Converting From Oil to Gas: What Changes in the Chimney

When a house converts from oil to gas, the chimney nearly always needs attention, and the reason is counter-intuitive: gas is the cleaner fuel but it is harder on a masonry flue. Gas combustion produces significantly more water vapor than oil does, and modern gas appliances run at higher efficiency, which means lower flue-gas temperatures. Wetter exhaust plus a cooler flue equals condensation on the flue wall, and that condensate is mildly acidic.

On top of that, the flue that vented the old oil appliance is usually far larger than the new gas appliance needs, which makes the whole problem worse.

Why oversizing matters so much

A flue has to be sized to the appliance it serves. Too small and it cannot move the volume. Too large and the exhaust slows down, spreads out and cools before it reaches the top — so it condenses on the way, and the draft the appliance depends on weakens.

Sizing is driven by the appliance's input rating in BTU, the height of the chimney, the vent connector run and the manufacturer's own venting tables. It is not a judgment call and it is not something to eyeball. The appliance manufacturer publishes what the appliance requires, and the installation has to meet it.

What happens if it is left alone

Acidic condensate attacks mortar joints from the inside. Over a few seasons it degrades the parging in the flue, works into the joints between clay tiles and, in older unlined chimneys, straight into the masonry itself. The visible signs arrive late: staining on the chimney breast, damp patches, white efflorescence on the exterior brick, mortar crumbling out of the stack near the top. By then the damage is inside where nobody has been looking.

What the conversion normally involves

In most cases the answer is a correctly sized liner. For a gas furnace, boiler or water heater that means either a furnace or boiler liner sized to the new appliance, or a B-Vent system where that is what the appliance and the route call for.

A few specifics that come up on almost every conversion:

• Orphaned water heaters. If the boiler moves to a direct-vent system but the gas water heater still uses the chimney, that water heater is now alone in a flue sized for two appliances. It is a classic oversizing situation and a common source of condensation problems afterwards.
• The old thimble. Where the oil appliance connected, there is a hole. It has to be properly closed, not stuffed.
• Existing liner condition. If there is a clay liner, its condition matters. Acidic condensate finds failed joints quickly.
• The rest of the chimney. Converting the heating system is a good moment to have the crown, cap and flashing looked at, because the chimney is being worked on anyway.

Who does what

A conversion normally involves a licensed gas fitter for the appliance and the gas line, and chimney work for the venting. ABC Chimney Solutions performs the chimney and venting side directly and can coordinate the qualified gas, plumbing or HVAC trades a project needs — or work alongside a contractor the homeowner has already chosen. Either way the point is that one organized project is easier to manage than three separate ones that each assume somebody else handled the flue.

If you have already converted

If the conversion happened some years ago and the chimney was never addressed, it is worth having looked at now rather than after a stain appears on a bedroom wall. A comprehensive inspection covers the chimney systems in the home, including the venting connected to heating appliances, which is exactly the part that gets missed.