How many homes still burn oil
The American Community Survey’s 2020–2024 estimates of main heating fuel vary a lot across the South Coast: about 42.5% of occupied homes in Acushnet, 30.2% in Dartmouth, 13.8% in Fairhaven and 6.8% in New Bedford heat with fuel oil or kerosene. In the towns, that is a large share of chimneys dedicated wholly or partly to an oil boiler or furnace.
Many of those flues are in masonry chimneys built in the 1950s, ’60s and ’70s, often with a fireplace flue alongside. Others are in much older houses, where the heating flue may share a big center chimney with fireplace flues that were never lined.
What oil does to a flue
Oil combustion leaves soot that contains sulfur. When that soot sits in a cool flue with moisture, from condensation or rain coming down an uncapped flue, it can form acids that attack the mortar between clay tiles and eventually the tiles themselves. The damage happens inside the flue, out of sight.
Signs include flakes of soot or tile in the cleanout at the base of the flue, staining on the chimney breast, crumbling mortar visible at the connector, and a burner technician noting draft problems. A heavy buildup of soot can also partly block the flue, which matters because the burner depends on the flue to carry its exhaust out.
What an inspection of an oil flue looks at
- The cleanout at the base, for soot, tile fragments and debris.
- The connector pipe and where it enters the chimney, for corrosion, gaps and proper fit.
- The visible flue from top and bottom, and by camera where something suggests damage.
- The cap and crown, since water coming in at the top speeds the acid damage.
- Any fireplace flue sharing the stack, which shares the same crown and weather.
NFPA 211 recommends inspecting chimneys and vents every year, and that includes the flue serving an oil appliance. Many owners have the burner serviced annually; scheduling the chimney inspection around the same time keeps the whole system checked before winter.
Switching from oil to gas
When an oil boiler is replaced with a gas one, the flue usually needs attention. Gas appliances produce cooler, wetter exhaust and generally need a smaller flue than the oil equipment they replace; put that exhaust in a large, cold flue and it can condense heavily. Old oil soot left in the flue can also loosen once gas firing begins.
The sequence that avoids trouble is to inspect the flue (NFPA 211 calls for a Level 2 inspection when the fuel or appliance changes), clean out the old soot, install a liner if the appliance or code requires one, then connect the new appliance. Some high-efficiency appliances vent through a side wall instead and leave the chimney unused, which raises its own question of how to close the flue properly.
Old oil flues in old houses
In older houses, the oil boiler’s flue is often one of several in a large brick chimney, and sometimes it is an old fireplace flue pressed into service decades ago. Those flues were rarely lined, and when they were, the tiles may not be the right size for the boiler. Oil soot sits on rough brick more easily than on smooth tile, and acid damage to soft old mortar can go a long way before anyone notices.
A camera inspection is the practical way to know what an old oil flue looks like inside. It shows whether the flue is lined, whether the joints are intact, and whether soot or fallen debris is narrowing it. If the boiler is due for replacement in the next few years, knowing the flue’s condition now helps plan the change: whether the new appliance can use it, needs a liner, or should vent another way.
Who does what
Oil burners and boilers are serviced by a heating technician, who cleans the burner and checks combustion. The chimney contractor inspects, cleans, repairs and relines the flue. Problems tend to show up in the gap between the two, so it helps if both know what the other found.
A practical routine
- Have the burner serviced and the flue inspected each year, ideally before the heating season.
- Make sure the flue has a cap so rain doesn’t add to the acid damage.
- Ask for a camera look if tile pieces appear in the cleanout or the technician reports draft problems.
- Have the flue inspected before any change of fuel or appliance, not after.
Sources we checked
- Census Reporter: ACS 2020–2024 table B25040, house heating fuel (New Bedford, Dartmouth, Acushnet, Fairhaven)
- NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances
- Chimney Safety Institute of America: The Three Levels of Chimney Inspections
- Mass.gov: Massachusetts law about carbon monoxide detectors
- U.S. Consumer Product Safety Commission: Carbon Monoxide Information Center
General information for homeowners from Chimney Champ’s New Bedford. It cannot stand in for an inspection of your own chimney, and questions about code are decided by your local building or fire official.