Utility Flue / Water Heater Liner | A Wilmette Customer Asks: Does My Water Heater Need a Chimney Liner?

A Wilmette homeowner recently asked us an important question: “Does my water heater need a chimney liner?”

The answer depends on the water heater, the chimney, and the other appliances connected to the same venting system.

An atmospheric gas water heater may send its combustion gases through a masonry chimney. In that case, the chimney needs a suitable flue passage that is intact, correctly sized, and compatible with the appliance.

However, that does not always mean the homeowner needs a brand-new liner.

A sound and properly sized existing liner may continue to serve the water heater. On the other hand, an unlined chimney, damaged clay tiles, an oversized flue, or a recent furnace replacement may create a completely different situation.

Therefore, the correct first question is:

How does the water heater currently vent?

⚡ Quick Answer: An atmospheric gas water heater that vents through a masonry chimney needs a suitable, properly sized venting passage. The existing chimney liner may be sufficient if it remains intact and matches the appliance. However, a new listed liner may be needed when the chimney is unlined, deteriorated, incorrectly sized, or left serving only the water heater after a furnace upgrade. Power-vent, direct-vent, condensing, and electric water heaters do not use a traditional masonry chimney in the same way.

Water Heaters and Chimney Liners: At a Glance

Water Heater Setup	Does It Use the Masonry Chimney?	What May Be Needed?
Atmospheric gas water heater connected to a masonry chimney	Yes	An intact and properly sized clay or listed metal liner
Water heater and conventional furnace sharing a chimney	Yes	A common venting system sized for both appliances
Water heater left alone after a high-efficiency furnace upgrade	Usually	Utility-flue evaluation and possible resizing or relining
Power-vent water heater	Usually no	The manufacturer’s dedicated powered vent system
Direct-vent or condensing water heater	Usually no	A listed intake and exhaust system designed for that appliance
Electric water heater	No combustion vent	No chimney liner for appliance exhaust

First, Identify What Type of Water Heater You Have

The term “water heater” can describe several types of appliances.

Although they all heat water, they do not handle combustion air and exhaust gases in the same way.

As a result, one water heater may depend entirely on a masonry chimney while another does not connect to the chimney at all.

Atmospheric Gas Water Heater

An atmospheric gas water heater usually has a metal draft hood on top.

A metal vent connector rises from that draft hood and may enter a masonry chimney or a listed metal vent.

This system relies on natural draft. In other words, the warm combustion gases rise through the venting system without help from a mechanical exhaust fan.

Because the appliance depends on that upward movement, the connector, chimney height, flue condition, liner size, and available combustion air all matter.

Power-Vent Water Heater

A power-vent water heater uses a motorized blower to move combustion gases outside.

Depending on the appliance, the vent may run horizontally through an exterior wall instead of rising through a masonry chimney.

Therefore, installing a chimney liner does not normally solve a power-vent appliance problem.

The correct vent material, length, fittings, termination, and installation instructions for that specific model control the venting arrangement.

Direct-Vent Water Heater

A direct-vent water heater draws combustion air from outside and sends its exhaust back outdoors through a dedicated venting system.

Because the appliance does not rely on household air or a traditional open chimney flue, it creates a different inspection and maintenance situation.

The intake, exhaust, connections, and exterior termination still matter. However, a conventional masonry chimney liner may not form part of the system.

Condensing Water Heater

A condensing water heater extracts additional heat from the combustion gases.

Consequently, the exhaust leaves the appliance at a lower temperature and produces condensate as part of normal operation.

These appliances require venting and drainage materials approved by the manufacturer. A traditional clay-lined masonry chimney should not automatically receive this type of appliance without a venting design made specifically for it.

Electric Water Heater

An electric water heater does not burn gas or oil.

Therefore, it does not produce combustion gases and does not require a chimney liner for exhaust.

However, an unused chimney may still need exterior maintenance if it remains part of the building.

What Does a Water Heater Chimney Liner Do?

A chimney liner creates the passage that carries combustion gases from the appliance vent connector to the outdoors.

In a utility chimney, the liner serves several important functions.

It helps:

• Contain combustion gases inside the intended venting passage
• Separate exhaust from the surrounding brick and mortar
• Provide a smoother route through the chimney
• Match the flue area more closely to the connected appliance
• Reduce the amount of cold masonry that the exhaust must heat
• Limit moisture-related deterioration inside the chimney
• Support reliable natural draft when the system is properly designed

However, a liner does not correct every appliance or venting problem.

