Oil Tanks – Internal & External – Furnace Service Maine

Overview

Heating oil (No. 2 fuel oil) is the dominant home heating fuel in Maine. Tanks are available in a range of sizes — 275, 330, 500, and 1,000 gallons are the most common for residential use. The tank must be properly installed to prevent leaks, spills, and fuel delivery problems. A leaking tank can contaminate soil and groundwater, resulting in extremely costly environmental remediation.

Maine DEP: Maine's Department of Environmental Protection regulates petroleum storage. Tanks of 660 gallons or more (above ground) or any size underground tank require registration. Even smaller residential tanks must be installed per NFPA 31 and local codes. Always obtain required permits before installation.

Indoor (Internal) Oil Tanks

Advantages of Indoor Tanks

Protected from freezing — No. 2 heating oil gels at approximately -10°F to -20°F, but in extremely cold conditions viscosity increases. An indoor tank eliminates this concern entirely.
Easier to check fuel level and inspect for corrosion.
No exterior exposure to UV, weather, or vandalism.
Generally lower installation cost — no outdoor pad, no burial.

Location Requirements for Indoor Tanks

Tanks must be located on a non-combustible surface or a surface protected with a non-combustible material (concrete basement floor is ideal).
Minimum clearance: 7 feet from any furnace, water heater, or other heat-generating appliance. This protects against flash fire during a spill.
Tanks must not block emergency egress from the basement.
The basement must have adequate ventilation — oil vapor accumulation is a fire hazard.
Tanks larger than 660 gallons in a basement require secondary containment.

Standard 275-Gallon Indoor Tank

The most common residential indoor tank is the 275-gallon oval steel tank, often called a "basement tank." It stands approximately 38" tall, 44" wide, and 27" deep — sized to fit through a standard basement door and bilco doorway. Newer models are often double-walled (tank within a tank) for secondary containment and leak detection.

Indoor Tank Installation Steps

Step 1

Select Tank Location

Choose a location on the basement floor that maintains all required clearances, is accessible for annual inspection, and is near the furnace fuel connection without requiring excessively long copper tubing runs.

Step 2

Place Tank on Legs or Pad

The tank must sit on its factory legs (usually included) or on concrete blocks to allow inspection under the tank. The tank must be level — use a level and shim the legs as needed. A tank that is not level may cause the fuel gauge to read incorrectly and can create pockets of water or sludge.

Step 3

Install Fill and Vent Pipes

A 2" fill pipe runs from the tank to the exterior of the home, terminating with a fill cap at a location accessible to the fuel delivery truck. A 1.25" vent pipe runs from the top of the tank to the exterior, terminating with a screened vent cap at least 2 feet above grade. The vent must not be blocked — a blocked vent creates vacuum in the tank during drawdown and pressure during fill, potentially causing a blowout.

Step 4

Install Fuel Lines

Connect 3/8" OD copper tubing from the tank's bottom outlet to the furnace burner. Use flare fittings — never compression fittings on fuel oil lines. Install an inline oil filter between tank and burner (a 10- to 25-micron cartridge filter is standard). Install a manual shut-off valve at the tank outlet and a fire-rated fusible link valve in the fuel line where it enters the furnace room (required by NFPA 31 in many situations).

Step 5

Two-Pipe vs. One-Pipe Systems

If the tank is above the burner (gravity feed), a single-pipe system works. If the tank is below or far from the burner, a two-pipe system with a return line allows the pump to pull fuel more reliably and returns excess oil to the tank. Most basements use a single-pipe system with a good fuel pump.

Step 6

Install Fuel Gauge and Leak Detection

A float-type fuel gauge (visible from the tank top) or an electronic gauge allows monitoring of fuel level. On double-walled tanks, install or test the leak detection sensor in the interstitial space — these typically have a small port with a dip stick or electronic sensor that signals if fuel is present between the walls.

Outdoor (External) Oil Tanks

Above-Ground Outdoor Tanks

When interior space is not available, tanks are installed outside on a concrete pad or heavy-duty plastic pad. Outdoor tanks must be rated for exterior use — many include additional corrosion protection or are constructed of fiberglass or polyethylene rather than steel. Steel outdoor tanks benefit from periodic inspection and repainting to prevent corrosion.

The tank must be on a solid, level, non-combustible pad.
Setback from the building per NFPA 31 and local ordinance — typically 5 feet minimum from any opening.
In Maine, outdoor tanks should be filled with an anti-gel additive in late fall to prevent viscosity problems in extreme cold.
The fill and vent fittings must remain accessible and labeled for the delivery driver.
Fuel lines running into the building must be protected from freezing and mechanical damage — use buried copper or a properly insulated line.

Underground Oil Tanks

Underground storage tanks (USTs) are rarely installed new in Maine for residential heating due to the high cost of installation, environmental regulations, and the devastating cleanup cost if they leak. Most homes with underground tanks are dealing with aging legacy installations.

Old Underground Tanks: If you have an aging underground tank (pre-1980s bare steel), the prudent course is to have it removed or abandoned in place by a licensed contractor before it leaks. Maine DEP has specific requirements for tank closure. Do not delay — a leak from a buried residential tank can cost $50,000 to $500,000 or more to remediate.

Oil Line Maintenance

Replace the inline oil filter annually (or more often in systems with older tanks that generate more sediment).
Inspect all fuel line flare connections annually for seepage.
Keep a record of fuel deliveries and consumption — a sudden increase in consumption may signal a leak or burner efficiency problem.
Consider adding a fuel oil additive each fall to prevent sludge accumulation in the tank bottom.

Heating Oil Delivery

For reliable oil delivery in Maine: Maine Energy Services.

Tank Installation & Service

For oil tank installation and fuel system service: BRF Services.