Steve wrote:

Pete C. wrote:
It sounds like he said the line comes from the bottom of the tank, goes
up to ceiling level and then back down to floor level where the burner
is.

As for the leak thing, it's quite possible to have a leak that lets air
in under suction conditions and does not leak any fuel outward when the
pump is inactive and the line is at a pretty neutral pressure.

Pete C.

Yes you have that right!

Then the leak would have to be up high somewhere? There are no joints
between the filter (down low) and the pump (down low).

I'll let you guys know what I find or do if it happens again. I am too
stubborn to call a repair man! Thanks!

Steve

The leak doesn't necessarily need to be up high. When the pump is
running and fuel is flowing, a bubble of air could be sucked in from a
leak anywhere along the line.

In a single line gravity feed setup where the line stays along the floor
all the way to the pump the line remains under pressure all the time and
there is little chance of sucking air in the line even if there is a
slight leak.

In a single line suction setup where the line goes up above tank level
(whether the line comes from the bottom or top of the tank makes no
difference) the reduced pressure allows air to be drawn in from any
leaks. These air bubbles will be very small, but they will be drawn
along and accumulate in the pump where they can eventually cause the air
lock.

With a two line suction system you can still draw in bits of air from
leaks, but when they get to the pump they don't accumulate to problem
levels because the excess fuel being pumped is returned to the tank via
a second line instead of being returned to the pump inlet as in a single
line system.

Other than the obvious advantage of finding any leaks in your feed line
before they become bigger, adding a return line from the pump to the
tank will most likely solve the air lock issues. Most pumps are designed
for this and adding the return line is generally as simple as removing a
plug on the pump and installing a fitting and the return line to the
tank.

Pete C.