Boiler is Potterton 200K BTU. About 15 years old.
Conventional fully pumped vented system.
Installed and serviced annually by the same engineer.

Through the winter it had a problem. Heat output considerably down, making
soot and noxious fumes. Boiler is located in a well vented external boiler
room.

Engineer came out and diagnosed the gas control valve as being worn out.
Next visit he serviced the boiler, de-coked the heat exchanger, dismantled
and cleaned the flue, fitted and adjusted the new gas valve. Left in working
order.

The boiler is certainly running much better. Good blue flame and somewhat
quieter. Water output temp has risen gradually and continues to rise.

Problem is that it is still making soot in appreciable quantities. Flakes up
to about 10mm across.
Have spoken to the engineer again and he is a bit stumped.

My only guesses are (1) the boiler is now working more efficiently and is
burning off residue from the H/E or (2) that the burner unit is worn,
burning improperly, and creating soot.

If (1) I hope the problem will go away, but it has now been 3 months.

Any ideas please?

Nick.