Oil to Gas in Connecticut: What Happens to the Chimney

Why the flue ends up the wrong size

An oil boiler moves a large volume of hot gas and the chimney was built to carry it. Replace it with a modern gas appliance and you have something producing perhaps a third of that volume, at a lower temperature, venting into the same masonry.

The gases lose heat to all that brick on the way up. Below a certain temperature water vapor in them condenses, and because combustion gas carries sulfur and other compounds, what condenses is acidic. It soaks into the mortar joints between flue tiles and into the tiles themselves, and it does that on every heating cycle for as long as the arrangement lasts.

The second problem is draft. A flue far larger than the appliance expects does not pull properly, which is a combustion issue rather than a masonry one and shows up as an appliance that will not run cleanly.

Sized from the appliance, not the chimney

The liner diameter comes from the appliance manufacturer venting tables — input rating, chimney height, number of offsets. It does not come from what happens to fit down the flue. Ask to be shown the table rather than taking it on trust in either direction.

When insulation is required

An exterior chimney, a chimney on an unheated wall, or a long run will cool the liner enough to condense even at the correct diameter. Insulation is what prevents that. On an exterior Connecticut stack it is generally necessary rather than optional.

The parts of the job that get left out

The liner is the obvious item. Three others routinely go missing from a conversion quote and all three matter.

The abandoned opening

Where the oil appliance connected there is now a hole in the chimney. It has to be filled properly with masonry, not covered with a plate. An unsealed opening lets flue gas into the basement and lets air into the flue, which affects the draft on whatever is still venting.

Anything else sharing the flue

A water heater very often vents into the same chimney. If the boiler leaves and the water heater stays, that water heater is now alone in a flue sized for two appliances — its own oversizing problem. It has to be in the calculation from the start.

A camera survey either side

Before, to establish what forty years of oil condensate has done. After, to confirm the liner is properly seated and connected. Both are quick and both are worth insisting on.

What it costs you to skip it

A conversion done without relining will run. That is what makes this easy to defer — nothing fails on the first cold night. What happens instead is slower: mortar joints wash out, flue tiles crack, and combustion products start reaching the chimney structure and eventually the house.

By the time it shows up as a stain on a ceiling or a failed inspection on a sale, the repair is considerably larger than the liner would have been. Getting it priced alongside the boiler is the cheapest time it will ever be quoted.