Electric baseboard heaters are not supposed to cause soot. However; dust settles on the elements. When the electricity is turned on, the dust will burn off. There will be a trace of soot from that dust.
Possibly the boiler is firing oil and thus the soot from the oil is spilling back into the gas flue
Yellow soot is obtained when the holes of the burner are not clean. The combustion is incomplete. The yellow soot or yellow flame is because of unburnt carbon particles.
Soot particles are the result of incomplete combustion of carbon or organic compounds.
Soot
If you think to black soot this is formed from tiny carbon particles.
Possibly the boiler is firing oil and thus the soot from the oil is spilling back into the gas flue
Heat, unburned gasses, carbon soot.
Soot's plural is soot.
Soot is often produced when burning wood.
Soot lowers the albedo of the ice (the amount of solar energy it is able to reflect), causing the ice to absorb more radiation and therefore warm up and melt more.
You still have some soot on the very tip of your nose.
soot doors is provide to clean the outside of the water tubes and to remove the soot.
soot
I think soot and smog come from the air pollution.
Soot is produced in incomplete combustion of hydrocarbons. Soot is black and leaves marks. It also causes health problems if in contact or inhaled!
Soot is the product of an incomplete combustion of carbon.
Be wary of that soot on the hearth.