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Question DetailsAsked on 10/10/2016

What could be causing the knocking sound in a home oil tank

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1 Answer


Most likely causes I can think of - sticking air vent (modern ones have a one-way valve, so air can get in to replace the volume of fuel burned, without letting the volatiles in the fuel evaporate, which degrades the fuel and presents a fire hazard. If it is sticking can give a rap or thump when the pressure differential finally gets to be enough to make it open.

Ditto to expansion valve - they have (or should have) an expansion release valve (should be connected to a standpipe that puts the fumes high enough that they are not a fire risk) - sometimes called poppet valves - as it heats up in the sun (above-ground outdoor tanks) as the pressure gets to the point where the valve opens, it "pops" or snaps open. Rarely, but can happen with basement tanks too if the area experiences a significant temperature differential - like is part of conditioned space but is allowed to fluctuate more than normal for conditioned space.

Ditto with respect to air trying to escape due to heating and lifting a hinged fill port or filler tube cover - either because vents are not venting properly, or during a dramatic change in atmospheric pressure causing a fill hatch or cover to bump open and clang (would take a pretty dramatic pressure change - but did hear this happening on tank cars once when a tornado was passing close by, and the pressure relief valves were popping off down the length of the tank car unit train).

Just thermal expansion and contraction of the tank and/or frame if in the sun/shade alternately, or in-house and exposed to significant temperature differentials.

Something hanging from/near the tank (like restraining chain on fill cap) blowing in the wind or such, slapping against the tank.

Fuel pump, if mounted at tank - if bearings are going bad, can "knock" or "tap" when running.

Tank going nearly dry, so pump is pulling air along with fuel - or sludge in bottom of tank causing similar effect of pump starving for fuel.

Sticking or "clunking" electric controlled valve, which would occur at start and/or end of firing cycle

Rarely, if the tank is leaking (at or near the bottom, below fuel level) as the fuel leaks out, if there is not a functioning makeup air valve, air bubbles will come into the tank through the leak and "pop" or "gurgle"as they reach the surface - just like air bubbles going into an upside down bottle as the liquid pours out. Can sound quite loud in an enclosed tank, which acts as an echo chamber - especially with metal tanks. Same thing can happen with a gravity feed tank without inlet air valve (or it is plugged) - bubbles passing through the fuel line back into the tank and popping in the tank.

Kids dropping rocks into the filler tube for basement or underground tank (had one case where group of kids completely filled up the filler tube with landscaping stones.)

If static-like or snap-like sounds, remotely possibly shorting out of an electric level gage or leak detection system (if multi-wall) - obviously not a good thing in or immediately at a fuel tank.

Leak detection system going off, but the battery is almost dead so instead of sounding the alarm or flashing the warning light, it is just giving a "clunk" or "click" sound. Commonly happens with alarm systems of all types that lose hardwired power so drain their backup battery - alarm sound commonly ends up just being a series of screches, clicks, or taps in the speaker.

With below-ground tanks or in-basement tanks, water leaking into the tank or into the filler pipe can cause a "ping" or "tap" or "drip" sound in the tank.

If you can't tie down the exact location by ear, try a stethoscope on the piping, tank, supports, etc.

Think of when these noises occur, whether regular frequency or not, if occurring when fuel is being taken out or not or if turning power off to any sensors or pump changes it, and if these thoughts don't correllate with any of the mentioned conditions or if your fuel flow becomes irregular or your tank is losing fuel, then you need a fuel oil system company to come look at it. If tank is leased, then who you lease it from probably. Fuel supplier if they own the tank. Otherwise, independent fuel system installation and repair company.

Answered 3 years ago by LCD

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