For example, a new liner cannot repair a malfunctioning burner, damaged gas valve, inadequate combustion-air supply, or incorrectly installed water heater.

Therefore, the entire venting path needs evaluation before someone selects a repair.

When Does a Water Heater Usually Need a New Chimney Liner?

A new liner may become appropriate when the existing flue cannot continue serving the appliance as intended.

Common reasons include:

• The masonry chimney has no liner
• The clay flue tiles have cracks, gaps, missing sections, or deterioration
• The flue is too large for the remaining appliance load
• The water heater became the only appliance using the chimney
• The chimney shows signs of chronic condensation or moisture damage
• The existing metal liner has corrosion, disconnections, or improper sizing
• The appliance type or input changed
• The venting path contains an obstruction or major offset
• A gas utility, HVAC contractor, plumber, or home inspector identified a venting concern
• The current installation does not match the appliance manufacturer’s instructions or applicable requirements

Nevertheless, the presence of an older clay liner does not automatically mean replacement is necessary.

First, someone needs to determine whether the liner remains complete and whether its size works with the connected water heater.

What Is an Orphaned Water Heater?

The term “orphaned water heater” usually describes an atmospheric water heater left venting into a masonry chimney after another appliance stops using that chimney.

For example, an older furnace and water heater may originally share one utility flue.

Together, both appliances send enough warm exhaust into the chimney to help establish draft.

Later, a homeowner replaces the furnace with a high-efficiency model that vents through an exterior wall. The atmospheric water heater remains connected to the original masonry chimney by itself.

At that point, the water heater has become “orphaned.”

The chimney may now be much larger than the venting load produced by the water heater alone.

As a result, the smaller amount of exhaust can cool before it reaches the chimney top. That cooling may weaken draft and encourage moisture to condense inside the flue.

Therefore, a furnace upgrade should trigger another question:

Can the remaining water heater still vent correctly through this chimney?

Why an Oversized Utility Flue Can Cause Problems

A larger chimney does not automatically create better draft.

An atmospheric water heater produces a limited amount of warm exhaust. Before that exhaust can rise effectively, it must begin warming the vent connector and the chimney interior.

If the flue is much larger than the appliance requires, the gases spread across a greater surface area.

Consequently, they may cool too quickly.

When the combustion gases cool, water vapor can condense inside the chimney. That moisture may combine with combustion byproducts and contribute to deterioration of the liner, mortar, brick, and metal components.

An exterior chimney may face additional challenges because outdoor temperatures keep the surrounding masonry colder during the heating season.

For that reason, sizing a utility flue involves more than measuring the water heater’s connector and installing a liner with the same diameter.

The appliance input, chimney height, connector length, lateral run, other connected appliances, and venting configuration all influence the answer.

Does a High-Efficiency Furnace Change the Answer?

Often, yes.

Many high-efficiency furnaces use dedicated vent pipes instead of the original masonry chimney.

When an installer removes the furnace connection from a shared utility flue, the water heater may become the chimney’s only remaining appliance.

That change can affect:

• The total heat entering the chimney
• The amount of exhaust moving through the flue
• How quickly the chimney establishes draft
• The likelihood of condensation
• The liner size needed for the remaining appliance

Therefore, the old chimney arrangement should not be assumed to remain suitable simply because the water heater connector still fits into the wall.

The venting system needs to match the new appliance combination.

Can a Water Heater and Furnace Share One Chimney?

They may share a properly designed common venting system when both appliances and the venting arrangement allow it.

However, the chimney must accommodate the combined appliance input, connector arrangement, height, and venting characteristics.

The system also needs a clear path to the outdoors.

Problems can develop when:

• One appliance receives a major efficiency upgrade
• The appliance input changes
• A connector enters the chimney incorrectly
• The common vent is too large or too small
• The clay liner has damage or missing joints
• The chimney contains debris or an animal obstruction
• Moisture has corroded the connector or liner

In addition, two appliances being located beside the same chimney does not prove that they share one flue.

A chimney structure may contain separate flue passages for different systems.

Does Every Water Heater Liner Have to Be Stainless Steel?

No single liner material fits every water heater and chimney.

The correct product depends on the appliance category, fuel, exhaust temperature, chimney condition, liner listing, and local requirements.

Some utility-flue applications use listed metal liners designed for non-condensing gas appliances. Other appliances require entirely different venting materials.

