On Sat, 07 Dec 2013 07:02:38 -0500, 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.

Those numbers like .75 is NOT "degrees". It's the GPH rate. (gallons
per hour). A .75 will deliver .75 or 3/4 gallon in an hour if it runs
continuously. Just look at the number on your old nozzle and use the
same size. A furnace is designed for that nozzle size.
UP sizing is probably not a good idea. But you could likely go down in
size. But for less heat value. I'd stick with the same number.