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Fuel Flow from Underground Fuel Tank

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My furnace keeps needing a reset. I traced the problem to the fuel line. (I’ve checked the fuel filter, strainer and gasket on the fuel pump, all good. Also, I cleaned the outside of the burner nozzle and inside the nozzle’s brass filter.)

Blowing compressed air into the line heading into the tank temporarily fixes the problem. The furnace will ruin for a few hours before stopping. I blow it out again and the furnace resumes. But I have to keep blowing out the line and it seems like the amount of time the furnace will run after each blowout is shorter and and shorter.

I have a 550 gal underground tank circa 1974. ( I know, time to get it out of the ground.). It has about 200 gallons of fuel or 19.75 inches when I dip the fuel stick.

It had two fuel lines originally allowing the oil to circulate back into the tank. About 20 years ago that was bypassed by the repairman when the flow was found not to be great, leaving only one line. Since then I’ve had problems off and on and had to bleed the pump every now and then especially when I’ve been away and the furnace doesn’t cycle on. Then about 5 years ago I installed a Tiger Loop, that made a huge difference for the better and everything has has been great until recently. I usually keep the tank full.

There has also been a history of water in the tank. That was because the original tank installer rigged the fill pipe connection because the threaded female inlet was off about ten degrees (when the tank was manufactured the threaded ring is welded into place. That ring was not plumb.) The installer hid problem from my father. When about an inch of water was discovered in the tank I dug it out the top and found the loose fill pipe connection. The tapered end of the pipe was resting on top of the treads and surrounded with plumbers putty. The pipe was braced with wire hangers in the soil. I was pretty pissed. After paying to have the water pumped from the tank I got a new fill pipe and screwed it in myself being sure to seal the threads. So now the fill pipe sits at a 10 degree angle instead of being plumb. No big deal. At least there wasn’t any more water getting in that way.

I have a Wayne MSR burner with a Suntec AV2A-7116 fuel pump. Nozzle is a Delavan 65 / 70A / N12 I was told the burn rate is .75 gallons per hour.

I haven’t checked the tank for water with the water test paste on the stick in a long time. I’ll do that next.

Do both fuel lines enter the tank at the same height? Is one higher and one lower?

Could I have 19” of sludge after 50 years? Or worse, water?

I am having the UST removed in a couple of months, but I’m trying to burn off the remaining oil first.

Thank you in advance for any insights.
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So you have an oil burner and you cleaned the nozzle, this is the first issue. They should be replaced every year or two. Next is an under ground tank. This is going to cause you a lot of problems. If you plan on keeping the oil burner, get the oil out of that tank and get an above ground tank. With out giving legal advice, just a warning - - the under ground tanks are known to leak. If by chance it is tested and found it leaked, the cost of clean up could range in the $20,000 to $100,000 area. Thus start now - drain the tank and hope for the best. The EPA treats this as a huge problem, forcing home owners to clean it up and gives out fines, big fines. Converting to LP is not that expensive and is much cheaper to operate. Wayne makes the conversion kit and costs in the area of $1500 from a contractor plus labor. You will not be able to install it, as it will require a combustion analyzer to tune it and set the corrected flow rate. A .75 nozzle will convert to around 90,000 to 120,000 BTU rated burner. What every you do, get the old tank out and have the ground tested. Hiding leakage is frowned upon, and can make things a lot worse. Good Luck
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