How to Handle the Chimney a Furnace Left Behind

Why this matters

When a heat pump replaces a gas or oil furnace, the furnace's flue does not disappear. It stays in the house as a masonry chimney or a metal vent with a hole at the bottom, an open top, and often one appliance still connected to it. Left as it is, it becomes one of three things: a cold, oversized flue that slowly poisons the water heater still using it, a rain and animal entry point with nothing drying it out, or a hole in the house's air barrier that undoes the air sealing the heat pump was sized around. Each of those turns into a callback that gets blamed on the heat pump.

This procedure is the decision about the chimney itself: keep it for something, reline it for what remains, or abandon it properly. Why the water heater suffers is owned by What Removing a Gas Appliance Does to the Ones Left, and this card spends one clause on it: a flue sized for two appliances, carrying one small one, runs cold and wet. Capping the gas line is owned by How to Cap and Abandon a Gas Line Correctly.

The steps

1. Test every appliance still connected before you plan anything

Put on a personal CO monitor, fire each appliance still vented into the chimney, and check for spillage at the draft hood and CO in the room. If the room reads 35 ppm or more, or an appliance spills and does not clear, shut that appliance off at its own gas valve, ventilate, tag it, and tell the customer in plain words not to relight it.

Why first: the plan you are about to make assumes the remaining appliance is merely at risk. If it is already spilling, the plan is an emergency, and a customer who hears "we'll reline it next month" will keep running it tonight. Skip it and you leave a water heater backdrafting through the weeks it takes to schedule a chimney crew.

2. Inventory the whole stack, not just the flue you used

Go to the roof line or look from the yard and count flue openings at the top of the chimney. Then go inside and find every thimble (the opening where a connector enters the chimney), cleanout door and capped hole at every floor. A single masonry chimney often carries two or three flues: the furnace's, a fireplace's, sometimes an abandoned one from a coal or oil boiler. Write down what each flue serves now.

Why: the decision is per flue, but the work is per chimney. A crown repair, a cap or a chimney removed above the roof affects every flue in the stack. Skip it and someone seals the top of what they think is the furnace flue and caps the fireplace flue with it, or relines the wrong one.

3. Get the flue inspected before you choose

Have a chimney professional inspect the flue that lost its furnace. NFPA 211, in the edition your jurisdiction has adopted, describes a Level II inspection for changes to a venting system, including a change in what it serves, and many chimney sweeps treat a furnace removal as that kind of change; whether your jurisdiction requires it is the code official's call. A camera run of the full flue is what separates a sound tile liner from one with cracked or missing tiles, mortar loss at the joints, or debris piled at the base.

Why: every option below depends on what is inside the flue. A cracked tile liner cannot be kept for a water heater as is, and debris at the base is how an orphaned water heater goes from spilling to blocked. Skip it and you are choosing between relining and abandoning without the one fact that decides it.

4. Decide the remaining appliance's path first

If a water heater or boiler still vents into the flue, you have three honest choices.

• Reline the flue for that appliance alone. A listed liner sized for the single appliance, read from the venting tables in the fuel gas code as adopted (IFGC Chapter 5 or NFPA 54, whichever your state uses), at this chimney's height and the appliance's input. A properly sized liner restores the flue gas velocity and keeps the condensate off the masonry.
• Move the appliance off the chimney. Replace it with a power-vent or direct-vent unit that vents through a wall, leaving the chimney free to abandon.
• Electrify it. A heat pump water heater removes the last combustion appliance from the flue, and often from the house. See related: Plug-In 120-Volt Heat Pump Water Heaters, for the version that may not need a new circuit.

Why this order: the chimney's fate follows from the appliance's. You cannot abandon a flue something still uses, and paying to reline a flue for a 12-year-old water heater that will be replaced in two years is money spent twice. Skip it and the chimney gets decided first and the water heater inherits whatever is left.

5. Keep a fireplace flue separate from any gas appliance

If the chimney has a working fireplace, its flue stays a fireplace flue. The fuel gas codes, as adopted, restrict connecting a gas appliance to a flue that serves a working fireplace and allow it only under narrow conditions that all have to be met (IFGC Section 501 lists them for your edition: a noncombustible seal below the connection so room air cannot enter the flue, access to the flue for inspection and cleaning, and a connector that discharges directly into the flue; a factory-built fireplace flue only where its listing allows it). So the water heater does not get moved into the fireplace flue because it is the one in better shape, and the old furnace flue cannot be borrowed for a wood stove insert without its own listed liner and inspection.

Why: a fireplace flue is sized for a fire's volume and an open damper, and a gas appliance sharing it can have its flue gas pulled back into the house when the fireplace runs. Skip it and you have created the shared-flue hazard on purpose.

