What Removing a Gas Appliance Does to the Ones Left
Why this matters
The furnace comes out, the heat pump goes in, and the job looks finished. The gas water heater beside it was never touched, so nobody tests it. Two winters later the clay tile at the base of the chimney has crumbled into the connector, the water heater spills flue gas into the basement every time it fires, and the household's CO alarm is the first instrument to notice. Nothing in that sequence was a bad installation. The heat pump was fine. What changed was the venting of the appliance that stayed, and that change happened the day the furnace left.
This card owns that consequence. Capping the line belongs to How to Cap and Abandon a Gas Line Correctly; the empty chimney itself belongs to How to Handle the Chimney a Furnace Left Behind.
First, on any visit where a gas appliance has been removed
Before you diagnose anything, wear a personal CO monitor and test every gas appliance still connected to a vent: fire it, check for spillage at the draft hood or the vent connector, and read CO in the room. If the monitor reads 35 ppm or above in the room, a common shop default taken from NIOSH's 35 ppm recommended exposure limit (an 8-hour average, so a short-test reading at that level is a signal, not a dose), or the appliance spills and does not clear, turn that appliance off at its own gas valve, ventilate, tell the customer in plain words that it is unsafe to run, and tag it. Do not leave a spilling appliance running because the customer needs hot water tonight. See related: Carbon Monoxide Testing + Response Reference, and How to Take a Combustion Sample That Means Something.
How a natural-draft vent works, briefly
A natural-draft appliance, which the fuel gas codes call Category I (non-positive vent pressure, not condensing in the vent), moves its flue gas by buoyancy alone. Hot gas in the flue weighs less than the cold air outside, and the difference over the height of the chimney is the draft. Draft gets weaker when the gas is cooler, when the column is shorter, and when the gas moves slowly enough to lose its heat to the flue walls before it leaves. See related: How Draft Actually Works.
Two things in that list are set by the flue's size relative to what it carries. A flue sized for a lot of flue gas, carrying a little, moves that little slowly. Slow gas in a large, cold masonry flue cools all the way up.
What happens to the appliance left alone
Picture the common shared chimney: a furnace and a water heater, both natural draft, joined into one masonry flue. The flue was sized, or at least passed, for their combined input. The furnace was the dominant user. When it runs, it fills the flue with hot gas, warms the masonry, and gives the water heater an established draft to join.
Remove the furnace and three things change at once.
The flow drops to a fraction. If the furnace was 100,000 Btu/h and the water heater 40,000, the flue was carrying 140,000 and is now carrying 40,000, about 29 percent of what it carried before. The same cross-section now moves less than a third of the gas, so it moves slower.
The flue starts cold every time. The furnace ran for hours in winter and kept the masonry warm. The water heater fires for minutes. Each time it starts, it starts into a cold column with weak draft, which is when it spills at the draft hood.
The gas condenses. Natural gas combustion produces a lot of water vapor. When flue gas cools below its dew point on the way up, the vapor condenses on the tile. That condensate is acidic. It eats mortar joints and clay tile liners, rusts the connector, and over a few seasons drops debris into the base of the chimney, where it can block the connector entirely. A blocked flue on a natural-draft appliance sends all of its flue gas into the room.
This is the orphaned water heater. It is worst in an exterior masonry chimney, one with a wall exposed to outdoor air, because that masonry is colder still. The fuel gas codes treat exterior masonry chimneys separately for this reason.
EXTERIOR WALL INTERIOR
| | | |
| | clay tile liner, sized| |
| | for furnace + WH | |
| | | |
| | WH gas alone: slow, | |
| | cools, condenses on | |
| | this cold face first | |
| | | |
| | old furnace thimble--+ | open: seal it,
| | | cold air in
| | WH connector thimble----+---- single wall
| | | [ water heater ]
| | base: tile and mortar |
| | debris collects here |
+--+---------------------------+ cleanout
What the fuel gas code actually asks
The question is not "did it work before." It is whether the remaining appliance, alone, is within the venting tables for the vent it now has. The Category I sizing tables in NFPA 54 (the National Fuel Gas Code, Chapter 13) and in Section 504 of the International Fuel Gas Code, in the edition your jurisdiction has adopted, give each combination of vent type, height and area a range of appliance input. For masonry chimneys they list a minimum input as well as a maximum, because below the minimum the flue is too big to stay warm. Exterior masonry chimneys get their own tables or limits, and in cold climates those are the most restrictive of the set; in some cases the code does not permit the unlined exterior chimney at all.
So the check after a removal is a table check: chimney height, flue internal area, whether it is interior or exterior, the connector's rise and length, and the remaining appliance's input. If the remaining appliance falls under the table minimum for that flue, or the table does not list the combination at all, the vent no longer complies, whatever it did yesterday. NFPA 54 and the IFGC are model codes; they bind only as adopted, often with local amendments, and the inspector reads the version on their books.
