it's not the crunchy stuff that's the problem but the black tarry deposits, which fur up the flue , like clogged arteries.
I wouldn't burn much more of the wood from the loft conversion, not because I think it is causing creosote build up (you would get this burning unseasoned wood or dipped fence posts etc) but because it will be treated with chemicals.
You can burn pallets ( I use them for kindling) but make sure they have HT on them- heat treated rather than chemically
My sweep didn't seem too fussed, but suggested that we burn a bit of smokeless fuel as well as apparently it helps break down deposits in the chimney. I'm just a bit wary of this crunchy sooty stuff and worried about chimney fires. I think the best thing is probably just to not burn it. Then I won't be worrying about it.
My chimney sweep was telling me that he had had to remove a lump of tar that completely blocked a flue because a customer had been burning old creosoted fence posts. It caused a lot of expensive damage. He said that if you are burning a lot of wood with a higher moisture content (unseasoned) then you will need to sweep more often due to greater soot deposits.
Our neighbour burns pallets and old bits of wood, his stainless steel flu is really black now on the outside. Our fire was put in the same time and in over a year its as shiny as day one, we only use good seasoned logs.
We talked to our fireplace fitter about this, and he said he avoids treated wood in his own fires. He said the kind of cheapo pallets you get stuff delivered on are usually fine, but anything that's meant to last a while has probably been treated with pesticides. Less good.
The loft conversion was done in the eighties I think. There's also roof trusses, which I think would be original and from the thirties. And some offcuts of new wood from the new conversion. Is that definitely dodgy?
We've been burning old wood from the old loft conversion, and I'm just getting a bit suspicious of it. I assumed that it wouldn't have any nasties in, but I had the chimney swept last week and I'm getting little crispy bits of soot falling down the flue, which I'm sure isn't good. Just a handful of little rice cake like bits over the last couple of days, but am thinking that this means there is creosote forming in my liner.
Is it the wood? I'm trying an ash fire tonight instead to see if it makes a difference. Or is there something else going wrong with the stove/liner. Obviously the scaffolders and builders were up by the chimney pot, but I can't really see what they could have done to it that the sweep wouldn't spot.