6. Abandon what nothing uses, at both ends

For a flue that will serve nothing:

• Close the thimble with masonry or a listed thimble cover, sealed to the surrounding wall, not a pie-tin cap pushed into the hole. The thimble is an air leak into the house and a path for flue debris and moisture.
• Close or cap the top. A cap that sheds rain and screens out birds and animals, or a top seal. Whether to seal the flue tight or leave it vented to dry depends on whether it shares a stack with a live flue and how wet the masonry is; that is the chimney professional's call from the inspection.
• Remove the old connector and any draft hood remnants from the utility room, so nobody later connects a new appliance to a flue that has been closed at the top.

Why both ends: an open top with a closed bottom fills with water and freezes in the masonry; an open bottom with a closed top is a cold air leak into the basement and the place a squirrel ends up. Skip it and the abandoned flue becomes a moisture and air problem within a season.

Hazard at this step: sealing a thimble puts you at the base of a flue that may hold soot and creosote, and an old chimney in a house built before the 1980s may have asbestos in a thimble cover, a cement flue pipe or the old connector insulation. Do not break, cut or sand a suspect material; leave it, and have it sampled before anyone disturbs it, since dry breakage releases fibers you inhale. Wear a fit-tested respirator under a written program per 29 CFR 1910.134 (reached in construction through 29 CFR 1926.103) if your work raises soot.

7. Decide whether the chimney stays above the roof

A chimney nothing uses can stay as an architectural feature, be capped and kept, or be taken down below the roof line with the roof patched. Taking it down is roofing and masonry work, usually needs a permit, and stays off the table while any flue in the stack is still live.

Why: an unused chimney above the roof keeps needing crown and flashing maintenance with no appliance justifying it. Skip it and the customer learns this from a roof leak three years out.

8. Record it where the next tech will look

Tag the closed thimble and the gas outlet, note the flue's status on the equipment record, and tell the customer which flues are live. See related: How to Document a Converted House for the Next Tech, which owns the tags and their wording.

Worked example: two flues, one water heater

A 1958 ranch with an interior masonry chimney: an 8 by 12 inch tile-lined flue that served an 80,000 Btu/h gas furnace and a 36,000 Btu/h water heater on a shared connector, and a separate fireplace flue on the other side of the stack. The furnace comes out for a ducted heat pump.

Step 1. The water heater drafts within a short time of firing, room CO is background, no spillage at the draft hood after it warms up. It is at risk, not failing.

Step 2. Two flues at the top. Inside: the shared thimble in the basement, a capped coal-era thimble behind the water heater, the fireplace on the main floor.

Step 3. The camera shows mortar loss at several tile joints in the lower third and a small pile of tile flakes at the base, the start of the condensation damage the orphaned water heater card describes. The flue is carrying 36,000 of the 116,000 Btu/h it used to carry, about 31 percent of its former load.

Step 4. The water heater is 9 years old. The customer is choosing between relining now and a heat pump water heater in a year or two. The tech lays out the cost ratio in the customer's terms: a sized liner is a meaningful fraction of a heat pump water heater, and it would serve an appliance with a few years left. The customer chooses to go straight to a heat pump water heater on the same visit cycle, three weeks out.

The gap. Three weeks of running an orphaned water heater on a flue already shedding tile. The tech clears the debris from the base through the cleanout, confirms the connector is clear, installs a CO alarm in the basement because none was there, and asks the customer to call if the alarm sounds or the pilot area smells of combustion. That is a bounded risk with a monitor on it, not a promise.

Step 5. The fireplace flue stays as it is; nothing is moved into it.

Step 6, three weeks later. The water heater is replaced, the shared thimble sealed with mortar, the old connector removed, and the coal-era thimble behind it confirmed sealed. The chimney professional caps the abandoned flue at the top with a cap shared with the fireplace flue, which stays open for the fireplace.

Step 7. The chimney stays above the roof because the fireplace still uses it.

What would have changed the call. A 3-year-old water heater, and relining for it alone would have been the right money. A cracked tile with gaps through to the masonry, and the three-week wait would not have been acceptable: the water heater would have been shut off and the customer given temporary hot water. An exterior chimney with a wall to outdoor air runs colder still, and pushes toward relining or electrifying sooner.

How to verify you got this right

Every flue in the stack has a stated status: live and serving what, or abandoned at both ends. No gas appliance shares a flue with a fireplace. The remaining appliance, if any, drafts with the house's fans running, which is the test in Combustion Safety After a Partial Conversion. The thimble does not move air when you hold a smoke pencil near it with the basement depressurized. And the customer can tell you which flue the fireplace uses.

References

• NFPA 211, Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, in the edition your jurisdiction adopts
• International Fuel Gas Code Chapter 5 or NFPA 54, as adopted by your state, for venting and liner sizing
• 29 CFR 1910.134 Respiratory Protection
• See related: What Removing a Gas Appliance Does to the Ones Left; Unlined Chimneys, Coal Chutes and Unused Flues; Combustion Safety After a Partial Conversion; How to Cap and Abandon a Gas Line Correctly