Outdoor oil tanks in winter
Heating oil does not freeze the way water does; it waxes. Chill it far enough and paraffin crystals cloud the fuel, plug the filter, and finally gel the line solid, usually on the coldest night of the year. Outdoor tanks and exposed lines take nearly all of these calls, and every one of them is cheaper to prevent than to thaw.
What is actually happening
Below its cloud point, wax crystals start forming in the fuel. They restrict flow through the line, blind the filter, and spoil atomization at the nozzle. Colder still and the fuel gels into what everyone calls a frozen line. Untreated #2 heating oil typically starts clouding in the teens Fahrenheit, though blends vary; your dealer can tell you what temperature their winter fuel is treated to. Water causes the same symptoms: free water sitting in a low spot of the line freezes solid. Per NORA, outdoor tanks with bottom-fed lines are the most susceptible of all, because the line leaves the tank at its coldest, wettest point.
Prevention that works
- Cold-flow improver, dosed before the cold. These additives lower the temperature where wax forms. Per NORA, dosed with each delivery they work very well; poured into a tank that has already gelled they mostly do not. If your tank is outside, ask your dealer to treat every winter delivery.
- Kerosene blending. The traditional fix: kerosene dissolves wax and flows in deep cold. Dealers in cold pockets blend it into winter fuel for exposed tanks; it costs a little more per gallon and leaves more margin before the fuel starts to wax.
- Insulate the exposed line from the tank to the wall, and have the piping pulled from a top tapping with the pickup set about 6 inches off the tank bottom, above the layer where water and the coldest fuel sit. The abandoned bottom valve gets closed and plugged, because tanks have thawed and drained through a valve that was only "closed".
- A tank enclosure. NORA recommends them for outdoor tanks: shelter from wind, snow, and ice in winter, shade against summer temperature swings, and a bumper against the plow. Those swings drive the condensation that puts water in a tank.
None of that changes where the tank sits. Placement matters more to how long a tank lasts than the brand on the plate does. Indoor, outdoor and buried placement are compared in the heating oil tank overview.
If the line is already gelled
That is a service call, and on a brutal night you will not be the only one in the queue. The tech will clear the line back to the tank, often reroute it to a top draw, insulate what stays exposed, and get the burner running. Resist the urge to improvise with heat sources around an oil tank or to crack burner-side fittings yourself; bleeding and restarting after a gel-up is licensed-tech work, the same boundary as every burner-side job on this site.
Common questions
At what temperature does heating oil gel?
Untreated #2 oil typically begins clouding in the teens Fahrenheit and gels somewhat below that, but the exact numbers depend on the fuel batch and blend. Treated or kerosene-blended winter fuel keeps flowing far colder, so ask your dealer what they treat to.
Will pouring in additive fix a frozen line?
Almost never. Per NORA, cold-flow improvers prevent waxing very well and rescue an already-gelled system poorly. The additive cannot reach wax that has already plugged a line. Prevention is a dose with each delivery; a gelled line is a service call.
Do outdoor tank covers actually help?
Yes. An enclosure shelters the tank and its fittings from snow load, ice, and wind in winter, and shades it in summer, which cuts the day-night temperature swings that drive condensation. NORA recommends enclosures for outdoor tanks and insulation for exposed lines.
Is moving the tank indoors worth it?
Where space and code allow, it is the most complete fix. NORA recommends indoor installation whenever possible: no gelling, no frozen lines, less condensation, longer tank life. If a replacement is in your future anyway, weigh it; the tank lifespan guide covers the rest of that decision.
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