Running out of oil: how to restart the boiler properly

Why it will not just start

An oil burner’s pump is designed to move liquid, not gas. When the tank empties, the pump pulls air through the supply line and fills it. Once you have a delivery, that air is still sitting in the pipe between the tank and the pump, and the pump cannot push it out on its own – it has no significant capacity to compress and expel a long column of air.

So the sequence runs, no fuel arrives at the nozzle, the photocell sees nothing, and the burner locks out. Resetting repeatedly will not clear it. The air has to be bled out at the pump.

What bleeding involves

Every oil pump has a bleed point – a small screw or nipple on the pump body, sometimes with a vent port. The procedure in principle:

1. Make sure there is actually oil in the tankAnd that the tank valve and fire valve are open. Bleeding against a closed valve achieves nothing.
2. Attach a tube to the bleed pointInto a container. Oil will come out and it will not be a small amount.
3. Open the bleed screwA turn or so – not out.
4. Run the burnerThe pump pushes air, then foamy oil, then clean oil. The sequence will lock out repeatedly while this happens, which is normal here.
5. Close the bleed when clean oil runs steadilyNo bubbles, no foam.
6. Reset and runThe burner should now light and stay lit.
7. Check the filtersThis is the part people skip, and it is where the run-dry actually costs you.

On some installations there is more to it. A two-pipe system, or a pump fitted with a bypass plug, behaves differently. A de-aerator changes the procedure. And some pumps need priming rather than just bleeding. It is a job a confident owner can sometimes do, and it is also a job that goes wrong in ways that are messy and occasionally expensive.

The sludge problem, which is the real cost

The bottom of an oil tank is where everything settles: water from condensation and the occasional wet delivery, sediment, degraded fuel components, and in older steel tanks, rust. Over years it forms a layer that sits below the outlet and, in normal use, is never disturbed.

Running the tank right down changes that. The outlet starts drawing from the bottom of the tank, and the layer goes straight into the supply line.

• Filters block, immediately or within days. Both of them.
• Water reaches the burner, which will not burn and causes lockouts that look like ignition faults.
• Sediment can reach the nozzle, whose orifice is measured in thousandths of an inch and blocks readily.
• Fine abrasive particles reach the pump, which is a precision component with close tolerances.
• Sludge coats the inside of the line, so it keeps shedding into the fuel for weeks afterwards.

That is why a run-dry in December frequently becomes a filter change in January and a pump in February. The restart was the easy part.

What to do after a run-dry

1. Let the delivery settle for an hour or two before firing up, so any disturbance in the tank can drop back down.
2. Bleed the line properly rather than repeatedly resetting.
3. Change both filters – tank-side and burner. Not optional after a run-dry.
4. Check the filter bowls for water, which tells you what is in the tank.
5. Watch it over the following weeks. If lockouts recur, the line is still shedding sludge and may need flushing.
6. Consider a tank inspection if the filters came out badly contaminated. That is the tank telling you something about its condition – see eight signs an oil tank is near the end of its life.

Not doing it again

• Fit an electronic tank gauge with a low-level alarm. They are inexpensive and they remove the whole problem. Float and sight gauges stick, and an electronic one that tells you at a quarter full is worth far more than it costs.
• Order at a quarter, not at an eighth. That reserve is what keeps the sludge layer undisturbed and gives you time when deliveries are busy.
• Know your winter consumption, so a quarter tank is a number of weeks rather than an abstraction.
• Order early in the season. Demand and lead times both rise sharply in a cold snap, and that is exactly when you cannot wait.
• Consider a top-up or monitoring service if the property is a second home or is unoccupied for periods.

More on buying sensibly in buying heating oil.