Photograph of a residential outdoor above ground oil tank - was this tank intended for outdoor use? Photo courtesy Arlene Puentes, Kingston NY Guide to Above Ground Outdoor Home Heating Oil Storage Tanks

  • ABOVE GROUND OUTDOOR OIL TANKS - CONTENTS: Above ground outside oil tanks: inspections, leaks, troubleshooting, Heating oil additives and chemicals for outdoor oil tanks to deal with water, icing, and waxing in outdoor oil storage tanks
  • POST a QUESTION or READ FAQs about installing, using, & maintaining an outdoor aboveground heating oil storage tank

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Above ground heating oil storage tank - AST inspections outdoors:

How to inspect above ground oil storage tanks (ASTs) when they are installed above ground outdoors: This article discusses the risks of heat loss or leaks when above-ground outdoor oil storage tanks are used outside. It explains how water gets into oil storage tanks no matter where they are located, above ground oil tanks, buried oil tanks, or even possibly indoor oil tanks. Photograph at page top courtesy of Arlene Puentes.

Green links show where you are. © Copyright 2015, All Rights Reserved.

Guide to Issues with Indoor-rated Oil Storage Tanks Used Outdoors

Aboveground outdoor oil tank (C) Carson Dunlop AssociatesThe article explains the problems caused by water in oil tanks and also the problems faced by owners of above ground oil storage tanks located in cold climates. This article is a sub-chapter of OIL TANK INSPECTIONS

[Click to enlarge any image]

This article, sketchs, and photographs give advice and example photos for the visual inspection of above ground oil tanks for leaks and damage including damaged or leaky oil storage tanks, improper oil tank piping, valves, and indoor-type oil tanks located outdoors.

Sketch of an above ground outdoor oil storage tank is courtesy of Carson Dunlop Associates.

Article Contents

Here are a some important indicators of tank condition that any home inspector can include when an oil storage tank is visible and accessible inside or at a building.

Oil tank label tag

Oil Tank Size and Rating for indoor use but Used Outdoors: If you find a 250g or 275g oil storage tank above ground outside, check its label and UL rating to see if that location was permitted.

Similarly, if you learn that a buried oil tank is an older, small-capacity tank such as 250g or 275g, it is reasonable to assume that an "indoor use only" oil storage tank was buried outside, as we have not located a single instance of a 20 year old 250/275 gallon oil storage tank which was tested, UL-Labeled, and rated for outdoor use above ground nor underground.

An oil tank lacking a rating for outdoor or buried use may lack adequate corrosion resistance and strength, risking rust-through, leaks, and even a dangerous collapse hazard.

See OIL TANK STANDARDS - UL for more about UL labeling & standards for heating oilstorage tanks.

Oil tank labelSuch oil tanks often need to be replaced. Unfortunately so many indoor oil tanks have been used outside, above ground or buried, that their use has been so popular that UL standards for labeling and controlling oil tank use are a bit more confusing.

Modern oil tank labels might no longer indicate if the tank is intended for outdoor use or not, and modern oil tank manufacturers may have rated tanks which were previously labeled "indoors" as now suitable for outdoor use or even buried-use.

Check with the manufacturer of your oil tank before moving it, using it outdoors, or burying it.


Problems Faced by Above Ground Outdoor Oil Storage Tanks

Photograph of an open oil outdoor oil tank vent and an oil tank filler pipe exposed to roof spillage.

How Water Gets Into Oil Storage Tanks

Water can enter an oil storage tank by more than one means:

  • Water may collect in an oil storage tank, above ground or buried, or in some cases even indoors, when the oil is left at low levels in the tank for long periods, particularly in periods of cool weather or in periods of alternating warm humid and then cool weather.

    As temperatures change around and then in the oil tank, warm moist outside air is drawn into the oil storage tank through its vent. Moisture in the air condenses on the cool tank interior surface, and can accumulate in the tank.
  • Water may enter an oil delivery truck by the same means as just discussed above for oil tanks.

    From the oil delivery truck water, along with oil, may then be pumped into the property owners oil storage tank. Some oil companies are quite diligent to avoid these problems or to include water filters on their delivery trucks.
  • Water may enter an oil storage tank by delivery of a "bad" batch of oil from the oil delivery company.

    Those huge commercial oil storage depot tanks you may see, perhaps near a railroad line or a river, may be deliberately maintained with a few feet of water in the bottom of the tank so that if a leak occurs it can be discovered as escaping water before the actual oil leaks out.

    When an oil company truck is being filled from one of these tanks, in some conditions some of the larger tank's water can enter the oil delivery truck tank where under some circumstances it can also be pumped into the homeowner's oil tank.
  • Water may enter an outdoor oil storage tank from roof runoff (rain or melting snow) spillage onto or near the oil filler or vent line for an oil tank.

    I've seen this condition both at above ground tanks which were placed under the house eaves (right where roof spillage fell onto the tank) and also at buried tanks whose filler pipe was set flush with the ground.
  • Water may enter a buried oil storage tank by groundwater leaks into the oil tank piping, fittings, or via leaks in the upper areas of the tank itself. Buried oil tanks whose fill or vent pipe are at, below, or close to ground level may also obtain water from surface runoff if the tank fill or vent are not adequately protected.
Photograph of a half buried outdoor oil tank. Was this oil tank intended for this exposure? Was it intended for ground contact.

