Oil Furnace and Boiler Flues

What builds up in an oil flue

CSIA explains that oil-fired appliances produce soot over a winter of operation, made up largely of carbon and sulfur. It accumulates on the chimney walls, can partly block the flue, and can fall to the base of the chimney or back into the appliance connector.

Unlike creosote from wood, oil soot is usually not a fuel for a chimney fire in the same way. The bigger concerns are blockage, acid damage to the liner, and the effect on the appliance's draft.

How acids damage the liner

When the sulfur in oil soot mixes with moisture in the flue, the result is acidic. CSIA describes two forms of damage:

• In clay tile liners, flaking or spalling of the tile surface.
• In metal liners, corrosion that can eat pinholes through the metal.

A liner that has deteriorated cannot reliably contain flue gases or heat, which raises both fire and carbon monoxide concerns. CSIA also notes that soot blockages can cause carbon monoxide to spill back into the house instead of going up the chimney.

Condensation and efficiency

Modern oil boilers and furnaces extract more heat from the exhaust, so the gases entering the chimney are cooler than they used to be. Cooler gases are more likely to condense in the flue, especially in exterior chimneys. One oil boiler manufacturer's manual we reviewed requires a listed corrosion-resistant metal liner when flue gas condensation occurs, recommends one for exterior chimneys and low-firing applications, and states there must be no signs of continuous wetness at the chimney.

What boiler manuals ask of the chimney

Manufacturers set the conditions their equipment needs. As an example, the oil boiler manual we reviewed:

• References NFPA 211 and NFPA 31 for chimney and installation requirements.
• Gives a typical minimum chimney size and height, unless code allows otherwise.
• Requires the existing chimney, liner and vent connector to be inspected and repaired or replaced as needed.
• Specifies draft readings at the breech and over the fire.
• Requires a barometric draft control, installed and adjusted per the manufacturer.
• Calls for annual cleaning by a qualified professional.

Your own appliance's manual may differ in details. The heating technician who services the burner uses these requirements to set it up.

Clues that the flue needs attention

• Soot around the barometric damper, the connector joints, or the burner area.
• A buildup of soot and scale at the chimney base or cleanout.
• Flakes of tile in the cleanout.
• White staining or damp spots on the chimney exterior.
• The heating technician reports draft readings out of range.

A sudden release of soot from the burner, often called a puffback, is a sign to stop and have the burner and venting checked. If a carbon monoxide alarm sounds, leave the house and call 911.

What an oil flue service covers

CSIA describes annual service of an oil-appliance chimney as including an inspection of the chimney interior, an evaluation of the exterior masonry, disconnecting and sweeping the connector pipe, and removing soot from the flue and base. NFPA 211, as summarized by CSIA, calls for chimneys to be inspected annually and cleaned and repaired as needed.

The chimney work and the burner tune-up are related but separate. The burner technician adjusts combustion; the chimney service makes sure the flue is clear and sound. Coordinating the two, usually before the heating season, makes sense.

The connector pipe and barometric damper

Between the appliance and the chimney is the connector pipe, sometimes called the breeching or smoke pipe. It is often where problems show first. Soot collects in horizontal runs and elbows. Joints can loosen. Corrosion can thin the pipe from the inside. The barometric damper on the pipe should swing freely; if it sticks, draft can fall out of range.

The connector should enter the chimney cleanly, sealed at the thimble, and should not stick into the flue far enough to block it. These are details the chimney service and the heating technician both look at.

Shared flues and changing appliances

In many older homes, the oil boiler and a gas or oil water heater share one flue. When either appliance changes, the flue that worked for the pair may no longer suit what is left. A high-efficiency replacement boiler may run cooler exhaust, and a water heater that loses its partner may struggle to draft. CSIA lists a change in fuel or the replacement of an appliance as reasons for a Level 2 inspection, which includes a video examination of the flue. If you are converting from oil to gas, the flue needs to be evaluated for the new fuel before the switch, since a liner suited to oil may not be the right size or material for gas.

When an oil flue needs relining

Relining is considered when the clay tile has spalled or cracked, the existing liner is oversized for a new, more efficient appliance, or condensation is wetting the chimney. Stainless liners for oil must be of an alloy listed for that fuel. When a boiler or furnace is replaced, New Jersey's chimney verification form asks whether the chimney is appropriately lined and sized for the new appliance. See liner options and, if a water heater shares the flue, the orphaned water heater problem.

Next step

Schedule your oil flue service before the heating season, or sooner if you see soot. Request a visit, and see our furnace and boiler flue cleaning and chimney liners pages.

Questions homeowners ask

Sources

1. CSIA: Annual Service of Your Oil-Fired Appliance Chimney
2. PB Heat WBV-e oil boiler installation manual (venting section)
3. NJ DCA Form F370: Chimney Verification for Replacement of Fuel-Fired Equipment
4. CSIA: Homeowner Resources and FAQ
5. CSIA: The Three Levels of Chimney Inspections

This guide is general information. It can't diagnose a specific chimney; an inspection can.