Photograph of  an oil tank with seepage around the filler pipe and a too-small vent pipe. Leaky Oil Tank Filler Piping
A Diagnosis & Repair Guide

  • OIL FILL PIPE LEAKS - CONTENTS: What causes leaks at oil tank filler piping? What to do about leaky oil tank vent or filler piping; oil tank piping sources of leaks and oil odors or smells
  • POST a QUESTION or READ FAQs about how to diagnose & repair leaks at the oil tank fill or vent pipe

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Oil tank leaks at the fill or vent piping:

Tthis article discusses the causes of leaks at oil tank fill or vent piping, what the leak and other hazards are, and what to do about oil tank piping leaks.

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Why Oil Storage Tank Piping Leaks & How to Fix Leaky Oil Storage Tank Piping

Photograph of a seepage leak at an oil tank filler pipe, wrapped with a rag

It is very common for there to be leakage around the oil filler pipe or vent pipe where they are connected to the top of an oil tank.

If the tank is buried these leaks may go undetected for some time, but on an above ground oil tank, inside or outside, the seepage is quickly visible and also can be smelled by most owners shortly after an oil delivery.

[Click to enlarge any image]

There are the reasons for the leakage:

Heating oil leak problems at oil tank fittings and pipe threads

The pipe threads (NPT) which are cut into an oil tank top tapping intended to accept the filler pipe or vent pipe are not precisely machined - they are a bit rough.

So are the threads on most large-diameter iron piping used to screw into the tapping.

Having installed a few of these, we can testify that it requires some very careful workmanship to make a leak-tight joint, combining cleaning of the threads, use of a high quality pipe dope rated for use on petroleum products, and turning the pipe into the tapping with sufficient force to seal it without damaging threads. In sum, often the connection is simply not very tight.

Heating oil leak problems due to pressurized oil storage tank fill procedures

Heating oil deliver with an attentive operator (C) Daniel Friedman Bottini Fuel Wappingers Falls NYIn the mind of the installer, "What the hey, after all, the tank is just going to be at atmospheric pressure and filled by gravity from above, right?"

Wrong. Oil companies recognize that the time needed to complete an oil delivery is part of their profit or loss picture. Modern oil delivery trucks are designed to pump heating oil into the tank under pressure in order to speed the delivery process.

Photo at left: the oil delivery operator is demonstrating the correct and thoughtful way to fill an indoor or buried oil storage tank.

During oil tank filling the driver is listening to the tank whistle or tank alarm to avoid over-filling the tank - a common source of seepage and occasional heating oil or fuel oil spills. If the plumber or installer locates oil fill and vent pipes where they are difficult to access, don't expect such careful oil deliveries.

In fact most modern oil tank filler caps have a special fitting, often different from one oil company to the next, that permits the installer to "lock" the filler hose to the filler pipe during the fill-up procedure.

Usually an oil storage tank is filled right to its top during an oil delivery. This is because the way that the delivery driver knows that the tank is full is that s/he is (supposed to be) listening at the filler pipe. Tank fill or vent valves are (supposed to be) equipped with a whistling noisemaking device that indicates when the tank is full as the heating oil reaches the sounding device.

So if, unlike the driver in our photo above, the oil delivery pumper is not listening, say s/he went aside to smoke a cigarette or make a cell call, or if the tank top fittings are not absolutely tight, it is common to see some leakage around the tank top after a fill-up. We discuss details of oil tank pressures created during the oil tank filling operation
at Oil Tank Pressures.

When and what should you do something about a leak at the top fittings of an oil tank?

Photograph of a small heating oil leak at a tank fire-o-matic safety valve at the tank bottom
  • Find the exact point of oil leakage: If you see oil on the floor, check the bottom of the tank, sides and welded seams, and be sure to check the oil lines to the boiler and in particular, check the oil tank safety valve or other tank bottom fittings as there are often leaks at these valves, as shown in this photograph.