Meanwhile, a condensing or positive-pressure appliance may require a vent system specifically listed for condensate and the pressure created by that appliance.

Therefore, homeowners should not choose a liner solely because an online chart recommends one material or diameter.

The installed system needs to match:

• The water heater manufacturer’s instructions
• The liner or vent manufacturer’s listing
• The connected appliance configuration
• The actual chimney dimensions and condition
• Applicable local requirements

⚠️ Signs Your Water Heater Flue Needs Attention

A utility-flue problem does not always produce an obvious failure.

However, several conditions deserve evaluation:

• ⚠️ Rust or corrosion on the draft hood or vent connector
• ⚠️ Moisture, dripping, or staining near the chimney connection
• ⚠️ White deposits or deteriorating mortar on the chimney
• ⚠️ Pieces of clay tile, mortar, or masonry near the cleanout
• ⚠️ A damaged or disconnected metal vent connector
• ⚠️ Soot or dark discoloration around the draft hood
• ⚠️ Exhaust odors near the water heater
• ⚠️ A gas utility or contractor shutting down the appliance
• ⚠️ A furnace recently removed from the shared chimney
• ⚠️ A home inspection identifying an unlined or damaged flue
• ⚠️ A carbon monoxide alarm activating

Do not continue operating the appliance simply because the water still becomes hot.

The water heater can produce hot water while the venting system remains unsuitable.

Gas and carbon monoxide warning: If you smell natural gas, leave the area immediately and call North Shore Gas at 866-556-6005 from a safe location. If a carbon monoxide alarm activates or occupants experience possible carbon monoxide symptoms, leave the building and call 911.

What Does a Utility-Flue Inspection Include?

The inspection should follow the venting path from the appliance connection to the chimney termination.

Depending on the system and requested scope, a technician may document:

• The type of water heater and visible venting arrangement
• The number of appliances connected to the utility flue
• The vent connector’s material, size, condition, and support
• The chimney entry point and surrounding masonry
• The presence and visible condition of a clay or metal liner
• The accessible flue interior with camera equipment when possible
• The approximate flue dimensions and chimney height
• Debris, nesting material, deterioration, or obstructions
• Visible moisture and corrosion
• The chimney cap or utility-flue termination
• The exterior brick, mortar, crown, and flashing
• Conditions that may require coordination with an HVAC technician or plumber

At Chimney Monkey, a chimney inspection can document the visible and accessible utility flue and identify whether the chimney needs further correction.

However, chimney evaluation does not replace appliance service by the appropriate licensed trade.

A plumber or HVAC technician may still need to address the water heater, burner, gas controls, combustion performance, or final appliance connection.

Inspection First vs. Installing a Liner Immediately

A homeowner may receive a recommendation that says, “The water heater needs a liner.”

Sometimes, that recommendation is correct.

Nevertheless, the next step should confirm why the liner is needed and what system the new liner must serve.

A useful evaluation should answer questions such as:

• Does the chimney currently contain a liner?
• What condition is the existing liner in?
• Which appliances connect to the flue?
• Did a recent furnace replacement change the venting load?
• Is the current flue oversized for the water heater?
• What liner or vent system matches the appliance?
• Does the chimney have offsets, obstructions, or access limitations?
• Does exterior masonry deterioration also need repair?

Without that information, a liner proposal may address only one part of the system.

The goal is not merely to place a pipe inside the chimney.

Instead, the goal is to create a complete venting path that matches the appliance and the building.

Does Wilmette Require a Permit for a Water Heater Liner?

The Village of Wilmette currently lists the 2018 International Residential Code, International Mechanical Code, and International Fuel Gas Code as adopted codes, with local amendments contained in Chapter 8 of the Village Code.

Wilmette’s permit information also makes an important distinction.

The Village’s permit guide lists a replacement-only water heater among work that does not require a permit. However, the same guide lists flues among work that requires a permit.

Therefore, replacing a water heater with no venting alteration is not necessarily the same project as installing or changing a chimney liner.

The exact requirement depends on the proposed scope.

Before liner installation or major venting changes, homeowners and contractors should confirm the current requirements with Wilmette Community Development rather than relying on the permit rules for a simple appliance replacement.

How Often Should a Water Heater Chimney Be Inspected?

An annual inspection is a strong baseline for a chimney or vent serving a fuel-burning appliance.

The Chimney Safety Institute of America cites NFPA 211 guidance that chimneys, fireplaces, and vents should receive inspection at least once a year for soundness, deposits, and correct clearances.

