Guide to Above Ground Outdoor Home Heating Oil Storage Tanks
InspectAPedia^®  -      

• Above ground outside oil tanks: inspections, leaks, troubleshooting
• Water in oil tanks - sources, problems
• Heating oil additives and chemicals for outdoor oil tanks to deal with water, icing, and waxing in outdoor oil storage tanks
• Questions & answers about installing, using, & maintaining an outdoor aboveground heating oil storage tank

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.

Such 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.

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.

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.

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.
  
  

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.