Do You Need a New Liner After an Oil to Gas Conversion?

What changes when you switch fuels

An oil burner sends hot, sooty exhaust up the chimney. A gas boiler or furnace sends cooler exhaust that is mostly water vapor and carbon dioxide. In a big, cold masonry flue, that vapor condenses on the walls before it reaches the top, and the chimney that stayed fairly dry on oil starts to run wet on the inside.

The flue also carries its history. Years of oil leave sulfur scale stuck hard to the tiles. When gas condensation soaks into that scale, it forms an acidic mix that breaks down mortar joints and clay faster than either fuel would alone. That is why a conversion calls for a close look at the chimney, not just the boiler.

Where the new boiler vents decides a lot

The first question is whether the new appliance uses the chimney at all. Many high-efficiency condensing boilers and furnaces vent through the side wall with plastic pipe, as their maker's instructions require, and cannot vent into an ordinary masonry chimney. Standard-efficiency gas boilers and furnaces do vent into the chimney, and so does a typical atmospheric gas water heater.

Each combination leaves the chimney with a different job, and that job sets what the flue needs.

• New boiler vents out the wall: the chimney may still serve the water heater, or nothing
• New boiler vents into the chimney: the flue must suit gas and be sized for the boiler
• Boiler and water heater share the flue: the size must work for both together
• Water heater left alone in the old flue: usually oversized and prone to condensation

Why the old oil flue is usually the wrong size

Chimneys in older houses throughout the region were often built for coal, then served a large oil burner for decades. A modern gas boiler, and especially a gas water heater, puts a fraction of that exhaust volume into the same opening. The gases move slowly in the wide flue, lose heat to the cold masonry and drop below the point where water condenses well before they reach the top.

Venting tables in the appliance instructions and the fuel gas code set the size range a flue can be for a given appliance. An old oil flue frequently falls outside that range on the large side. Relining with a smaller, sized liner is how the flue is brought back into range.

Signs the chimney is struggling after a conversion

Problems can take a season or two to show. Water at the cleanout, rust streaks on the vent connector, white staining on the chimney in the basement, or a damp patch on the chimney breast upstairs all point to condensation. Crumbling mortar or bits of tile at the base mean the acid is already working on the flue.

A water heater draft hood that spills warm air when you hold your hand near it, or a carbon monoxide alarm that sounds when exhaust fans or the dryer are running, means draft is weak. That one is a call to the gas utility or 911 first, then a chimney inspection.

Choosing a gas boiler chimney liner

For a gas boiler, furnace or water heater venting into the chimney, the usual fix is a metal liner sized from the appliance maker's venting tables. Stainless steel suits gas and handles condensation well. Aluminum liners are listed for some gas appliances and are lighter duty. Whatever the material, the liner maker's listing has to match the appliance and the fuel.

When a boiler and water heater share the flue, the liner is sized for the combined load and the connectors are joined as the instructions allow. When only the water heater remains, the liner is sized for it alone, which usually means a much smaller diameter than the old flue.

Cleaning comes before lining

Before any liner goes in, the old flue has to be cleaned of oil soot and scale, and the base has to be emptied. Lining over that residue leaves acidic material packed around the new liner and loose debris that can block the bottom. The technician also checks for obstructions, broken tile shelves and offsets that affect how the liner runs.

At the top, the liner is fixed with a top plate that seals the old flue around it, and a cap goes on. At the bottom, a tee and cleanout cap or a sealed connection is fitted, and the thimble where the connector enters the masonry is made tight.

If the chimney is no longer used

When the new boiler and water heater both vent out the wall, the chimney may serve nothing. It still stands through your roof, so it still needs a cap or a full cover to keep rain and animals out, and the old connector opening in the basement should be sealed properly with masonry, not a pie plate or tape. A chimney that sits unused and wet deteriorates faster than one in service, because nothing warms or dries it.

Some condensing boilers allow their plastic vent and intake pipes to run up an unused chimney as a chase, instead of exiting through the wall. Whether that is allowed depends entirely on the boiler maker's instructions. If it is, the chimney still needs to be cleaned first and capped in a way that suits the pipes, and the masonry around them still needs its crown and flashing kept in repair.

Planning the conversion

Bring the chimney into the plan early, before the new boiler is installed. The heating contractor chooses the appliance; a chimney technician looks at the flue it will vent into, cleans out the oil residue, and sizes and installs the liner the appliance needs. Coordinating the two keeps the boiler from running for weeks into a flue that is wrong for it.

Ask the heating contractor for the model and its venting section before the chimney visit. The installation follows the appliance maker's instructions and the code your town has adopted, and local permit requirements for the boiler swap vary, so confirm them with your building department.