Soot is burnt fuel that did not leave. A cord of seasoned hardwood makes a known amount, and I weigh what I take out. That flue gives me about nine kilos a year on a fire the family lights maybe twelve times.
I have swept chimneys for twenty six years and I am registered, which means I issue a certificate that an insurer will accept.
Soot is the whole of this, so here is what soot is.
When fuel burns incompletely, the unburnt carbon and the tars go up with the flue gas and some of it condenses on the flue wall. How much depends on the fuel, the moisture in it, and how the fire is run: a slow smouldering fire on wet wood makes a great deal, and a hot fire on seasoned hardwood makes very little.
There are numbers for it. A tonne of well seasoned wood burnt properly leaves something like one or two kilos in the flue. Burnt badly, five or six.
I weigh what I take out, because I have a set of scales in the van and because I started doing it for my own interest years ago and never stopped. It goes on my copy of the certificate.
The house is a farmhouse, part sixteenth century, with four flues.
Three of them are ordinary. The kitchen stove flue is swept twice a year and gives me the right amount for a stove that runs all winter. The two bedroom flues are capped and dead.
The fourth serves an inglenook in the hall with an open fire in it.
That fire is lit at Christmas and for a birthday and when there is a power cut. The family have told me so and I believe them, and you can tell anyway: an open fire that is used gets a particular polish on the fireback and this one does not have it.
Call it twelve fires a year, of an armful each. Perhaps a hundred and fifty kilos of wood in a year, and it is good wood, because he has eleven acres and a moisture meter.
At the worst rate in the book that is under a kilo of soot.
I take nine.
Nine kilos, give or take a kilo, every year, for eleven years, out of a flue that burns a hundred and fifty kilos of dry wood.
You cannot get nine kilos of soot out of a hundred and fifty kilos of wood. There is not that much carbon in it that can fail to burn and still leave you a fire.
What kind of soot it is.
It is what stopped me telling myself the scales were wrong.
Wood soot is light. It is flaky and black and it goes everywhere and it weighs almost nothing for its volume, which is why a sweep's bags are enormous and light.
What comes out of that flue is heavy. It is dense and slightly greasy and it is not black, it is a very dark brown, and it packs down in the bag.
It is coal soot. I have swept coal flues for twenty six years and I would know it in the dark.
That house has not burnt coal since at least 1974, which is when the family bought it, and there is no coal on the place. I have asked, in a way that did not sound like an accusation, and he took me to the outbuildings himself to show me, which I had not asked him to do.
The flue.
Swept, camera surveyed twice, and relined in 2019 because I recommended it.
A liner is a stainless steel tube that goes down the flue from the top and is sealed at both ends. After a reline, the only thing that touches the inside of that liner is what comes off that fire.
The first sweep after the reline, twelve months later, gave me eight and a half kilos.
Out of a brand new stainless liner, sealed, in a flue that had been cleaned back to brick before it went in.
Where it comes from.
There is no other appliance connected. The camera survey confirms one flue, one opening, no shared stack and no breaches.
The chimney is on a gable and there is nothing on the other side of it but the outside.
I have measured the stack from the roof and from inside and the flue accounts for the space in it, which is the check you do when you suspect a second void.
What I do about it, and this is the only part where I have not been straightforward.
Nine kilos a year in a flue that should hold under one is a fire risk on paper. It is not, in practice, because I take it out annually and it never gets near the depth that matters, and the liner is sound and the survey says so.
But the certificate has a field for the condition of the flue and a field for the amount removed, and I have been writing a number in that second field for eleven years that no other sweep in this country would believe.
Nobody has ever queried it. An insurer wants to see that the chimney was swept by somebody registered, and it was, by me, properly, and the flue is safe when I leave.
The one thing I have changed is when I go.
I used to do that house last, on the way home, because it is out on the moor road and it made sense.
I do it first now, at eight in the morning, and I have the bags in the van and out of the van before ten, and I take them straight to the yard instead of leaving them in the van overnight the way I do with everybody else's.