Annual inspection becomes especially important when:

• The service history is unknown
• The home recently changed ownership
• The furnace or water heater was replaced
• The chimney sits on an exterior wall
• Moisture or corrosion has appeared
• A gas utility or contractor identified a concern
• The appliance has remained unused for an extended period

Inspection does not mean that the chimney automatically needs cleaning or relining every year.

Instead, the inspection determines what maintenance or repair the actual condition requires.

Why This Question Comes Up So Often in Wilmette

Many Wilmette homes contain masonry utility chimneys that originally served older furnaces, boilers, and atmospheric water heaters.

Over time, homeowners replace those appliances at different intervals.

For example, a high-efficiency furnace may move to sidewall venting while the older water heater remains connected to the chimney.

Another home may still have its original clay utility-flue liner but no clear inspection history.

From the basement, the vent connector may look unchanged.

However, the conditions inside the chimney can be completely different from the original installation.

That is why the presence of a metal pipe entering the wall does not answer the liner question.

First, identify the appliance.

Then, determine which flue it uses.

Finally, evaluate whether that flue still matches the appliance and its current venting load.

Frequently Asked Questions

Does a gas water heater need a chimney liner?

An atmospheric gas water heater that vents through a masonry chimney needs a suitable venting passage. An intact and properly sized existing liner may be sufficient. However, an unlined, damaged, or oversized chimney may need a listed replacement liner.

Does every water heater use the chimney?

No. Atmospheric gas water heaters may use a masonry chimney or listed metal vent. Power-vent, direct-vent, and condensing appliances often use dedicated vent systems. Electric water heaters do not require combustion venting.

Can my existing clay liner vent the water heater?

Possibly. The clay liner needs to remain intact and appropriately sized for the connected appliance or appliances. Cracks, gaps, missing sections, deterioration, or excessive flue area may lead to a different recommendation.

What happens when a furnace stops using the chimney?

The remaining atmospheric water heater may become an orphaned water heater. Because the water heater produces less heat and exhaust than the original appliance combination, the chimney may draft poorly or develop condensation. The utility flue should receive evaluation after the change.

Can a water heater and furnace share one chimney liner?

They may share a common venting system when the appliances and vent have been designed and sized for that arrangement. A furnace upgrade, appliance replacement, damaged liner, or connector change can alter the answer.

Does a tankless water heater need a chimney liner?

Many tankless water heaters use dedicated direct-vent or power-vent systems rather than a traditional masonry chimney. However, the exact venting requirements depend on the model and manufacturer. Never connect a tankless appliance to an existing chimney based only on the previous water heater’s setup.

Can I keep using an unlined utility chimney?

Do not assume an unlined masonry chimney is suitable for a gas water heater. The appliance and chimney need evaluation to determine the required venting correction before continued reliance on that flue.

Does a water heater liner prevent carbon monoxide?

A suitable liner helps contain and vent combustion gases, but it is only one part of the system. Proper draft, appliance operation, combustion air, connectors, terminations, and working carbon monoxide alarms also matter.

Does a carbon monoxide detector replace a chimney inspection?

No. A carbon monoxide alarm provides an important warning when it detects a dangerous condition. It does not evaluate liner damage, corrosion, obstruction, incorrect sizing, or moisture inside the utility flue.

Do I need a permit for a water heater liner in Wilmette?

Wilmette lists flues among work requiring permits, although a replacement-only water heater appears on its no-permit list. Because a liner installation changes the flue rather than simply replacing the appliance, confirm the current project requirements with Wilmette Community Development.

Who should I call if I smell gas near the water heater?

Leave the area immediately. Do not operate switches, phones, appliances, or anything that may create a spark while inside. From a safe location, call North Shore Gas at 866-556-6005. For a carbon monoxide alarm or suspected carbon monoxide exposure, leave the building and call 911.

Serving Wilmette and the Surrounding Communities

Chimney Monkey is based at 741 Hastings Dr in Buffalo Grove and helps homeowners throughout Wilmette inspect utility flues, water-heater chimneys, liners, and exterior masonry.

We also serve Kenilworth, Winnetka, Evanston, Glenview, Northfield, Northbrook, Glencoe, Highland Park, Lake Forest, Lake Bluff, Deerfield, and nearby Cook and Lake County communities.

Not sure whether the water heater needs a new liner or whether the existing flue can remain in service?

Start by identifying the appliance, the connected flue, and the condition inside the chimney.