micky wrote:

All these years, I've save the plastic cylinders that oil furnace
nozzles come in, and I finally looked at what they say.

All start with .75 80^ (degrees) but there is a lot of variety in what
comes next. A,B, W, and the other brand's HA and Hx. (I can't find
it right now.)

I had in my mind that the proper nozzle was listed in the owner's
manual, but now that I'm looking, it seems not. (It says 0.75, but
nothing else) How do these technicians decide what nozzle to use,
just whatever the previous guy used? If so, how come there are so
many different containers left behind?

Thanks.

..75 is gallons per hour firing rate (at 100 psi)
80deg is the spray angle
The letters after indicate the flame pattern eg. solid cone, hollow
cone, more hollow cone, semi-solid cone, etc.

Do a search on "oil burner nozzle cross reference" and you'll find some
charts and pictures that explain the patterns better.

The pattern selection mostly depends on the techs experience and what
nozzles they have on hand. The firing rate is the most important, and
the spray pattern selected will be based on the size and shape of the
chamber.

Another complicating factor is that some newer burners don't run at 100
psi and instead have high and low firing rates, requiring some
calculation to get the correct firing rate nozzle for the higher
pressure high rate firing.