Do You Need a Chimney Liner for an Oil Furnace?

What oil exhaust does to a masonry flue

Oil exhaust carries soot and sulfur compounds. As it cools in the flue, it leaves a scale that is acidic and attacks the lime in mortar and the inside face of clay tiles. The scale flakes off, drops to the base and has to be cleaned out. Season after season, the flue gets rougher, the joints open and pieces of tile come loose.

A chimney with no liner at all, which some very old houses still have, offers no protection. The exhaust works directly on the brick and mortar, and through any crack it can reach the walls and rooms around the chimney. In a brownstone or rowhouse, where the stack runs up a party wall, those cracks can open toward a neighbour's side as well.

Signs the oil flue needs attention

Most of these show up at the cleanout or around the boiler before anything is visible from outside. The yearly burner service and the chimney inspection are the two moments they tend to be found.

Smell is another clue. A faint oily or sooty odor near the boiler between service visits, or fresh soot on the wall around the barometric damper, deserves a call to the oil service company and a look at the flue in the same week.

• Curved pieces of orange or tan clay tile in the cleanout
• The base filling with scale and debris faster than it used to
• Damp or stained patches on the chimney breast or the basement wall
• Soot around the barometric damper, poor draft readings or burner lockouts
• Crumbling mortar, flaking brick or white crust near the top of the chimney
• No liner visible at the base or the top at all

Liners that suit oil

Stainless steel is the common choice for relining an oil flue. Each liner maker lists which fuels each of its alloys is made for, and the one installed has to be listed for oil. Flexible stainless liners follow the bends and offsets many older chimneys have. Rigid sections suit straight flues. On an outside-wall chimney, the liner's instructions may call for insulation to keep the exhaust warm.

Aluminum liners are made for certain gas appliances and do not suit oil. A cast-in-place lining, poured around a form inside the old flue, is another option for some chimneys, especially where the masonry itself needs sealing, provided the finished size still suits the burner.

Whichever liner is used, the bottom tee has to stay reachable through a cleanout, because oil scale collects there first and needs clearing at each service.

Sizing to the burner, not the chimney

The appliance, not the room inside the chimney, decides the liner size. The boiler or furnace maker's venting instructions, together with the chimney's height and the length of the connector pipe, set the flue size. Too big, and the exhaust cools and lingers, which means more scale, weaker draft and more condensation. Too small, and the burner cannot vent its exhaust properly.

Many oil boilers on Long Island, in north Jersey and around Philadelphia vent into a flue built for a much older, larger heating plant, sometimes a coal furnace. A correctly sized liner often lets the burner run cleaner simply because the draft becomes steady.

The burner and the chimney work as one system

A poorly adjusted oil burner soots up a flue quickly, and a flue with weak or unsteady draft makes a burner hard to tune. That is why the oil service technician and the chimney technician are looking at two halves of one system. The service technician measures draft with a gauge and sets the barometric damper, the swinging plate on the smoke pipe that steadies the draft. A new liner changes the draft, so the burner should be checked and adjusted after relining.

The worst case is a puffback: a delayed ignition inside the boiler that blows soot out through the house, sometimes after repeated restart attempts or with a blocked flue. If the burner locks out, call for service rather than pressing the reset button again and again, since each attempt can leave unburned oil in the chamber.

Planning for a fuel switch

Oil-to-gas conversions are common across the region, and they change the venting. Gas exhaust is cooler and wetter than oil exhaust, gas appliances are sized from their own venting tables, and the new burner or boiler may need a smaller flue than the oil unit did. Old oil scale left in a flue can also loosen and fall once it starts getting wet from gas condensate.

If a switch is on the horizon, say so before relining. A liner whose alloy is listed for both oil and gas, sized with the future appliance in mind, can avoid relining twice. If the new appliance will vent out the side wall instead, the chimney may only need to serve the water heater, which changes the size again.

If the house is moving off oil and gas altogether, to heat pumps, the old flue may end up serving nothing at all. It then needs a cap at the top and a sealed thimble at the bottom rather than a liner.

How the installation is done

The flue is swept first to clear soot and scale. The liner goes down from the top, where it is sealed to the crown with a top plate and fitted with a cap. At the bottom it connects to a tee or connector that lines up with the thimble, the opening where the smoke pipe enters the chimney, and the tee has a removable cap so debris can be cleaned out later. The smoke pipe is then connected and sealed at the thimble, and the burner is fired to confirm the draft.

Our own crew does the work, and the size, alloy, insulation and full scope are spelled out in writing before the job is scheduled.

Relining mid-winter or waiting for spring

The findings decide the timing. Tile falling into the cleanout, a flue that is partly blocked at the base, soot at the barometric damper or any sign of exhaust in the basement mean the flue should be dealt with now, even in January, because the boiler cannot simply be switched off for the season. Liner work can be done in winter, with the heat off only for part of a day while the connection is made.

A flue with worn joints but sound tile, steady draft and a clean base can often be cleaned, watched and relined in spring or summer, when scheduling is easier and the roof is dry. A technician should say clearly which of these two situations your flue is in.

Carbon monoxide and an oil flue

A cracked, blocked or oversized oil flue can let exhaust spill into the basement, and that exhaust contains carbon monoxide, which is dangerous and cannot be seen or smelled. Keep CO alarms near the boiler and on every sleeping floor and test them. If one sounds, or people in the house feel unwell, get out first and call 911 from outside.