An oil fired beast, right?
Make sure the flue isn't obstructed with a bird nest, spalling masonry (assuming it doesn't have stainless liner) or a dead raccoon or something. If it's all clear, make sure it has the manufacturer's recommended jet sizing on the burner, or just call back the tech to re-tune the thing, or even down-size the jet (to within manufacturer's parameters), since nearly all oil boilers are ridiculously oversized for the space heating loads, causing it to run at a low duty cycle and poor AFUE. They may have installed the wrong jet, or failed to properly adjust the air/fuel mixture.
Oil burner mixture & combustion efficiency usually drifts over the course of a couple of years, and it's not crazy to have somebody with the experience & combustion analyzer tools to dial it back in at least every other year, if not annually. It's not rare to find an 85% efficiency boiler running at 75% combustion efficiency after a few years of benign neglect. But once it has been tuned and serviced you shouldn't have problems of this magnitude for years unless they screwed it up , or maybe your chimney is clogged/collapsing.