Oil supply pipework and fire valves

What the fire valve is for, and why so many would not work

A fire valve is a simple, entirely mechanical device. A sensor sits close to the burner. If the temperature at that sensor rises above a set point – meaning there is a fire at the appliance – the valve in the oil line snaps shut and stops fuel feeding it. There is no electricity involved and nothing to fail electronically. It is one of the most elegant safety devices in a domestic property.

It only works if the sensor is in the right place. We regularly find sensors that have dropped out of their clip and are dangling behind the boiler, sensors cable-tied to a convenient pipe two metres away, sensors that have been unclipped for a service and never put back, and capillary tubes that have been kinked to the point of being useless. In every one of those cases the valve is decorative.

Fire valve operation is checked as part of every oil boiler service we carry out. If yours has never been checked, it is worth asking.

Materials and how the line should be run

Above ground

Copper or steel, properly clipped, with expansion allowed for on long runs. Plastic is not acceptable for an above-ground oil line in an exposed position – it is not fire resistant and it degrades in sunlight. A short, purpose-made flexible connection at the burner is normal and correct; a length of garden-variety flexible hose is not.

Below ground

A buried line has to be protected against mechanical damage and corrosion. Plastic-coated copper or a purpose-made plastic oil line in a duct is the usual approach, laid at a sensible depth, with no joints below ground wherever it can possibly be avoided – because a buried joint that weeps is invisible until the ground tells you about it, by which point you have contaminated soil. Where a buried line passes under a driveway or hardstanding, it needs ducting so it can be replaced without excavation.

Through the building

Where the line passes through a wall or floor it needs sleeving and sealing, both for fire and to stop the pipe being abraded by the structure. Runs inside the building should be kept as short as is practical and should be visible where possible – buried oil pipe inside a screed is a problem waiting to happen.

Single-pipe or two-pipe

Most modern installations are single-pipe with a tank above the burner, which is simpler and avoids the return line. Two-pipe systems exist where the tank is below the burner level, and they bring their own considerations – not least that a two-pipe pump converted to single-pipe without removing the bypass plug will damage itself. It is a detail that gets missed on burner changes.

Valves, filters and de-aeration

A properly finished oil line has isolation you can actually reach:

• A valve at the tank outlet, so the tank can be shut off without draining anything.
• A filter with a replaceable element, normally at the tank end, ideally somewhere you can kneel next to it. The burner has its own filter as well, and both get changed at service.
• A valve at the appliance, so the burner can be worked on without the whole line draining back.
• A fire valve outside the building, as above.

On installations that suffer repeatedly from air – long runs, tanks below burner level, or lines with a high point – a de-aerator is often the right answer. It separates air out of the fuel and returns it to the tank rather than letting it reach the pump, and it converts a two-pipe requirement into a single-pipe run. Where a boiler locks out intermittently for no apparent reason and the fault always clears after bleeding, air is very often the cause and a de-aerator is the fix rather than another service visit.

What tends to go wrong

The pattern worth noticing is that the oil line frequently presents as a boiler fault. A great deal of money gets spent on burner components for problems that were actually a five-pound seal or a twenty-minute re-route.