What is the purpose of top plumbing on the oil tank?

It still has to go to the bottom of the tank to draw the oil.

Because it won't go all the way to the bottom. I have a problem with sludge clogging the bottom outlet that will get solved by drawing from the top.

What happens when your new line fills with air?

"I have a problem with sludge clogging the bottom outlet ..."

Might be time to drain the tank and flush the crud out.

If the sludge contains any moisture it will start to corrode the tank (and possibly has already).

Brick,

Where I live, it costs almost as much to clean out a tank as it does to replace it, so I am looking to extend the life prior to going through that big expense. The tank is only 9 years old.

The issue I am having is that sometimes, I do not get enough oil flow from the tank to support both the furnace and the (separate) hot water heater, and the furnace starts to sputter. The only remedy that has worked is to have the lines blown backward into the tank. We are therefore assuming a bad fill at some point in the past and a collection of sludge on the bottom of the tank.

More than one of the heating oil guys I deal with have recommended this replumbing effort, so i am trying to get some idea of average cost so I know I'm not getting gouged.

You are going to have a problem with air lock.

Do it right clean/fix or replace the tank.

Drawing from the top is an OK procedure, but always difficult to know how far to stick the inlet pipe down, if your wishing to stay off the sludge.

The complications are that the oil pump(s) now has to have that additional suction capability to draw it up.

My advice would be to leave it on the bottom, convert or change your oil pump(s) to 2-line, meaning there is a return, recycling oil back through the top of the tank, which promotes oil turn over & filtration. Then if you have adequate filters, you can get that tank cleaned up at the same time with frequent filter changes.

zl,

I'm unclear how filtration would be improved with a return line when I have only one filter at the outlet on the bottom of the tank. Would the return line also be filtered? (It would seem to me that any flow back into the top would have negligible effect on 200 gallons of oil pushing out through the bottom.)

Would I have any luck staying as I am and changing the bottom filter, say, once per month during the heating season?

No, just the oil would flow more turning over and mixing the tank as the excess returns versus just draw and use what the burner needs. Certainly if you are finding dirt or gelled oil (oil & water) in the filter I would change it more often.