One of the "experts" that visited our home trying to cure a
hard-starting Riello said that he preferred a single line because it
gave the oil a better chance to warm up before going to the burner.
He claimed that the oil had to be over 50 degrees for some reason or
another so he switched us from a double to a single line system, but
since he was never able to cure the hard start, I have no idea if he
was correct.

How would a single line system heat the oil any better than a two line
system? This doesn't make sense to me...

Also, I have two oil burners feeding off the same line. We had a problem
with one line starving the other. I had check valves put in and that solved
the problem. I also read that you can put in a switch that will only allow
one burner to fire at a time. On my hot water heater I now have a two line
system and on the boiler's burner I have a one line system. I'll let you
guys know if I have any problems with both appliances this winter. It's
still to mild to tell if my installation is golden. My old Beckett head had
a .80 nozzle, was low rpm and ran dirty. The new Riello has a 1gph nozzle
and really heats the boiler to temp faster. The tech also performed a smoke
test and there was no smoke residue on the paper. I was told that the
Reillo will run super clean compared to my 25 year old Beckett head. I hope
to see a 20-30% savings. The tech said that my old Utica boiler was built
like a tank and is made better then the newer Utica.