your vent should just be a pipe from on of the tanks back outside near the fill pipe - it should have a screen on the outlet and a cap (usually a combination screen/cap).

if the vent is plugged it's one of two options, the whistle (combination whistle gauge located on the tank where the vent pipe enters it) or the screen cap.

having a plugged vent is a VERY dangerous condition, during a fill (older tanks especially) have split open because of plugged or inadequate venting leading to hundreds of gallons of oil in the house and a serious nightmare. Don't underestimate this hazard - if you "fix" it yourself, be absolutely sure the venting issue IS fixed before you try to fill it with oil again. It's like a lot of things, if you have to ask - you may be better off calling a plumber.

Thanks, Berlin--the information is much appreciated. One way or another, I'll find a plumber.

I am surprised that the oil company wouldn't handle this for you. While it is your tank, you are their account. Are you buying from a discounter or a full service oil company? Paired tanks can be problematic if not piped properly and the previous comment about hundreds of gallons filling your basement is a very real possibility. Unfortunately I saw a guys house get turned into a superfund site in this manner. Call a servicing oil company, describe the problem, have them analyze and repair as necessary and buy oil from them. Oil has a significantly higher service requirement than gas. This is probably a pretty easy fix, as the system has functioned before, right?

I'm sorry I'm so late updating this thread. It turned out that my gauge was stuck at 1/4 of a tank, when in fact the tank was full. A plumber came and figured this out for me. What a nice Christmas bonus--$1,500 worth of heating oil I didn't know about!

Anyway, problem solved. Thank you for your comments and advice.

alisande,

Do you not keep track of your oil deliveries? 350-400 gallons seems a lot of oil to "find." ;-)

Nope, I don't keep track of my oil deliveries. I heat mostly with wood, using the oil burner only briefly first thing in the morning. I normally don't go through very much oil.

Last month, however, I ran out of firewood for the first time in memory. I've been heating with oil, and appreciating my oil bonanza. :-)