Problems Caused When Water Enters an Oil Storage Tank?

What happens when an oil tank is outdoors? Water can enter the oil storage tank where it causes possibly serious problems for the heating equipment.

Water in the fuel oil or simple exposure of the oil to cold temperatures can lead to loss of heat and resultant damage to a building by several means:

The photo shows an oil tank which is half buried outdoors under a deck. Was this tank intended for outdoor use at all? If so, was it intended for use when in contact with the ground? Probably not. The risk is tank rusting, water entry, oil leaks, and related problems we've already listed.

  • In cold climates, water in the oil tank, which will reside at the bottom of the tank, may freeze in the tank or in the oil line, particularly for installations at which the oil line is attached to the bottom of the tank.

  • In warm, moderate, or cold climates water in the oil storage tank can cause rusting which is pulled into the heating equipment where it clogs the filter, fuel pump, or oil burner nozzle

  • In some modern high efficiency systems a special filter is used which will stop flow completely (such as the "System 2000" heating boiler), shutting down the burner in response to water or debris buildup in the filter - a move to protect the equipment.

  • In cold climates, heating oil in an outside oil tank supply line may "Jell" and stop flowing at cold temperatures.

  • Fire risk: Outside above ground oil storage tanks in cold climates exposed to jelling of the heating fuel may be (in error) fitted with a heating tape in an attempt to avoid freeze-up in the oil line itself. This is a potential fire hazard. Heat tapes should not be used on heating oil lines.

Photograph of a dual use outdoor oil tank with leaks at the oil lines.

Outdoor oil tanks in cold climates risk loss of heat from freezing water or jelling fuel as we just cited. In the photo shown here the oil lines are taken off of the top of the oil tank so as to avoid picking up water that may be present in the tank, avoiding the icing problem.

Note that no heat tapes are in use on the oil lines (good, that reduces a fire risk). It looks as if there has been some seepage around these oil lines, that the tank is old, and that one tank is being used to serve two oil-fired devices (two sets of oil lines leaving the tank).

Water in oil tanks also often leads to internal corrosion and leaks in the tank itself, regardless of the tank location, though buried tanks and indoor oil tanks are less prone to water accumulation due to in-tank condensation in response to temperature changes than a tank located above ground outside.

Oil storage tanks usually fail from rust perforation due to combination of water inside the tank with sulphur in the fuel oil. External rust, unless very heavy, isn't highly correlated with internal rust. A new tank, when required, may cost more $2000. installed, including removal of the old oil tank.

When an outdoor tank is exposed to these conditions and even for an indoor tank which we suspect has had a dose of water and sludge, we recommend regular use of a fuel oil additive such as 4 in 1 Hot(TM) to absorb water and to help break up sludge. The best solution is to locate the tank indoors or to build a heated shelter over the outdoor tank.

Heating or Fuel Oil Additives for Outdoor Oil Tanks

In cold climates, heating or fuel oil additives for above-ground outdoor oil tanks can help prevent loss of heat by adding a pour point depressant which lowers the temperature at which the heating oil will form waxes or jell, and by adding a chemical, typically an alcohol, to remove [small amounts] of water from the oil.

I've used a product called "4-in-One Hot" which contains both a sludge break-up chemical and alcohol to help remove water from the heating oil. Such additives may indeed help break up sludge which tends to clog old heating oil lines.
See HEATING OIL CLOUD WAX GEL POINT for details about heating oil waxing or jelling

Watch out: two warnings about using heating oil additives and chemicals for outdoor oil tanks:

  1. No oil additive is going to remove a large quantity of water from an oil tank. Measure the amount of water present. If it's inches, your oil company can pump the water out (leaving the heating oil intact).

  2. Oil additives that break up sludge might in some circumstances precipitate frequent clogging of the oil filter installed at your heating boiler or furnace since an increased amount of debris is being freed and sent along the oil lines.

    If you find that your boiler stops working shortly after receiving an oil delivery, check to see if the problem was a clogged oil filter. That would suggest that your tank has a lot of debris and that the debris or sludge were being stirred up whenever oil was delivered. Discuss this concern with your heating service contractor.

Pour point depressants for heating oil tanks are about the same as similar products used by owners of diesel fuel powered automobiles and trucks in cold climates, but except in dire emergency I would not recommend substituting one for the other as there are some differences in these fuels and chemicals.

Note: these tips are not a complete oil tank installation guide. Proper installation must be done by trained service technicians and must comply with local building codes.

Readers whose heating system uses an above-ground outdoor oil storage tank should be sure to also


Details about the various types of oil storage tank additives, treatments & chemicals and some warnings about their use are



Continue reading at OIL FILL & VENT PIPING or select a topic from the More Reading links shown below.


Suggested citation for this web page

ABOVE GROUND OUTDOOR OIL TANKS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

More Reading

Green link shows where you are in this article series.


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Publisher's Google+ Page by Daniel Friedman

Click to Show or Hide Citations & References