We've got thicker than normal black smoke coming out of the chimney, and if we run the burner for long - you can smell it in the house.

Is this:

a clogged chimney?

a clogged nozzle on the burner?

something else?

and how hard is it to clean or replace the nozzle?

Burning rich/under combustion. Check the air intake / filters, then check the nozzles.

could need a new electrode too...oil companies will do a cleaning service on it for like $100.

they're pissed at you for locking in such a low price so they are making your furnace flood with oil so you have to purchase more from them

When was the last time you had your burner serviced and your chimney cleaned? If you're like most, it's been too long.

Black smoke, when coming from a car, means you're burning rich. Not in an oil burner. So unless you changed the nozzle, it's almost certain that soot has built up in the burner and in the exhaust ductwork. You should call a serviceman soon.

You are responsible for global warming.

actually, it was done just before the winter last year, but we also let the tank run down almost to empty, so I'm thinking some sediment from the bottom of the tank might have clogged the nozzles, but I've never poked around with this stuff before.

I'm usually pretty handy with kind of stuff, d o my own car repairs for the most part, or at least I did when I had shitboxes that actually needed repairs more often.

Our tank has a filter at the output valve before it hits the line to the furnace. That's supposed to stop any sediment.

Sounds like it needs another tuneup/adjustment. We do ours every year.

You have filters, probably 2 of them, in the oil line leading from your tank to the burner. Even though there most certainly is residue in the bottom of the tank, it wouldn't have gotten past these filters. Even so, a clogged nozzle wouldn't result in black smoke and a backdraft into your house. Your burner would just not fire.

Oil burners are not complicated, but if you've never worked on them before, now might not be the time. Still, it can't hurt to change the filters, which is easy to do. I don't know how old your burner is, but if it's a newer model, take the top off and see if your heat exchanger is clogged with soot. Take a look in the ductwork for the same problem.

Despite what smcd says...it is an incomplete burn that causes black sooty smoke. You can buy a new nozzle kit yourself and it is a simple fix. You are going to need a pressure gauge though - so you'd be better off hiring someone.

gotcha.

man, with so many posts from smcd, and I couldn't find a reason to post this as a response: