The blower fan is an integral part of the gun that includes the motor, electrode, nozzle, oil pump, and fan.
The obstruction is that 90-degree vent pipe attached to the back of the boiler which force exhaust against the vent seam on the side. Vent pipe sections do not always make a tight fit and need three small sheet metal screws and hi-heat duct tape to seal it up. ......
I graduated from Fast Eddy's 12-Week School of Oil Burner Technology where the emphasis was all about doing it FAST ...... and ABLE to convince the customer that you know what the heck you are doing down in their basement with thaht olde oil burner because you got it done, FAST, and talked all about the fuel oil combustion/filthy dirty OIL soot build-up/electrode/nozzle/oil pump/fan blower, all synchronized together in perfect combustion harmony as you was hitting the oil burner with a FAST tune-up. ....
Steve Lavimoniere the oil burner man: North Dartmouth, Massachusetts; Sept 17, 2020 .....
www.youtube.com/watch?v=1T1s9JSHol4 ..... Smell ...
... of fumes coming from oil burner/water heat boiler
So, why do people go with natural gas by pipe or liquid propane in the tank when it costs more and has less btu's than residential fuel oil? Because it burns cleaner and usually needs way less service calls including no yearly boiler/furnace clean-out/tune-up.