or bubble, they light up quickly and outgas well. Flue temp as measured on the singlewall steel black stovepipe three inches from the stove collar is kept at (350 f. by magnetic bimetal thermo = 450 f. by IR thermo). Don't have a probe in the stovepipe, but that 350 f. is the magic point where only heat waves come out the stack. Never occured to me that all the cold startups might cause the creo, but it makes sense - during start, it's really no diff. than a pre-EPA airtight.

oldspark said:

I do cold starts a lot, especially this warm winter and in the fall and spring and no problems with creosote, (not with the old chimney and Nashua either) but I do burn my flue hot on start ups (sometimes I have no choice in the matter) which can be 600 (surface single wall pipe), are you burning the flue hot for periods of time when you burn?

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So far, I back off the intake air during cold starts to keep the stovepipe surface thermo from going above 500 or so because the magnetic thermo shows 450 and above to be "too hot". During reload (damper-open charring burn) it may get up to 550 but not for long because I get nervous and reduce the air. The IR thermo measured there always reads 100 degrees more than the magnetic device, so I guess it does hit 600 but not for long. Is it OK to let it sit at 600 surface for awhile?

The replies make good sense. Thank you. Indeed, the creosote is all found in the top 6' of flue where it would be coolest. Our stove cannot maintain more than 400 f. flue temp while running well in cat mode because it dissipates alot of heat before the exhaust gets out of the collar. It has had 830 f. (Measured on back shoulders) overburn a couple times and the flue thermo was 400f. I'll make hot open-damper fires that burn awhile and watch for a cleaner pipe.

The firewood does not sizzle as it's C/S/S outdoors in single rows in open windy field, 3 years for the oak. Only the kindling sizzles a little sometimes. Firewood is stored after seasoning in an unheated space totally surrounded by cement, no weather gets to it. The 6" chimney pipe (singlewall) exits the stove horizontally for 2 feet, through the thimble and it T's into the rigid 6" stainless singlewall liner which is 25' tall vertical. The liner is inside an internal masonry chimney (clay lined) and thermix has been poured between the liner and masonry chimney. I am trying to measure flue temp with a simple magnetic thermo stuck on top of the stovepipe 3 inches out from the collar. I also confirm the pipe temp with an IR gun - it usually reads about a hundred above what the magnetic thermo says. IAW the responses so far, I am leaving the damper open much longer at hotter temps before engaging the cat. I will be going after hotter blurns now instead of fuel-conservation burns and we'll see how the creosote is reduced from that.