The ways out
| Option | What it does | Watch for |
|---|---|---|
| Listed metal liner sized to the remaining appliance | Gives the gas a flue its own size, which stays warm | Liner sized from the tables, not by eye; some installations need an insulated liner; the liner is a listed product installed to its listing |
| Type B vent run up the chimney as a chase | Double-wall vent inside the masonry, sized to the appliance | Same table check, on the Type B tables |
| Replace with a power-vent or direct-vent water heater | Takes the appliance off the chimney entirely | Needs a 120-volt circuit, sidewall termination clearances, condensate handling on some models |
| Electrify the water heater too | Takes the last user off the chimney | Often the plan's next step anyway; see the sequence card |
The trap in the first row is the liner sized "the same as the old connector." A liner is a venting system, and it is sized from the same tables as any other vent.
Combustion air changes too, usually not where you expect
Removing an appliance lowers the total input in the room, so on paper the space's combustion air situation improves. The ways it gets worse come from the work around the removal:
- The furnace closet gets sealed. The installer air-seals the mechanical room or replaces louvered doors with solid ones because the heat pump air handler does not need combustion air. The water heater in the same room still does. Combustion air openings stay until the last combustion appliance leaves. See related: Combustion Air Requirements Reference.
- The envelope gets tightened. Air sealing is step one of a good conversion, and it reduces the infiltration a natural-draft appliance has been quietly relying on.
- A new exhaust appears. A large kitchen hood installed with the induction range can depressurize the house enough to reverse a weak water heater draft. See related: Kitchen Hood Makeup Air.
- The new air handler leaks. Return-side leaks in the mechanical room pull that room negative whenever the blower runs.
This is why the test after a removal is done under worst-case conditions: exterior doors and windows shut, exhaust fans and the dryer running, the air handler on, interior doors set to make the appliance's room most negative. A water heater that drafts with the basement door open and the house quiet tells you nothing about the night the hood and the dryer run together.
Worked example: the furnace job that found the water heater
A 1950s two-story. Exterior masonry chimney with a clay tile liner, shared by an 80 percent natural-draft furnace at 100,000 Btu/h and a natural-draft tank water heater at 40,000 Btu/h. The furnace is being replaced by a heat pump and air handler; the water heater is three years old and staying.
Before the furnace comes out. The HVAC tech reads the chimney: exterior, about 25 feet, tile liner in fair shape, debris at the cleanout. He checks the adopted edition's exterior masonry chimney provisions for a single 40,000 Btu/h appliance with that flue and height. The water heater alone falls below the listed minimum, or is outside what the tables permit at all. With the furnace gone, the vent will not comply. That is known before anything is removed, which is the point of looking first.
The options, priced in time. A stainless liner sized from the tables for the water heater alone: half a day for a chimney crew. A power-vent water heater: a new 120-volt circuit, a sidewall vent, and replacing a three-year-old appliance. A heat pump water heater, which the owner's plan already called for within two years.
What happened. The owner chose the heat pump water heater now, moving step 2 of the plan forward rather than buying a liner for an appliance with two years left in the house. It could not be installed the same day. So the question for the gap was whether the water heater could run in the meantime.
The test. With the furnace disconnected and its thimble sealed, the tech ran the worst-case test: doors shut, bath fans and dryer on, the new air handler running. The water heater spilled at the draft hood and did not clear, and CO in the basement climbed while it fired. He shut the water heater off at its gas valve, tagged it, and told the owner there would be no hot water until the new unit went in, three days later. The plumber's visit was moved up.
The failure this avoided. Had the tech removed the furnace, tested the heat pump, and left, the water heater would have kept firing into a cold, oversized, exterior flue. It would probably have worked, mostly, for a while, while the condensate went to work on the tile. The household would have had no reason to suspect anything until a CO alarm or a blocked flue told them.
What would have changed the answer. An interior chimney with a smaller flue might have kept the water heater within the tables, and then the work is a passing test and a note in the file. A water heater ten years old makes the replacement obvious. A house with no plan to electrify the water heater makes the liner the right call.
The other direction
When the water heater comes out first and the furnace stays, the furnace is left alone in a flue it already dominated. That is usually within the tables, but usually is not a test. Run the same table check and the same worst-case test before you leave.
References
- NFPA 54, National Fuel Gas Code, Chapter 13 (Category I vent sizing), and International Fuel Gas Code Section 504, in the edition your jurisdiction has adopted
- NIOSH recommended exposure limit for carbon monoxide (35 ppm, 8-hour time-weighted average)
- See related: Carbon Monoxide Testing + Response Reference
- See related: How to Handle the Chimney a Furnace Left Behind
- See related: The Whole-House Electrification Sequence