Switching Heating Fuel: What Happens to the Chimney

Why a fuel switch is also a chimney project

A chimney flue is sized and lined for the exhaust it was built to carry. Oil exhaust brings soot and sulfur. Burning gas makes exhaust heavy with water vapor. Modern high-efficiency equipment sends cooler exhaust up the flue, or skips the chimney entirely and vents through a side wall. Change the fuel or the efficiency and the flue is suddenly working under different conditions, even though nothing about the brick has changed.

The industry and the state both treat this as a trigger. CSIA's description of inspection levels says a Level 2 inspection is required when changes are made to the system, naming a change in fuel type and adding or replacing an appliance that differs in type, input rating or efficiency. New Jersey's form for replacement fuel-fired equipment, UCC F370, has check boxes for oil to gas conversion, gas to oil conversion, gas appliance replacement and oil to oil replacement.

Five common switches at a glance

Oil to gas

When a gas appliance will keep using the chimney, the switch touches it in three ways.

The old deposits. Years of oil firing leave soot and scale on the flue walls and at the base of the chimney. The F370 verification for a conversion includes a statement that the chimney has been substantially cleaned of residue where that applies, so the flue is normally swept and the base cleared before the new appliance is connected.

The size of the flue. A flue that suited an older oil burner can be too large for a modern gas appliance. In DuraVent's homeowner guide on relining for gas, draft suffers when an efficient gas furnace exhausts into an oversized masonry flue, and that the cooler exhaust reaches its dew point inside the chimney. The water that condenses carries acids that attack tile, brick and mortar.

Where the chimney sits. Exterior chimneys run colder. One New Jersey borough's notice to contractors, from East Rutherford's construction and plumbing subcode official, points out that replacing an appliance that vents into an exterior masonry chimney calls for a sizing review under the fuel gas code's venting tables, and that an alternate means of venting will often be needed because newer equipment produces cooler exhaust.

Many oil-to-gas projects end with a new liner sized for the gas appliance and any water heater sharing it, or with appliances that vent through the wall instead. Which one fits depends on the equipment chosen, so the chimney question belongs in the conversation before the equipment is ordered.

When the new unit vents through the wall

High-efficiency furnaces and boilers frequently exhaust through plastic pipe in a side wall, leaving the chimney behind. That sounds like it solves the chimney question. It can create one instead, because the water heater, and sometimes a second appliance, often stays connected to the old flue.

The F370 form anticipates this. For a direct-vent installation, the person verifying the chimney confirms that the new appliance is direct-vent and that the existing chimney or vent is appropriately lined and sized for any remaining appliances. In other words, the paperwork looks past the new unit to what is left behind. The orphaned water heater guide explains why a small appliance alone in a large flue can draft poorly and condense.

Moving to a heat pump or electric heat

A heat pump or electric boiler burns nothing, so it needs no flue. What happens to the chimney then depends on what else is connected to it:

• A gas or oil water heater still uses it. The flue now serves only that appliance and should be reviewed the same way as any orphaned water heater.
• A fireplace flue sits beside the heating flue. The fireplace side is unchanged, but the chimney's maintenance still covers every flue in it. See chimneys with more than one flue.
• Nothing is left. The connector comes out, the opening into the flue is closed properly, and the chimney becomes an unused structure that still takes weather. It can be capped and maintained, or taken down; should you remove an unused chimney? walks through those choices.

Before anything is closed off, every flue's job should be confirmed. Sealing a flue that a forgotten appliance still uses is a carbon monoxide hazard.

Gas to oil, and adding wood or pellets

Gas to oil is less common but appears on the F370 form. Oil brings soot and sulfur back into the flue, so the liner needs a listing that covers oil and a size that suits the burner, and the chimney returns to a schedule of annual cleaning.

Adding a wood or pellet stove to lower heating bills is a fuel switch for part of the house. A solid-fuel appliance generally needs its own flue or a listed chimney, and the F370 form has a separate verification statement for solid-fuel appliances and fireplaces. See insert or freestanding stove? and pellet stove venting.

Putting the work in the right order

1. List what uses the chimney nowFurnace or boiler, water heater, fireplace, stove. Note which flue each one uses, if you know.
2. Have the chimney inspected before signingThe findings show whether the flue can serve the new plan, needs a liner, or will be left orphaned. That can change which equipment makes sense.
3. Agree on the venting plan with the heating contractorWho handles the liner, who handles the connector, and what happens to any appliance left on the old flue.
4. Do the chimney work before the new unit runsCleaning, repairs and relining are easier, and safer, before the appliance is commissioned.
5. Complete the F370 and the permit inspectionThe form notes that it must accompany the permit application for minor work, or be presented before final inspection for full permits, and that no verification is required for direct-vent to direct-vent replacements. A homeowner who chooses not to submit it must be present for the inspector to remove and reinstall the vent connector.

Municipal pages show what the inspection looks at locally. Lawrence Township's page on replacing a furnace, as one example, lists a chimney certification form among the application documents and a final inspection of venting, chimney and connections.

Who handles which part

A fuel switch involves several trades, and gaps between them are where chimney problems slip through. The heating contractor selects and installs the appliance. Gas piping is handled by a plumber or the utility. The construction office issues the permit and inspects. The chimney side, cleaning, inspection, relining and the condition of the masonry, is where we come in. We run a camera through the flue wherever it fits, send the findings and photos ahead of any scheduling, and install stainless, cast-in-place and clay tile liners when relining is called for. See inspection before a new appliance and furnace and boiler flue cleaning.

Questions homeowners ask

Sources

1. NJ DCA Form F370: Chimney Verification for Replacement of Fuel-Fired Equipment (PDF)
2. CSIA: The Three Levels of Chimney Inspections
3. DuraVent: Home Safety, Relining Chimneys for Gas Appliances (PDF)
4. Borough of East Rutherford, NJ: Chimney Notice (construction and plumbing subcode official)
5. Lawrence Township, NJ: Replacing a Furnace

This guide is general information. It can't diagnose a specific chimney; an inspection can.