Water in your oil tank
Every oil tank collects some water over its life. Oil floats, so the water sinks to the tank bottom and sits on bare steel, right where the fuel pickup draws from. Checking for it costs two dollars. Left alone, it corrodes the tank from the inside and feeds the sludge that clogs your filter.
How water gets in
- Condensation. The tank breathes through its vent, and moist air condenses on cool steel walls, worst in the air space of a half-empty tank through summer, when day and night temperatures swing. Outdoor tanks condense the most.
- A bad fill cap or gasket. Rain and snowmelt run straight down the fill pipe. On buried tanks, NORA found that water inside usually came from a defective fill cap or a corroded vent connection rather than a hole in the tank shell.
- Delivered with the fuel. A small amount of moisture can come in suspended in the oil itself.
What water does in the tank
Microbes live at the boundary where water meets oil. They feed on the fuel and multiply, and their byproducts are the sludge that clogs filters and the acids that pit the tank bottom from the inside out. In NORA's research that pitting is the leading cause of tank failure. Outdoors, standing water also freezes in bottom valves and low fuel lines. A bottom valve that filled with water and froze can crack and drain the tank.
Finding it: the paste test
Water-finding paste, a couple of dollars at any plumbing counter, changes color on contact with water. Coat the bottom few inches of a tank stick, lower it gently to the tank bottom per the sticking guide, and read two lines when it comes up: the oil line for gallons, the color change for water depth.
Two setups can keep water out of sight, per NORA: tanks with top-draw fuel lines, and tanks pitched the wrong way. Either one can hold a real layer of water for years without anything showing up at the filter. Check with paste once a season rather than waiting for a symptom.
Getting it out
- A properly pitched bottom-feed tank sheds small amounts of water continuously into the filter. A wet filter element at service time usually means that is where the water went, so the fix is a filter change and a look for the source.
- A bottom drain valve, where one exists, lets a tech draw the water layer off directly. On outdoor tanks the valve must be a freeze-tolerant type, and it stays plugged when not in use.
- A pump-out through the fill or a top tapping handles larger amounts, buried tanks included. That is a service visit, and the water and sludge must be disposed of properly, never poured on the ground or down a drain.
- Then find the source. NORA's procedure is removal first, diagnosis second: a tank that took on water once will do it again until the cap, gasket, vent, or pitch that let it in is fixed.
Common questions
How much water is too much?
Any paste line at all is worth fixing at the next delivery or service; more than an inch is worth a call this week, especially on a buried tank. Time matters as much as depth, because the microbes keep working for as long as the water sits there.
Does water in the tank mean my tank is leaking?
Usually not. Water nearly always comes in from above: a tired fill gasket, a loose cap, a corroded vent connection, or plain condensation. On buried tanks NORA specifically notes the fill and vent components are the usual suspects rather than the shell. If water keeps coming back after those are fixed, have a technician look further.
Can my oil company check for water?
Yes, and the good ones do it as routine. Ask for a water check at your next delivery or tune-up; the driver or tech has the paste and the stick on the truck.
Why does my buried tank keep showing trace water?
Small or trace amounts in an underground tank may be impossible to remove completely, per NORA, and a buried tank pitched away from the fill can hide a little from the stick entirely. Underground, the goal is to stop the inflow and keep the trend flat rather than chase the last cup.
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