    Before addressing tank top filler pipe fittings as a leak source, make sure the oil leakage is from the top of the oil tank.

    Other tank leaks such as at a seam or at perforations indicating a rusted tank need immediate evaluation by an expert, since these could spell major trouble like a big and costly oil spill should the tank break or a fitting blow off.
  • Check for more than one leak point: Make sure that you know where the seepage is coming from. There may be separate spillage outside at the filler pipe from that occurring inside at the tank top or other fill or vent pipe fittings.
Photograph of copper piping used for an oil tank fill and vent pipe
  • Check condition of oil piping: Make sure that the piping you're using is proper in material (steel is safer against rupture than plastic), and in diameter (2" or larger) for both vent and fill pipes.

    [The soldered copper fill and vent piping at the oil tank in the photo just above was unusual and may not be permitted. At the top of this page, the silver-colored (galvanized steel) oil tank vent pipe in the tank is probably too small for modern tank fill methods.]

    An older-installed small diameter vent or fill pipe [like the one shown at the page top photo] risks overpressuring the tank during fill-up, and risks a costly oil spill should an old tank or fill or vent line rupture. Ask your oil company to review the condition of the piping and the condition of the tank.
  • Small oil leaks at the heating oil storage tank: If the seepage is trivial in amount, say simply a light stain just a few inches around the tank top, clean it off and live with it. Or re-make the joints when other plumbing work is planned.

    (It's quite a bit of trouble to re-make these joints, especially if the filler and vent piping have been routed through and cemented into a masonry wall.)

    On our oil tank which recently "developed" a seep after our oil company bought a new, improved, higher pumping pressure delivery truck, I cleaned the tank surface and tied a rag around the filler pipe at the top of the tank to try intercepting the leak before it spreads over the tank and smells up our garage. I am doubtful that this will be effective long term but changing the rag after each tank fill was easier than scrubbing off the whole side of the tank.
  • Video: An Easy Cure for Small Oil Tank Fill or Vent Pipe Leaks & Odors illustrates how to contain oil seepage and smells from leaks at the pipe fittings at the top of an oil tank. Ultimately we traced this leak to seepage at the plastic-threaded oil tank gauge cover. Sealing those threads cured the problem.

    Other videos: VIDEO GUIDES at

Photograph of severe seepage leaks at all top fittings of a heating oil storage tank
  • Significant oil piping leaks: If the seepage is enough to wet the tank or run down the whole tank and drip onto the floor this is an unacceptable condition that needs correction.

    It's likely that the piping will have to be removed and (possibly new) piping installed.

    I am doubtful that any "band-aid" approach like pushing epoxy putty or other sealers around the existing fitting is going to work. The surfaces will be rough, rusty, and oily.

    If a reader or product manufacturer has a sealant that is actually effective in this application, dealing with the rust, oil, surface shapes, and fill pressure, I'll be glad to report on that product here. It would indeed be less trouble than re-piping the tank.
    Make sure the new piping is properly sealed and secured. If heating oil has been dripped on the floor, ask your oil company to clean it up and deodorize the area.

Photograph of a basement heating oil storage tank leak onto the concrete floor
  • Actual oil spills: If there is an actual oil spill, indoors or out, most state departments of environmental protection require that they be notified within 24 hours of the spill, an inspection is performed, and an expert specifies the amount of cleanup needed.
  • Remove un-used oil filler and vent piping: Never ever leave an oil and vent pipe installed at a home where the oil tank has been removed.

    There is a risk that a mistaken oil delivery driver will attempt to deliver to the wrong pipes, flooding and ruining the home with heating oil.
  • Enclosing indoor oil tanks: Don't try simply enclosing a tank that is leaking - you prevent inspection of it, can miss a deteriorating condition, may interfere with service procedures for the heating equipment, and you won't control the odors. If an oil tank is going to be covered for aesthetic reasons, make the covering easily removable for inspection and service.


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